# Schedule

All seminars are hosted as webinars on Zoom at 12:00 PM PST (3:00 PM EST) on Fridays, and can be accessed at these links:

Note: Attendance for the Zoom Webinar is capped at 500 participants on a first come first serve basis. Please connect on time to guarantee your participation. The YouTube link is unlimited.

## Upcoming seminars:

### See you again in Fall 2023!

## Past seminars:

### Friday, April 28th, 2023:

Dan Stamper-Kurn, University of California, Berkeley & Lawrence Berkeley National Laboratory [video]

Exploring measurement and feedback with ultracold atoms in an optical cavity

By placing ultracold atoms within an optical cavity, we couple a many-body quantum system with a single mode of light. This light serves a dual role, both as an actuator on and mediator of interactions among the atoms, and also as an information carrier that allows sensitive measurement, potentially in vivo, of the quantum system. I will discuss experiments performed on optical cavities containing either large atomic ensembles or mesoscopic atomic systems constructed atom-by-atom using optical tweezer arrays. We demonstrate how the response of an optical cavity to the collective spin of an ensemble can effect autonomous feedback stabilization to a non-equilibrium steady state. Sensitivity to single atomic spins enables mid-circuit measurement on an atom tweezer array, a prelude to implementing feedback and error correction. Lastly, we explore the collective optical response of a structured array of atoms, demonstrating super- and sub-radiant Rayleigh scattering. This collective optical response sets the stage for future experiments on light-mediated forces and a quantum-gas-microscope approach to studying phase transitions in open quantum systems.

### Friday, April 14th, 2023:

Sebastian Hofferberth, Universität Bonn [video]

Waveguide QED with Rydberg superatoms

The field of waveguide QED investigates how light in a single mode propagates through a system of localized quantum emitters. If the coupling between individual photons and individual emitters is sufficiently strong, this at first glance simple situation can turn into an interesting quantum many-body system where the photons mediate an effective interaction between the emitters, or vice versa the cascaded interaction with saturated emitters can be interpreted as a photon-photon interaction.

We realize effective two-level emitters by exploiting the Rydberg blockade effect of atomic ensembles. By confining N~10.000 atoms to a single blockaded volume, the ensemble only supports a single excitation creating a so-called Rydberg superatom. Due to the collective nature of the excitation, the superatom effectively represents a single emitter coupling strongly to single photons. The directional emission of the superatom into the inital probe mode realizes a waveguide-like system in free-space without any actual light-guiding elements.

This talk will discuss how we scale this system from one to few strongly coupled superatoms to study how the propagation of quantized light fields through a small emitter chain results in photon-photon correlations and entanglement between the emitters. We also show how controlled dephasing of the collective excitation into collective dark states can be used to subtract exact photon numbers from an incoming light pulse.

In this VAMOS talk I will review recent experiments, in which a trapped-ion, cooled to the ground-state of a Paul trap was interacting with ultracold neutral atoms. When applying the toolbox of trapped-ion quantum computing to these systems, atom-ion interactions can be studied with great sensitivity. In this talk I will review two aspects of ultracold atom-ion interaction that are very different from those of interactions in neutral gasses. The first is that, due to the relatively long-range of the atom-ion polarization potential, the required temperature for s-wave scattering is very low and often out of reach. However, the other side of this long-range interaction, quantum interference effects can be observed at a temperature which is orders of magnitude above the s-wave regime. Secondly, unlike neutral atoms, here the ion is strongly confined by the trap. The presence of the trap for one of the interacting particles breaks translational invariance and leads to the formation of molecules in binary, elastic, collisions.

### Friday, March 17th, 2023:

Joseph Thywissen, University of Toronto [video]

Orbital interactions between strongly confined fermions

Exchange-antisymmetric pair wavefunctions in fermionic systems hold the promise of new types of quantum simulations, topological quantum gates, and exotic few-body states. However, p-wave and other antisymmetric interactions are weak in naturally occurring systems, and their enhancement via Feshbach resonances in ultracold systems has been limited by three-body loss. Here we revisit p-wave interactions in the presence of strong confinement.

In a first scenario, we study the two-body correlation strength of quasi-one-dimensional (q1D) ensembles of spin-polarized fermionic potassium. The strength and spatial symmetry of interactions are tuned by a nearby p-wave Feshbach resonance and by confinement anisotropy. Surprisingly, we find a scattering channel that has even particle-exchange parity along the q1D axis. These emergent s-wave collisions are enabled by orbital singlet wave functions in the transverse directions, which also confer high-momentum components to low-energy q1D collisions.

In a second scenario, we create isolated pairs of spin-polarised fermionic atoms in a multi-orbital three-dimensional optical lattice. We measure elastic p-wave interaction energies of strongly interacting pairs of atoms and find pair lifetimes to be up to fifty times larger than in free space. We demonstrate that on-site interaction strengths can be widely tuned but collapse onto a universal single-parameter curve when rescaled by the harmonic energy and length scales of a single lattice site. Since three-body processes are absent in this scenario, we are able to observe elastic unitary p-wave interactions for the first time. Observations are compared both to an analytic solution for two harmonically confined atoms interacting via a p-wave pseudopotential, and to numerical solutions using an ab-initio interaction potential.

### Friday, March 3rd, 2023:

Andrei Derevianko, University of Nevada, Reno [video]

Fragility of life or what would happen if the speed of light were smaller

The anthropic principle implies that life can emerge and be sustained only in a narrow range of values of fundamental constants. We show that anthropic arguments can set powerful constraints on transient variations of the fine-structure constant alpha (or, colloquially, speed of light) over the past 4 billion years since the appearance of lifeforms on Earth. The regime of transient variation of fundamental constants is characteristic of clumpy dark matter models. We argue that the passage through Earth of a macroscopic dark matter clump with a value of alpha inside differing substantially from its nominal value would make Earth uninhabitable. We demonstrate that in the regime of extreme variation of alpha, the periodic table of elements is truncated, and water fails to serve as a universal solvent. Thereby, the anthropic principle constrains the likelihood of such encounters on a 4-billion-year timescale. This enables us to improve existing astrophysical bounds on certain dark matter model couplings by several orders of magnitude.

### Friday, Feb 17th, 2023:

Mackillo (Mack) Kira, University of Michigan [video]

Lightwave attoclocking in solids

Extremely strong lightwaves move semiconductor quasiparticles coherently much faster than scattering occurs, which paves the way for detailed control and transport of electronic quantum coherences, resembling the capabilities of atomic attosecond science. I will overview the new frontiers lightwave electronics opens for accessing and controlling multi-electron quantum information in solids. Examples include flipping valleytronic qubits in less than 5 femtoseconds, detecting quantum-material bands (including topology) with crystal-momentum combs, and direct attosecond clocking of many-body effects in diverse quantum systems. I will also speculate how these advancements could yield ultrafast access and control of electronic quantum information in solids, possibly at petahertz rates.

### Friday, Feb 3rd, 2023:

Andrew Ludlow, NIST [video]

Enhanced cooling and quantum control of divalent atoms for the next generation of optical lattice clocks

Atomic clocks operating in the optical domain are now capable of measuring time at up to eighteen digits of precision, and fundamental limits offer significant potential for even higher performance. To reach this goal, the next-generation of optical lattice clocks will require more advanced control of lattice-trapped atomic samples. To this end, here we consider two novel laser cooling strategies that exploit the extensive atom-laser coherence possible with divalent atomic structure. The first is a pulsed cooling process that replaces two-photon Raman cooling techniques with single-photon velocity-selection using the narrowband clock transition. The second is an excited state Sisyphus cooling mechanism that offers efficient three-dimensional cooling in a lattice. We demonstrate sub-recoil, nK-regime cooling of ytterbium in both cases, which aid in loading very shallow lattices that are less susceptible to lattice-induced light shifts. We also implement coherent delocalization of ultracold ytterbium through controlled tunneling in a Wannier-Stark lattice, which helps reduce atomic interactions. Finally, we also discuss a parallel effort on development of portable ytterbium lattice clocks for applications beyond the lab.

### Friday, Jan 20, 2023:

Hartmut Häffner, University of California, Berekley [video]

Towards a trapped electron quantum computer

We explore the feasibility of processing quantum information encoded in the spin of electrons trapped ion a Paul trap. The main idea is to replace the ions in a QCCD (quantum charge-coupled device) ion trap quantum computer with electrons. The combination of the low mass and simple internal structure enables high-speed operation while allowing for high-fidelity operation. In particular, our simulation of common two-qubit error sources show that error rates of less than 1E-4 at clock speeds of close to 1 MHz for transport and quantum gates might be feasible.

Towards this goal, we trap single to few electrons in a millimeter-sized quadrupole Paul trap driven at 1.6 GHz in a room-temperature ultra-high vacuum setup. Electrons with sub-5 meV energies are introduced into the trap by near-resonant photoionisation of an atomic calcium beam and confined by microwave and static electric fields for several tens of milliseconds. A fraction of electrons remains trapped and shows no measurable loss for measurement times up to a second. Electronic excitation of the motion reveals secular frequencies from several tens to hundreds of MHz.

### Friday, December 16th, 2022:

Speaker: Hannes Bernien, University of Chicago [video]

Building dual-species quantum processors and quantum networks atom-by-atom

Reconfigurable arrays of neutral atoms are an exciting platform to study quantum many-body phenomena and quantum information protocols. Their excellent coherence combined with programmable Rydberg interactions have led to intriguing observations such as quantum phase transitions, the discovery of quantum many-body scars, and novel quantum computing architectures. Here, I will introduce new methods for controlling and measuring atom arrays that open up new directions in quantum state control, quantum feedback and many-body physics. First, I will introduce a dual-species atomic array in which the second atomic species can be used to measure and control the primary species. We use an array of cesium qubits to correct correlated phase errors on an array of rubidium data qubits [1]. Crucially, by combining mid-circuit readouts, data processing, and feed-forward operations, these correlated errors are suppressed within the execution of the quantum circuit.

An alternative, hybrid approach for engineering interactions and scaling these quantum systems is the coupling of atoms to nanophotonic structures in which photons mediate interactions between atoms. Such a system can function as the building block of a large-scale quantum network. In this context, I will present quantum network node architectures that are capable of long-distance entanglement distribution at telecom wavelengths [2].

[1] Singh*, Bradley*, Anand*, Ramesh, White, Bernien arXiv:2208.11716 (2022)

[2] Menon, Singh, Borregaard, Bernien NJP 22, 073033 (2020)

### Friday, December 2nd, 2022:

Linda Young, Argonne National Lab / University of Chicago [video]

Stimulated X-ray Raman Spectroscopy with Free-Electron Lasers

X-ray free electron lasers (XFELs), with widely tunable output pulses possessing joint temporal and spatial resolution on the atomic scale and peak intensities up to 10^20 W/cm 2 , have revolutionized x-ray science. These characteristics have enabled multiphoton physics in the x-ray regime and the observation of nonlinear phenomena such as atomic x-ray lasing, second harmonic generation and optical/x-ray wavemixing. Going beyond these demonstrations, XFEL experiments have been proposed as a site-specific probe of ultrafast electronic excitations which can inform a wide variety of processes on their natural timescales, e.g. photosynthesis, and light harvesting. Here the use of nonlinear two-photon x-ray Raman techniques enables one to beat the core-hole lifetime that limits standard x-ray absorption and emission methods. In analogy with optical coherent Raman scattering techniques that probe ultrafast vibrational processes, x-ray nonlinear Raman techniques probe ultrafast electronic processes. Building from two pillars, propagation in dense media to amplify spontaneous Raman signals by ~10^8 and narrowband spectral spikes inherent in self-amplified spontaneous emission (SASE) pulses, we demonstrate high-resolution stimulated x-ray Raman spectroscopy (bandwidths ~0.3 eV) using broadband SASE pulses (~7 eV bandwidth). This ability to use broadband SASE pulses to achieve high-resolution, ultrafast x-ray Raman spectral snapshots allows one to rapidly acquire data in parallel and paves the way toward an understanding of site-specific electronic dynamics.

Work supported by U.S. Department of Energy, Office of Science, Basic Energy Science, Chemical Sciences, Geosciences and Biosciences Division under contract number DE-AC02-06CH11357.

### Friday, November 18th, 2022:

Ofer Firstenberg, Weizmann Institute of Science [video]

Quantum vortices of strongly interacting photons

Have you ever tried dragging a plate through the surface of a water pool?

Quite excitingly, you would form a pair of vortex and antivortex, which would propagate steadily on the water’s surface!

In optics, vortices manifest as phase twists of the electromagnetic field, usually formed by the interaction with matter. Vortex formation due to light interacting with light requires strong optical nonlinearity and has therefore been confined, until now, to the classical regime.

We will discuss the experimental realization of strong, effective photon-photon interaction in a quantum nonlinear medium based on ultracold Rydberg atoms. This interaction results in a faster phase accumulation for copropagating photon pairs. Similarly to a plate pushing water, the accumulation of localized excess phase produces a quantum vortex-antivortex pair within the two-photon wavefunction. The “conditional” π phase localized between these vortices can be used for deterministic quantum logic operations. Moreover, triplets of photons produce vortex lines and a vortex ring, giving rise to a 2π conditional phase. The deviation from the 3π conditional phase, expected for a quantum Kerr-nonlinear medium, attests to genuine three-photon interaction.

### Friday, November 4th, 2022:

Aephraim Steinberg, University of Toronto [video]

Quantum archaeology: How much time do atoms spend “under the tunnel barrier," and how much time do photons spend trapped “inside” atoms?

One of the most famous tidbits of received wisdom about quantum mechanics is that one "cannot ask" how a particle got to where it was finally observed, e.g., which path of an interferometer a photon took before it reached the screen. What, then, do present observations tell us about the state of the world in the past? I will describe two experiments looking into aspects of this “quantum retrodiction."

The main experiment I will focus on addresses a century-old controversy: that of the tunneling time. Since the 1930s, and more heatedly since the 1980s, the question of how long a particle spends in a classically forbidden region before being transmitted has been a subject of debate. Using Bose-condensed Rubidium atoms cooled down to a nanokelvin, we have now measured just how long they spend inside an optical beam which acts as a “tunnel barrier” for them. I will describe these ongoing experiments, as well as proposals we are refining to study exactly what happens during the time it takes to “collapse” an atom to be in the barrier.

I will also say a few words about more recent experiments, which revisit the common picture that when light slows down in glass, or a cloud of atoms, it is because the photons “get virtually absorbed” before being sent back along their way. We have carried out an experiment that lets us distinguish between the time spent by transmitted photons and by photons which are eventually absorbed, asking the question “how much time are atoms caused to spend in the excited state by photons which are not absorbed?”

[1] Measuring the time a tunnelling atom spends in the barrier, Ramón Ramos, David Spierings, Isabelle Racicot, Aephraim M. Steinberg, Nature 583, 529 (2020).

[2] Observation of the decrease of Larmor tunneling times with lower incident energy, David C. Spierings & Aephraim M. Steinberg, Phys. Rev. Lett. 127, 133001 (2021).

[3] Measuring the time atoms spend in the excited state due to a photon they do not absorb, Josiah Sinclair, Daniela Angulo, Kyle Thompson, Kent Bonsma-Fisher, Aharon Brodutch, Aephraim M. Steinberg, PRX Quantum 3, 010314 (2022).

[4] How much time does a resonant photon spend as an atomic excitation before being transmitted?, Kyle Thompson, Kehui Li, Daniela Angulo, Vida-Michelle Nixon, Josiah Sinclair, Howard M. Wiseman, Aephraim M. Steinberg, in preparation

### Friday, October 21st, 2022:

Piet O. Schmidt, PTB & Leibniz University Hannover [video]

An Optical Atomic Clock Based on a Highly Charged Ion

Optical atomic clocks are the most precise and accurate measurement devices ever constructed, reaching fractional systematic uncertainties below one part in 10 18 [1]. Their exceptional performance opens up a wide range of applications in fundamental science and technology. The extreme properties of highly charged ions (HCI) make them highly sensitive probes for tests of fundamental physical theories [2, 3]. Furthermore, these properties make them significantly less sensitive to some of the leading systematic perturbations that affect state-of-the-art optical clocks, making them exciting candidates for next-generation clocks [4, 2]. The technical challenges that hindered the development of such clocks have now all been overcome, starting with their extraction from a hot plasma and sympathetic cooling in a linear Paul trap [5], readout of their internal state via quantum logic spectroscopy [6], and finally the preparation of the HCI in the ground state of motion of the trap [7], which allows levels of measurement accuracy to be reached that were previously limited to singly-charged and neutral atoms. Here, we present the first operation of an atomic clock based on an HCI (Ar 13+ in our case) and a full evaluation of systematic frequency shifts [8]. The achieved uncertainty is almost eight orders of magnitude lower than any previous frequency measurements using HCI. Measurements of some key atomic parameters confirm the theoretical predictions of the favorable properties of HCIs for use in clocks. The comparison to the 171 Yb + E3 optical clock [9] places the frequency of this transition among the most accurately measured of all time. Furthermore, by comparing the isotope shift between 36 Ar 13+ and 40 Ar 13+ to improved atomic structure calculations, we were able for the first time to resolve the largely unexplored QED nuclear recoil effects. Finally, prospects for 5 th force tests based on isotope shift spectroscopy of Ca + /Ca 14+ isotopes and the high-sensitivity search for a variation of the fine-structure constant using HCI will be presented. This demonstrates the suitability of HCI a references for high-accuracy optical clocks and to probe for physics beyond the standard model.

References

[1] Brewer, S. M. et al., Phys. Rev. Lett. 123, 033201 (2019).

[2] Kozlov, M. G. et al., Rev. Mod. Phys. 90, 045005 (2018).

[3] Safronova, M. S. et al., Rev. Mod. Phys. 90, 025008 (2018).

[4] Schiller, S., Phys. Rev. Lett. 98, 180801 (2007).

[5] Schmöger, L. et al., Science 347, 1233–1236 (2015).

[6] Micke, P. et al., Nature 578, 60–65 (2020).

[7] King, S. A. et al., Phys. Rev. X 11, 041049 (2021).

[8] King, S. A. et al., http://arxiv.org/abs/2205.13053 .

[9] Lange, R. et al., Phys. Rev. Lett. 126, 011102 (2021).

### Friday, October 7th, 2022:

Susanne Yelin, Harvard University [video]

Dense arrays: a novel quantum tool

The physics of cooperative atoms/radiators in regular 2D arrays is dominated by two properties: first, a strongly frequency-selective reflectivity and second, the ability to confine polariton modes cleanly on the surface. This makes such a system highly sensitive to and controllable by light fields. Applications of these systems include beam steering, quantum information processing, metrology, and nonlinear single-photon techniques. I will introduce the basic physics aspect of such a system, explain some of the implementations with atoms and solid materials before introducing some applications.

### Friday, Sept 23th, 2022:

Elizabeth Goldschmidt, University of Illinois Urbana-Champaign [video]

Quantum photonics with emitters in solid-state

Optically active and highly coherent emitters in solids are a promising platform for a wide variety of quantum information applications, particularly quantum memory and other quantum networking tasks. Rare-earth atoms, in addition to having record long coherence times, have the added benefit that they can be hosted in a wide range of solid-state materials. We can thus target particular materials (and choose particular rare-earth species and isotopes) that enable certain application-specific functionalities. I will discuss several ongoing projects with rare-earth atoms in different host materials and configurations. This includes investigations of inhomogeneous broadening in rare-earth ensembles, which is highly host-dependent and plays an important role in which quantum memory protocols can be implemented in any given system. I will present results on our efforts to identify and grow new materials with rare-earth atoms at stoichiometric concentrations in order to reduce the disorder-induced inhomogeneous broadening. I will also discuss our work investigating photonic integration of rare-earth doped samples that aims to increase the light-atom interaction for practical quantum devices. I will show results from our work with rare-earth atom dopants in thin-film lithium niobate, which admits standard nanofabrication techniques, and show the suitability of this platform for quantum applications.

### Friday, May 27th, 2022:

Michael W. Zuerch, University of California Berkeley [video]

Nonlinear Extreme Ultraviolet Spectroscopy: A Novel Probe for Surfaces and Symmetry Breaking with Elemental Resolution

Extreme ultraviolet second harmonic generation spectroscopy (XUV-SHG) is an emerging technique used to study inversion symmetry breaking with core-state specificity. This novel technique was only recently demonstrated for the first time measuring the surface spectrum of carbon films [1]. Pushing nonlinear spectroscopic techniques to the XUV and soft X-ray regime has several advantages. For example, light pulses in these regimes can penetrate materials providing access to buried interfaces and symmetry-broken states in bulk material with specificity to a single atomic species. Recent experiments demonstrated quantification of the interfacial bond geometry of an organic-inorganic interface [2] and measurement of a surface spectrum of titanium [3]. Measuring the angular distribution of SXR-SHG has enabled additional sensitivities such as to the nature of the symmetry state itself [4]. Recently, we utilized XUV-SHG spectroscopy to investigate the polar metal phase of LiOsO3 [5]. In polar metals the coexistence of polarity and metallicity is unexpected as the itinerant conducting electrons in metals are expected to screen long-range electrostatic forces that are typically required to stabilize a macroscopic polarization. The large difference of atomic number renders it challenging to study this material with electron and X-ray scattering techniques. We apply XUV-SHG to study the symmetry properties in this material with specificity to the lithium atoms in the lattice. In the experiment we focus an intense femtosecond X-ray laser beam obtained by a free-electron laser onto the material with photon energies in the range of 28 to 33 eV, which enables reaching a resonance condition for the Li 1s electrons around the K-edge at ~54 eV. A sensitivity to broken inversion symmetry appears above the Li K-edge. We compare the experimental spectra with numerical calculations based on time-dependent density functional theory that show how the spectrally-resolved SHG varies with Li-displacement. As the first demonstration of XUV-SHG spectroscopy around a phase transition, these results pave the way for using nonlinear XUV methods to investigate broken symmetry from an element-specific perspective. I will also briefly discuss recent results on surface-specific XUV-SHG spectroscopy on a solid-state electrolyte where we find reduced Li ion mobilities at the surface and its relation to phonon modes providing important input for the design of future all-solid-state batteries [6]. Advancements of enabling even shorter pulse durations in the attosecond regime at FELs provide interesting new opportunities as the inherently increased intensity ideally combines with exploiting nonlinear material responses as they are facilitated in XUV-SHG. In addition, inherent femtosecond to sub-femtosecond temporal resolution will enable studying phase transitions on the electronic timescale and provide unique opportunities for studying fundamental physical phenomena, chemical dynamics at interfaces, and materials sciences.

[1] R. K. Lam, et al., Phys. Rev. Lett. 120, 023901 (2018).

[2] C. P. Schwartz, et al., Phys. Rev. Lett. 127, 096801 (2021).

[3] T. Helk, et al., Sci. Adv. 7, eabe2265 (2021).

[4] C. Uzundal, et al., Phys. Rev. Lett. 127, 237402 (2021).

[5] E. Berger, et al., Nano Letters 21, 6095–6101 (2021).

[6] C. Woodahl, et al., “Structure of Lithium at a Perovskite Interface Probed by Second Harmonic Extreme Ultraviolet Spectroscopy” (2022), submitted.

### Friday, May 13th, 2022:

Mark Saffman, University of Wisconsin - Madison [video]

Atom arrays for computation, communication, and sensing

Advances in our ability to control the quantum state of individual atoms are enabling new quantum enhanced devices. I will present our recent progress in neutral atom quantum computing, and point to work in progress using atom arrays for quantum communication and sensing.

One of the daunting challenges in developing a computer with quantum advantage is the need to scale to a very large number of qubits while maintaining the fidelity and isolation of pristine, few qubit demonstrations. Neutral atoms are one of the most promising approaches for meeting this challenge, in part due to the combination of excellent isolation from the environment and the capability to turn on strong two-qubit interactions by excitation to Rydberg states. We program circuits with a universal set of quantum gates using microwave and optical control of Cs atom qubits. Two-qubit gates are implemented using Rydberg interactions. We show that weak interactions, outside the blockade radius, can form the basis of a fast entangling gate protocol with high fidelity, using simultaneous excitation of atom pairs. With this approach several quantum algorithms have been demonstrated on a small scale computer. Work in progress aimed at scaling up the array, mid circuit measurements for error correction, quantum networking, and sensing, will also be described.

### Friday, April 29th, 2022:

Kai-Mei Fu, University of Washington [video]

Quantum point defects: Can these defects be less, well, defective?

Point defects in crystals are the solid state analog to trapped ions. Thus these “quantum defects”, which can be integrated into solid-state devices, have gained popularity as qubit candidates for scalable quantum networks. In this talk, I will introduce some of the basic quantum defect properties desirable for quantum network applications. I will highlight my own group’s efforts at understanding and controlling the properties of defects in diamond including (1) synthesis, frequency and emission control of deep-level vacancy complexes in diamond and (2) properties of shallow-level donors in ZnO, including single donors and intentionally synthesized donors in ZnO.

### Friday, April 15th, 2022:

Alexandra Landsman, The Ohio State University [video]

Photoemission delays in molecules

Attosecond chronoscopy has revealed small but measurable delays in photoionization, characterized by the ejection of an electron on absorption of a single photon. Ionization-delay measurements in atomic targets provide a wealth of information about the timing of the photoelectric effect, resonances, electron correlations and transport. However, extending this approach to molecules presents challenges, such as identifying the correct ionization channels and the effect of the anisotropic molecular landscape on the measured delays. This talk will focus on some experimental measurements of ionization delays in molecules and their theoretical interpretation using recently developed Classical Wigner Propagation (CWP) method. It will be shown how molecular ionization delays can be used to probe electron localization and molecular landscape.

### Friday, April 1st, 2022:

Tara Fortier, NIST [video]

Time and the precise measurement and application of atomic clocks

TBA.

### Friday, March 18th, 2022:

Shelby Kimmel, Middlebury College [video]

Speed-ups for Quantum Algorithms with Easier Inputs

The difficulty of solving a problem is often input dependent. For example, if you are searching an unordered list for an item, it is easier to find one if there are multiple copies. Quantum algorithms should also do better on easier inputs, but prior work for an important class of query algorithms only gives an improvement if you know ahead of time that you have an easier input. We designed an algorithm that matches the complexity (up to log factors) of the prior algorithm, but without knowing the difficulty of the input in advance. This work opens the door for average-case analyses of a broad range of quantum query problems. (Joint work with Noel Anderson and Jay-U Chung.)

### Friday, March 4th, 2022:

Marianna Safronova, University of Delaware [video]

Searches for New Physics with Quantum Sensors in the Laboratory and in Space

The development of atomic clocks with systematic uncertainties in the 10-18 range enables searches for the variation of fundamental constants, dark matter, and violations of Lorentz invariance. I will give an overview of dark matter searches and other fundamental physics studies with atomic and nuclear clocks and focus on development of clocks with the highest sensitivities to new physics. I will discuss recent advances in theory of novel clocks based on highly-charged ions and efforts to develop a nuclear clock. Recent ideas on dark matter searches and test of general relativity with clocks in space will be discussed. I will also report a release of the new version of an online portal for high-precision atomic data and computation. Future plans to add data for more systems as well as to release computer codes are discussed.

### Friday, February 18th, 2022:

David Schuster, University of Chicago [video]

Building materials from microwave photons

The study of condensed matter systems in electronic systems within solid-state materials has a long history. More recently, it has become possible to realize synthetic systems out of controllable components in ultracold atom or photonic systems. In this talk, I will discuss how we can use superconducting circuits to make single photons act like strongly interacting hard-core bosons. Dissipation and disorder commonly compete with or mask many-body effects, and one of the primary challenges in both synthetic and physical condensed matter systems is to control these forces. Here we will try to leverage our control over dissipation and disorder to use them as tools to stabilize and prepare a lattice of strongly interacting microwave photons. In addition to exploring strongly interacting photons, we will see how to engineer synthetic magnetic fields for photons, allowing them to experience analogs of the quantum Hall effect seen in charged particles

### Friday, December 17th, 2021:

Ana Maria Rey, JILA/CU Boulder [Video]

Optical lattice clocks: From state-of-the-art time keepers to an exquisite platform for probing many body physics.

Harnessing the behavior of complex systems is at the heart of quantum technologies. Precisely engineered ultracold gases are emerging as a powerful tool for this task. In this talk I will explain how ultracold strontium atoms trapped by light can be used to create optical lattice clocks – the most precise time keepers ever imagined. I am going to explain why these clocks are not only fascinating, but of crucial importance since they are helping us to answer cutting-edge questions about the quantum world, to unravel big mysteries of our universe and to build the next generation of quantum technologies.

### Friday, December 3rd, 2021:

Ashvin Vishwanath, Harvard University [Video]

Creating Highly Entangled Quantum States in Synthetic Matter

In contrast to ordered states such as crystals, magnets and BECs, qualitatively different patterns of quantum entanglement emerge in states with intrinsic topological order (ITO). Examples include fractional Quantum Hall states, quantum spin liquids and fracton phases. Their unique properties hold promise for a range of applications - most notably in the quest for fault-tolerant quantum computing. However, ITO is are notoriously hard to realize experimentally. For example, quantum spin liquids were proposed by Anderson in 1973, but remain to be clearly identified in any magnetic material. Key obstacles include the propensity of spin systems towards order and the lack of straightforward signatures associated with ITO. Synthetic platforms, such as Rydberg atom arrays, with their high tunability and access to non-local `string' correlations can help circumvent these barriers. In this talk, I will describe our theoretical proposal to create ITO in Rydberg atom arrays, akin to realizing BECs of `quantum loops', methods to characterize their subtle signatures, and the experimental progress towards these goals. Finally, I will describe our ongoing exploration of new approaches for realizing ITOs in synthetic matter.

### Friday, November 19th, 2021:

Sarang Gopalakrishnan, Penn State University [Video]

Surprises in high-temperature transport

One might expect high-temperature transport to be incoherent and diffusive. However, many experimentally relevant one-dimensional systems, such as the Heisenberg and Hubbard models, feature long-lived quasiparticles that give rise to anomalous transport. I will present a quantitative theory of high-temperature transport in such systems, in terms of a dense gas of stable quasiparticles that scatter elastically off one another. This theory resolves some puzzles, but many remain; I will discuss how observables beyond linear response, which are straightforward to measure in ultracold atomic gases, can shed light on these.

### Friday, November 5th, 2021:

Bryce Gadway, UIUC [Video]

Hamiltonian engineering in synthetic dimensions

Artificial quantum materials made up of atoms, molecules, and light open up many exciting opportunities to explore many-body physics in exotic regimes. This has already enabled the realization of new states of matter relevant to condensed matter physics, and this approach further promises to make connections to fields like high energy and nuclear physics. Many techniques have been added to the Hamiltonian engineering toolbox over the past decades, serving to expand the kinds of systems and phenomena that can be explored in AMO laboratories. In this talk, I'll review approaches to Hamiltonian engineering based on "synthetic dimensions," which can be broadly seen as utilizing extra degrees of freedom - such as the spin states of an atom - to open up a new dimension for studying problems of transport and quantum dynamics. I will discuss some of our recent results on disordered systems and interaction effects in synthetic lattices of atomic momentum states. and will further highlight some of the more exotic kinds of systems and phenomena that may be brought to life in synthetic lattices based on cold atoms and other platforms.

### Friday, October 22nd, 2021:

Monika Schleier-Smith, Stanford University [Video]

Atoms Interlinked by Light: Programmable Interactions and Emergent Geometry

Interactions govern the flow of information and the structure of correlations in quantum systems. Typical interactions decay with distance, resulting in a network of connectivity that is dictated by geometry. Yet a variety of applications in quantum state engineering, quantum simulation, and combinatorial optimization demand more versatile control of the graph of interactions, including nonlocal connectivity. I will report on the realization of programmable nonlocal interactions in an array of atomic ensembles within an optical resonator, where photons convey information between distant atomic spins. We program the coupling graph by tailoring the spectrum of an optical drive field, probing the resulting connectivity by observing spatial correlations arising from light-mediated spin mixing. As illustrative examples, we explore frustrated interactions, non-trivial topologies, and an emergent treelike geometry inspired by concepts of quantum gravity.

The future global quantum internet will require high-performance matter-photon interfaces. The highly demanding technological requirements indicate that the matter-photon interfaces currently under study all have potentially unworkable drawbacks, and there is a global race underway to identify the best possible new alternative. For overwhelming commercial and quantum reasons, silicon is the best possible host for such an interface. Silicon is not only the most developed integrated photonics and electronics platform by far, isotopically purified silicon-28 has also set records for quantum lifetimes at both cryogenic and room temperatures [1]. Despite this, the vast majority of research into photon-spin interfaces has notably focused on visible-wavelength colour centres in other materials. In this talk I will introduce a variety of silicon colour centres and discuss their properties in isotopically purified silicon-28. Some of these centres have zero-phonon optical transitions in the telecommunications bands [2], some have long-lived spins in their ground states [3], and some, including the newly rediscovered T centre, have both [4].

[1] K. Saeedi, S. Simmons, J.Z. Salvail, et al. Science 342:830 (2013).

[2] C. Chartrand, L. Bergeron, K.J. Morse, et al. Phys. Rev. B 98:195201 (2018).

[3] K. Morse, R. Abraham, A. DeAbreu, et al. Science Advances 3:e1700930 (2017).

[4] L. Bergeron, C. Chartrand, A.T.K. Kurkjian, et al. PRXQuantum 1 020301 (2020).

### Friday, September 24th, 2021:

Tim Taminiau, TU Delft [Video]

Quantum processors based on spins in diamond

Electron-nuclear spin systems based on optically active defects in diamond provide a promising platform for quantum networks, quantum simulations and quantum computation. We have recently shown that it is possible to detect and image large numbers of nuclear spins around a single NV center [1]. In this talk, I will review our recent progress in leveraging these systems for quantum simulations of many-body physics [2] and for the development of quantum computations [3].

[1] M. H. Abobeih et al., Nature 576, 411 (2019)

[2] J. Randall et al., arXiv:2107.00736 (2021)

[3] C. E. Bradley et al., Phys. Rev. X. 9, 031045 (2019)

### Friday, September 10th, 2021:

Bill Phillips, NIST/University of Maryland [Video]

The Quantum Reform of the International System of Units.

The International System of Units (SI), the modern metric system, has recently undergone its most revolutionary change since the beginning of the metric system during the French Revolution. The nature of this revolution is that all of the base units of the SI are now defined by fixing values of natural constants. The realization of the units is now, both philosophically and practically, strongly quantum and specifically connected to AMO. This talk will describe why this reform was needed and how it is done.

### Friday, May 28th, 2021:

Jack Harris, Yale University [Video]

Measuring the higher-order phonon statistics in a nanogram volume of superfluid helium

We detect the individual sideband photons produced by an optomechanical device consisting of a nanogram of superfluid helium confined in a cavity. We use the photon-counting data to probe the phonon-phonon correlations (up to fourth order, and for both normal-ordering and anti-normal-ordering) in a single acoustic mode of the superfluid. The data is consistent the assumption that the acoustic mode is in a thermal state with mean phonon number ~ 1. We also show that the acoustic mode can be driven to a coherent amplitude corresponding to several thousand phonons with no decrease in the purity of the acoustic state. We will discuss a number of applications of these results, including to the distribution of entanglement over kilometer-scale optical fiber networks.

### Friday, May 21st, 2021:

Christian Gross, Max Planck Institute of Quantum Optics [Video]

Spin and density cross-correlations in the Hubbard systems

Quantum gas microscopes provide experimental access to novel observables, foremost, single particle resolved multi-point correlation functions. These offer a novel microscopic window into the physics of strongly correlated many-body physics. In our setup we developed a method for the simultaneous detection of spin and density, which allows us to compute their cross correlations. In one dimension this technique revealed hidden spin-charge correlations, incommensurate magnetism and dynamical spin-charge separation by tracking the spin and charge wave fronts. Here we report on recent experiments building on this technique, in which we study a two-dimensional Hubbard system for varying doping levels. We observe the formation of magnetic polarons at low doping levels, their gradual disappearance for increasing doping, eventually leading to signals expected from a fermi liquid at high doping levels.

### Friday, May 14th, 2021:

Benjamin Lev, Stanford University [Video]

Topological pumping of a 1D dipolar gas into strongly correlated prethermal states

Long-lived excited states of interacting quantum systems that retain quantum correlations and evade thermalization are of great fundamental interest. We will present our observation of nonthermal states in a bosonic 1D quantum gas of dysprosium. These are created by stabilizing a super-Tonks-Girardeau gas against collapse and thermalization with repulsive long-range dipolar interactions. Stiffness and energy-per-particle measurements show that the system is dynamically stable regardless of contact interaction strength. This enables us to cycle contact interactions from weakly to strongly repulsive, then strongly attractive, and finally weakly attractive. We show that this cycle is an energy-space topological pump (caused by a quantum holonomy). Iterating this cycle offers an unexplored topological pumping method to create a hierarchy of increasingly excited prethermal states akin to scar states.

### Friday, May 7th, 2021:

Martin Centurion, University of Nebraska - Lincoln [Video]

Imaging ultrafast dynamics in molecules with electron diffraction

The conversion of light into chemical energy and heat at the molecular level drives many processes in nature, such as vision and photosynthesis, and has important implications for solar energy conversion and storage. After absorbing a photon, molecules can very efficiently convert energy by undergoing structural transformation through coherent nuclear motions, breaking bonds and making new ones. In order to understand and control these processes, it is essential to be able to observe them on their natural spatio-temporal scale of Angstroms and femtoseconds. Over the last few years we have developed the method of ultrafast electron diffraction, where we use a femtosecond laser pulse to trigger a reaction and a femtosecond electron pulse to capture a diffraction pattern, from which the structural changes can be retrieved. We are now able to observe nuclear wavepacket motions in the excited state, determine the structure of transient states and follow coherent vibrations that persist after the molecule returns to the electronic ground state. In this talk, a few representative examples will be discussed where we have imaged the dynamics of rotational and vibrational wavepackets, bond breaking and structural transformations.

### Friday, April 30th, 2021:

Prineha Narang, Harvard University. [Video]

Predicting correlated and non-equilibrium light-matter interactions in quantum systems

### Friday, April 23rd, 2021:

### Localization and delocalization in kicked quantum matter

A sizable fraction of the vast field of condensed matter physics consists of the exploration of the effects of spatial periodicity on quantum mechanics. It is now widely recognized that the effects of temporal periodicity give rise to a related array of phenomena, and that the interplay of temporal and spatial periodicity with many-body interactions creates particularly rich new possibilities. In this talk I will describe two recent experiments in this area: realization of a many-body kicked quantum rotor with tunable interparticle interactions, and exploration of Aubry-André localization in a kicked lattice which is "almost always" periodic. The results illuminate a variety of phenomena ranging from the interplay of ergodicity and localization to new techniques of quantum control.

### Friday, April 16th, 2021:

### Quantum Information Nondemolition Networks

The talk will highlight photon-nondestructive information-processing modules for quantum computation and communication. This will be exemplified with nonlocal quantum logic [Daiss et al., Science 371, 614 (2021)], teleportation without ex-ante entanglement [Langenfeld et al., PRL 126, 130502 (2021)], miniaturized qubit memories [Brekenfeld et al., Nature Physics 16, 647 (2020)], and qubit tracking detectors [Niemietz et al., Nature 591, 570 (2021)]. The unparalleled performance of these modules brings the quantum internet a step closer.

### Friday, April 9th, 2021:

Synthetic quantum materials with superconducting circuits

Superconducting circuits have emerged as a rich platform for emulating synthetic quantum materials composed of artificial atoms and photonic lattices. Here, we apply this toolbox to a variety of systems to showcase the flexibility of using fabricated devices for quantum simulation. We highlight the property that these lattice sites are deformable and permit tight-binding lattices which are otherwise unattainable, even in solid-state systems. Networks of resonators can create new classes of materials, including lattices in an effective hyperbolic space, and are particularly well-suited to the implementation of line graphs. I will present connections between the geometric properties of these lattices, the spectrum, and the presence or absence of fragile topology in their flat bands. Finally, we will explore the physics of a quantum impurity coupled to the many modes of a photonic crystal; in particular, probing transport through the waveguide reveals that the propagation of a single photon becomes a many-body problem as multi-photon bound states participate in the scattering dynamics. Furthermore, we study the effective photon-photon interactions induced by the impurity by probing the inelastic scattering spectrum. The measured correlations in the emitted quadrature fields at each waveguide mode reveal signatures of multi-mode entanglement.

The bottom row of the periodic table is full of little-explored radioactive elements. Many host exotic nuclei and have unique electronic structure, granting them discovery potential. Ion trapping and laser cooling provides a means to efficiently work with small quantities of radioactive material. Radium ions are an option for utilizing (and producing) heavy radioactive molecules, and are an attractive system in their own right. We will present measurements of the radium ion's atomic structure that are essential for advanced experiments and applications, and some of the unique features and opportunities that stem from radioactivity. We will also discuss ever finer levels of control over radium, as well as our progress with heavy radioactive molecules and their prospects for precision measurement.

### Friday, March 26th, 2021:

### Probing Topological Spin Liquids on a Programmable Quantum Simulator

Quantum phases with topological order, such as quantum spin liquids, have been the focus of explorations for several decades. Such phases feature a number of remarkable properties including long-range quantum entanglement. Moreover, they can be potentially exploited for the realization of robust quantum computation, as exemplified by the paradigmatic toric code model. While some indications that such phases may be present in frustrated condensed matter systems have been previously reported, so far quantum spin liquids have eluded direct experimental detection. In this talk, I will show how a programmable quantum simulator based on Rydberg atom arrays can be used to realize and probe quantum spin liquid states. In our approach, atoms are placed on the links of a kagome lattice and coherent evolution under Rydberg blockade enables the transition into frustrated quantum states with no local order. We detect the onset of a quantum spin liquid phase of the toric code type by measuring topological string operators in two complementary bases. The properties of this state are further revealed using a lattice with non-trivial topology, representing a step towards the realization of a topological qubit. Our observations open the door to the controlled experimental exploration of topological quantum matter, and could enable the investigation of new methods for topologically protected quantum information processing.

Quantum science with molecules is poised to advance the state of the art in fields from quantum simulation to ultracold chemistry and fundamental physics. A key aspect of enhancing these capabilities is creating ultracold molecules and optically trapping them in a way that preserves coherence between their quantum states. We apply optical techniques to create microkelvin samples of diatomic strontium molecules in the absolute ground state. Using "magic" optical trapping, coherence of quantum states separated by tens of terahertz can be maintained for over 100 ms, leading to uncertainties below a part per trillion on interatomic force measurements. We explore science with these van der Waals molecules, and discuss how they could contribute to metrology and to our understanding of fundamental forces.

### Friday, March 12th, 2021:

### From many-body physics to quantum optimization with Rydberg atom arrays

Individually trapped neutral atoms provide a promising platform to engineer quantum many-body systems in a controlled, bottom-up approach. They can be readily manipulated in large numbers and interact strongly when excited to Rydberg states. In this talk I will discuss several approaches to use such systems for probing quantum many-body phenomena and implementing quantum algorithms. I will first review the basic physics of arrays of Rydberg atom, including the phase equilibrium phase diagram of one and two dimensional atom arrays, focussing on critical properties of the accessible quantum phase transitions. Then I will discuss a relation between these Rydberg atom arrays and certain combinatorial optimization problems. In particular, I will show how one can encode the solution to maximum independent set problems in the ground state of properly positioned atoms. This allows to directly implement various heuristic quantum optimization and sampling algorithms with minimal experimental overhead, and study their performance for system sizes that can’t be simulated on classical computers.

### Friday, March 5th, 2021:

### Correlating materials analysis with qubit measurements to systematically eliminate sources of noise

The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. I will describe our recent efforts to correlate direct materials characterization with single spin measurements to devise methods to stabilize highly coherent NV centers within nanometers of the surface. We also deploy these shallow NV centers as a probe to study the dynamics of a disordered spin ensemble at the diamond surface. Our approach for correlating surface spectroscopy techniques with single qubit measurements to realize directed improvements is generally applicable to many systems, and time permitting, I will describe our recent efforts to tackle noise and microwave losses in superconducting qubits.

### Friday, February 26th, 2021:

### Listening to gravitational waves above the quantum din

The Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves for the first time in 2015. Since then dozens more astrophysical observations have been confirmed. To detect these spacetime ripples requires measurement with sub-attometer precision. I will describe the quantum technologies that make such a measurement possible and enable present and future discoveries.

### Friday, February 19th, 2021:

### Spin dynamics of ultracold atoms in optical lattices

In a Mott insulator, the motion of atoms is frozen out, and the study and control of the spin degree of freedom emerges as a new frontier. Ultracold atoms in optical lattices are an ideal platform to realize Heisenberg spin models and probe their dynamics. Until very recently, all experimental studies addressed the special case of an isotropic Heisenberg model. Using lithium-7 atoms and Feshbach resonances to tune the interactions, we have created spin ½ Heisenberg models with adjustable anisotropy, including the paradigmatic XX-model which can be exactly solved by mapping it to non-interacting fermions. Spin transport changes from ballistic to diffusive depending on the anisotropy. For transvers spin patterns, we have found several new dephasing mechanisms related to a superexchange induced effective magnetic field.

Using rubidium atoms and two atoms per site, we have realized spin 1 models. The onsite interactions give rise to a so-called-single-ion anisotropy term proportional to (S_z)^2, which plays an important role in stabilizing magnetism for low-dimensional magnetic materials. In the spin dynamics, we observe a resonant effect when this term and superexchange are comparable.

### Friday, February 12th, 2021:

### Topological control of quantum states through the non-Hermitian dynamics of a superconducting qubit.

A system described by a non-Hermitian Hamiltonian will, in general, have complex energies and non-orthogonal eigenstates. The degeneracies of such a system are known as exceptional points. Near these degeneracies, the complex energies are described by Riemann manifolds whose topology enables new methods of control over the system. Using a superconducting circuit QED platform we employ dynamical control over an effective non-Hermitian Hamiltonian to utilize the topology of its complex energy surfaces to control quantum state vectors. If a quantum system is initialized in one eigenstate, and the Hamiltonian parameters are varied slowly such as to encircle an EP, returning to the initial parameters, the topology of the Riemann manifold predicts that adiabatic evolution will switch the state to a different eigenstate. I will describe experiments where we observe this quantum state transport and use a quantum phase reference to measure the chiral geometric phases accumulated after this dynamical control.

### Friday, February 5th, 2021:

### Entanglement between living bacteria and quantized light

Many macroscopic phenomena rely on the laws of quantum physics. The solid-state physics, for instance, started with the realization that both electrons and vibrations must be treated quantum mechanically to even begin to be able to understand the thermodynamical behavior of many-body systems. A growing body of evidence now suggests that living systems too could be utilizing quantum coherence, superpositions, and even, in some cases, quantum entanglement to perform some tasks with higher efficiency. However, it is an exciting open question to what degree quantum effects can be maintained and controlled at the macroscopic level. This is interesting not just for our quest to realise scalable quantum computers, but also for engineering special-purpose programmable nano-machines.

I will explain the basics of witnessing entanglement and I will put this into the context of our present understanding of macroscopic quantum phenomena. I will then present the single molecule spectroscopy experiments we are currently undertaking in our laboratory to obtain a better understanding of quantum effects in complex (bio)molecules. This will include our recent observation of the vacuum Rabi splitting in a living bacterium strongly coupled with the electromagnetic field. I will also discuss how these experiments can be scaled-up, as well as how we can design artificial and hybrid biomimetic structures that capture the underlying fundamental quantum behavior of complex systems.

### Friday, January 29th, 2021:

### Quantum simulation of spin models with tunable arrays of 200 single Rydberg atoms

In this seminar, I will present how arrays of up to 200 single atoms held in optical tweezers and made to interact by exciting them to Rydberg states are an almost ideal platform for quantum simulation of quantum magnetism. I'll first review briefly the experimental realization of such arrays, and then illustrate, with recent experiments, how we can use it simulate the behavior of Ising or XY quantum magnets.

### Friday, December 11th, 2020:

### Hardware-efficient Quantum Error Correction

In the two decades since the beginning of the field, dramatic progress has been made towards realizing solid-state systems for quantum information processing with superconducting circuits. Superconducting qubits have improved their coherence by more than a million-fold, and they can be controlled and manipulated to perform quantum algorithms. These devices have also proven to be a wonderful platform for exploring the concepts of entanglement, quantum information, and quantum measurement.

The next challenge for the field is demonstrating quantum error correction (QEC) that actually improves the lifetimes of superpositions and entangled states and makes these systems robust by increasing the fidelity of gates. Developing practical schemes for quantum error correction is a requirement for building more complex systems and realizing the potential of quantum computing.

At Yale, our team [1] has been pursuing a novel, “hardware-efficient” approach for quantum error correction, based on encoding information in the multiple energy levels of a harmonic oscillator such as a microwave cavity. By allowing for redundancy without necessarily introducing more error channels, the “cat code” and other such bosonic error correction codes allow us to experimentally explore the concepts and the practice of QEC today, with smaller and less complex systems. This simplification has many benefits, reducing the overhead, relaxing the performance requirements, and allowing for more extensive calibration and characterization. It also leverages the extensive physics knowledge and techniques that have been developed in circuit-QED and quantum optics.

We have seen steady progress in hardware-efficient error correction, demonstrating the creation of a correctable or logical qubit by encoding and decoding of information in complex multi-photon states, the manipulation of these states to perform gates on individual logical qubits, and the generation of multimode entangled states. In addition, the dispersive coupling of transmon qubits to cavities has afforded not just the ability for the single-shot measurement of microwave photons, but also error syndromes such as the photon number parity. With the ability to track the parity more rapidly than the naturally-occurring decoherence (photon loss), we have shown the extension of the lifetime of a quantum bit, reaching breakeven for a memory operation, limited mainly by propagation of errors or non-fault tolerance of the measurement.

In this talk, I will review the development of hardware-efficient error correction based on bosonic codes, and how the concept of hardware-efficiency can be extended to prevent error propagation and realize fault-tolerant measurements and operations. By replacing the usual two-level systems used as ancillas with a correctly designed three-level system (simply a transmon with carefully chosen parameters), we can have shown a fault-tolerant measurement of an error syndrome, increasing the number of queries that can be made before a fault is induced. Finally, I will show recent results on a fault-tolerant gate for logical qubits, inspired by a proposal by Liang Jiang and co-workers. This gate allows for arbitrary rotations of a correctable bosonically encoded logical qubit, while suppressing both types of errors in the transmon ancilla, resulting in a significant improvement in the gate fidelity.

[1] This work has been performed together with the entire circuit-QED team at Yale, and especially my colleagues Michel Devoret, Steve Girvin, Liang Jiang, Leonid Glazman, and Mazyar Mirrahimi.

### Friday, December 4th, 2020:

### Driven-dissipative quantum systems and hidden time-reversal symmetries

Quantum systems subject to both driving and dissipation often have complex, non-thermal steady states, and are at the forefront of research in many areas of physics. For classical systems, microscopic time-reversal symmetry leads to open systems satisfying detailed balance; this symmetry makes it extremely easy to find their stationary states. In this talk, I’ll discuss a new way to think about detailed balance in fully quantum settings based on the existence of a “hidden” time-reversal symmetry. I’ll show how this symmetry has a direct operational utility: it provides a direct way to find exact solutions of non-trivial states. This symmetry is present in a number of experimentally-relevant systems. and has clear observable signatures. I’ll try to give a gentle introduction to these ideas, with a particular focus on models of driven, interacting bosonic modes (as can be realized in superconducting circuits and a variety of quantum optical platforms).

### Friday, November 20th, 2020:

### Exciting Exotic Atoms: Precision Spectroscopy of Muonium at PSI (Zak Burkley)

### Expanding the molecular ion control toolbox with quantum logic (Alejandra Collopy)

Abstract (Zak Burley): Recent and ongoing developments of low energy muon beamlines, paired with the development of efficient muon-muonium converters and laser advancements, is heralding a new era of precision Muonium spectroscopy. In this talk I will give an overview of the current status and future prospects of two new experiments currently underway at the LEM beamline at PSI and ETH, Mu-LaMB and Mu-MASS, which aim to improve the precision of both the Muonium Lamb shift and 1S-2S transition by several orders of magnitude, respectively. I will highlight our recent developments critical for such improvements, i.e., efficient production, tagging and detection of metastable Muonium in vacuum, and a robust, high power 244 nm laser system that will enable CW, doppler-free two-photon excitation of the Muonium 1S-2S transition.

*This work is supported by ERC/818053 - PI: Paolo Crivelli

Abstract (Alejandra Collopy): Quantum logic enables state preparation, readout and spectroscopy of otherwise difficult-to-study ions. Using the coupled harmonic motion shared between co-trapped ions, information can be transferred from the "spectroscopy" ion to the easily manipulated "logic" ion. With this technique we perform high-resolution terahertz-scale rotational spectroscopy of a single CaH+ molecular ion in pure states by leveraging a Ca+ ion as our logic ion. Entangled states between a CaH+ and a Ca+ have also been demonstrated, making molecular ions possible components of hybrid systems for quantum information processing. Because all our laser operations on the molecule are driving stimulated Raman transitions with high detuning, the technique promises to be generalizable to a wide variety of molecular ion species. Additionally, we study the systematic effects of the trap RF electric field on molecular levels and use it to obtain a measurement of the dipole moment of the charged molecule

C.-W. Chou, A. L. Collopy, C. Kurz, Y. Lin, M. E. Harding, P. N. Plessow, T. Fortier, S. Diddams, D. Leibfried, D. R. Leibrandt, Science 367,6485 (2020)

Y. Lin D. R. Leibrandt, D. Leibfried, C.-W. Chou, Nature 581, 273-277 (2020)

### Friday, November 13th, 2020:

### Building fully-controlled single molecules

Advances in quantum manipulation of molecules bring unique opportunities, including the use of molecules to search for new physics, harnessing molecular resources for quantum engineering, and exploring chemical reactions in the ultra-low temperature regime. In this talk, I will focus on our work developing techniques to build single ultracold molecules in optical tweezers as a versatile platform for quantum simulation and quantum computation. Starting with the first proof-of-principle demonstration of building a sodium-cesium molecule from two atoms, we progress to obtain full internal and external quantum state control of a single rovibrational ground-state molecule recently. I will explain how these single molecules will be used as valuable resources for future quantum entanglement applications due to their rich internal degrees of freedom and strong dipolar interactions. If time allows, I will touch on a separate work in the area of ultracold chemistry, where we work to obtain a detailed microscopic picture of chemical transformation of molecules from one species to another.

### Friday, November 6th, 2020:

### Coherent Control of Non-Adiabatic Dynamics in Molecules

Electronic coherence decays much more rapidly in molecules than atoms as a consequence of averaging over the different rates of phase advance for different internuclear separations associated with nonlocal nuclear wavefunctions. This is particularly dramatic for polyatomic systems that have many degrees of vibrational freedom over which this averaging occurs. Furthermore, if there is non-adiabatic coupling between electronic states, then it is not clear how the phase between states is maintained. I will discuss experiments that show via interference that the electronic coherence of a wave function in a molecule that undergoes internal conversion via non-adiabatic (non-Born Oppenheimer) coupling between electronic states is maintained [1]. The measurements demonstrate that this coherence can be used to control the population of different electronic states, and measure its decay time.

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.053202

### Friday, October 30th, 2020:

### Extremely out-of-equilibrium 1D gases

I will describe a series of experiments with 1D Bose gases out of equilibrium. Except in the limits of zero and infinite coupling, dynamical solutions for these nearly integrable systems could not be calculated until an approximate technique was invented in 2016, generalized hydrodynamics (GHD). We experimentally test GHD in the strong and intermediate coupling regimes by suddenly changing trap depths so that the 1D gases rapidly compress by as much as a factor of 35. Over many trap periods, we study the evolution of the distribution of rapidities, which are the momenta of the quasiparticles in these many-body systems. We find excellent agreement between our experiments and GHD theory.

### Friday, October 23rd, 2020:

### ATMs, CaSH, SupER molasses and other methods of modern molecular physics (Ivan Kozyryev)

### Quantum-programming a discrete time crystal (Matteo Ippoliti)

Abstract (Ivan Kozyryev): Our understanding of optical cycling properties for molecules with various structures, constituents and symmetries has advanced significantly in the past five years. The notion of an optical cycling center (OCC) not only unified various initial “educated guesses” for laser-coolable polyatomic molecules but also led to further advances in employing ultracold polyatomic molecules for diverse applications. I will briefly review the current state of polyatomic laser cooling and describe our work on characterizing the optical cycling properties of asymmetric top molecules (ATMs), including calcium hydrosulfide (CaSH). As the composite molecular mass grows, so does the number of photon scattering events necessary to decelerate and confine molecular beams using laser light. We recently proposed a novel Suppressed Emission Rate (SupER) molasses cooling scheme that utilizes coherent momentum exchange between light fields and molecules to significantly reduce experimental overhead for trapping polyatomic molecules of increasing complexity with an OCC.

References: Augenbraun, Doyle, Zelevinsky and Kozyryev, Phys. Rev. X 10, 031022 (2020); Wenz, Kozyryev, McNally, Aldridge and Zelevinsky, arXiv:2007.01776 (2020).

Abstract (Matteo Ippoliti): Recent progress in the realm of noisy, intermediate scale quantum (NISQ) devices opens exciting opportunities for many-body physics. It is natural to ask which kinds of new phenomena may be observed by leveraging NISQ devices' unprecedented control and measurement capabilities. In this talk I will focus on a paradigmatic example: discrete time crystals (DTCs), novel non-equilibrium states of matter that break time translation symmetry and are possible only in driven, disordered systems.

I will briefly review the theory of DTCs and the results of past experiments (on platforms ranging from solid state to trapped ions), in which transient ('prethermal') cousins of the DTC were realized. Key lessons from these experiments allow us to sharpen a checklist of requirements for the observation of a fully-fledged, stable incarnation of this phase. These requirements turn out to be a remarkably goot fit for a new generation of digital quantum simulators, such as Google's Sycamore processor, which can be programmed to realize the DTC phase and to experimentally detect its signature spatiotemporal order, even in the presence of noise and decoherence.

Reference: MI, K. Kechedzhi, R. Moessner, S. L. Sondhi and V. Khemani, arXiv:2007.11602 (2020).

### Friday, October 16th, 2020:

### Connecting and scaling semiconductor quantum systems

At the core of most quantum technologies, including quantum networks, quantum computers and quantum simulators, is the development of homogeneous, long lived qubits with excellent optical interfaces, and the development of high efficiency and robust optical interconnects for such qubits. To achieve this goal, we have been studying color centers in diamond (SiV, SnV) and silicon carbide (VSi in 4H SiC), in combination with novel fabrication techniques, and relying on the powerful and fast photonics inverse design approach that we have developed. We illustrate this with a number of demonstrated devices, including efficient photon interfaces for color centers in diamond and in SiC, and spectrally reconfigurable quantum emitters.

### Friday, October 9th, 2020:

### Bosonic Quantum Information Processing with Superconducting Circuits

Bosonic modes are widely used for quantum communication and information processing. Recent developments in superconducting circuits enable us to control bosonic microwave cavity modes and implement arbitrary operations allowed by quantum mechanics, such as quantum error correction against excitation loss errors. We investigate different bosonic encoding and error correction protocols, and provide a perspective on using bosonic quantum error correction for various applications.

### Friday, October 2nd, 2020:

### Stability Means Never Being Excited (and that's no fun, is it?)

Atomic physics gives quantum information science the unique capability to confidently predict the performance of machines that have yet to be built. For trapped ions, where the nature of the two-qubit interactions is universal (i.e. Coulomb repulsion), we already know many of the fundamental limitations of the commonly used techniques and species, long before we actually reach those limits. Looking forward to the time when the technology hits the physics ceiling fan, it makes sense to explore new techniques and species now that are not subject to the same constraints. Through the eyes of Scaredy the Squirrel, we will explore some of the approaches being designed by unstable thinkers to extend past these limitations.

### Friday, September 25th, 2020:

### Exploring dissipative many-body dynamics with cold atoms

In recent years, the exceptional control available in experiments with cold atoms opened new opportunities to explore many-body quantum dynamics both in and out of equilibrium. While much of the focus has been on coherent (Hamiltonian dynamics), a key element of the control we have relies on the possibility to control and even engineer dissipative dynamics in these systems. In analogy with the quantum optics of single atoms and photons, we can often work in scenarios where the separations of relevant energy scales allows us to write microscopic models for open quantum systems - but now with strong inter-particle interactions. I will discuss how we calculate dissipative many-body dynamics, and how this can be used as a tool in a variety of contexts for systems of cold atoms – including to characterise decoherence and to help in the verification of coherent dynamics. This also provides new possibilities to prepare desired many-body quantum states, and to study dissipative many-body phenomena, including quantum transport, and non-Markovian effects in open quantum systems.

### Friday, September 18th, 2020:

### Chiral interactions in optically coupled Bose-Einstein condensates

Chirality - the fact that Nature distinguishes left from right - manifests itself in very diverse phenomena, from the rotation direction of cyclones, to the motion of electrons in magnetic fields or the symmetry of organic molecules. Engineered AMO systems provide an excellent playground to realize novel types of chirality at the quantum level. In my talk, I will focus on ultracold quantum gases, and report on the generation of chiral interactions – interactions that depend on the momentum of the atoms – in optically coupled Bose-Einstein condensates. I will show that having chiral interactions is equivalent to realizing an artificial gauge field that is not static, but instead has back-action from matter. This density-dependent gauge field is a one-dimensional analogue of the gauge field generated by fractional excitations in 2D quantum Hall systems. I will present two experimental manifestations of chiral interactions: the existence of chiral solitons –self-bound wavepackets that propagate without dispersion only when moving along one direction- and the asymmetric expansion of the condensate due to the emergence of a density-dependent electric field.

### Friday, September 11th, 2020:

### Quantum sensing and imaging with diamond spins

Solid state spin qubits, in particular the nitrogen vacancy (NV) center in diamond, offer a path towards truly nanoscale imaging of condensed matter and biological systems with sensitivity to single nuclear spins. Here I discuss our NV-based magnetic imaging experiments as applied to condensed matter systems, where we have imaged current flow patterns in graphene as well as skyrmions, nanoscale spin textures. A grand challenge to improving the spatial resolution and magnetic sensitivity of the NV is mitigating surface-induced quantum decoherence, which I will discuss in the second part of this talk. Decoherence at interfaces is a universal problem that affects many quantum technologies, but the microscopic origins are as yet unclear. Our studies guide the ongoing development of quantum control and materials control, pushing towards the ultimate goal of NV-based single nuclear spin imaging.

### Friday, September 4th, 2020:

### Supersolidity in the ultracold: when atoms behave as a crystal and superfluid at the same time

Over more than two decades, ultracold quantum gases have enabled the observation of fascinating quantum phenomena. Research directions are ever increasing with the development of novel optical-manipulation techniques and the gain of an exquisite control over the inter-particle interactions.

Recently, a novel class of atomic species, possessing an exceptionally strong magnetic dipole moment, has entered the stage (Cr, Dy, Er). This offers new opportunities to study dipolar quantum phenomena, driven by the long-range and anisotropic interaction between particles. I will review the recent developments in atomic dipolar quantum gases from the Innsbruck perspective. In our laboratories, we work with dipolar Bose-Einstein condensates and Fermi gases of either Erbium or Dysprosium, or with dipolar quantum mixtures of both elements.

Particular emphasis will be given on our recent observations [1] of the elusive and paradoxical supersolid state of matter, using both Er and Dy ultracold gases. Such paradoxical phase, in which crystal rigidity and superfluid flow coexist, has intrigued scientists across different disciplines for decades. It now became possible to create supersolidity in the ultracold thanks to the unique interplay between long-range dipolar interactions, contact interactions, and a powerful stabilization mechanism based on quantum fluctuations.

[1] Observation of dipolar supersolids has been simultaneously reported by the group led by T. Pfau (Stuttgart) and the one led by G. Modugno (Pisa).

### Friday, August 28, 2020:

### Quantum gas of reactive molecules: evaporation, collision resonance, long lifetime

The preparation of a degenerate Fermi gas of polar molecules sets the stage to explore novel many-body physics. We apply precisely controlled electric fields to turn on elastic dipolar interactions by orders of magnitude while suppressing reactive losses. Efficient dipolar evaporation leads to the onset of quantum degeneracy in an array of two-dimensional optical traps. When the electric field is used to tune excited molecular states into degeneracy with the scattering threshold, we observe sharp collision resonances that give rise to three orders-of-magnitude modulation of the chemical reaction rate. Using this resonant shielding, we realize a long lifetime of polar molecular gas at very high electric fields.

### Friday, August 21, 2020:

### Single, few, and many photon physics in 1D atomic arrays

Tightly packed ordered arrays of atoms exhibit remarkable collective optical properties, as dissipation in the form of photon emission is correlated. In this talk, I will discuss the single-, few- and many-body out-of-equilibrium physics of 1D arrays, and their potential to realize versatile light-matter interfaces. For small enough inter-atomic distances, atomic chains feature dark states that allow for dissipationless transport of photons, behaving as waveguides for single-photon states. Atomic waveguides can be used to mediate interactions between impurity qubits coupled to the array, and allow for the realization of multiple paradigms in waveguide QED, from bandgap physics to chiral quantum optics. Due to the two-level nature of the atoms, atomic waveguides are a perfect playground to realize strong photon-photon interactions. At the many-body level, collective decay leads to superradiance even for finite inter-atomic separation, and generates directional photon-photon correlations that can be measured with current experimental capabilities.

### Friday, August 14, 2020:

### Dynamics of quantum systems with long-range interactions

Atomic, molecular, and optical systems often exhibit long-range interactions, which decay with distance r as a power law 1/r^alpha. In this talk, we will discuss bounds on how quickly quantum information can propagate in such systems. We will then discuss applications of these bounds to numerous phenomena including classical and quantum simulation of quantum systems, prethermal phases in Floquet systems, entanglement area laws, sampling complexity, and scrambling.

Cold atoms in optical tweezer arrays have emerged as a versatile platform for quantum science. I will review these developments, including atom-by-atom assembly as a method to generate defect-free arrays [1] as well as quantum simulation of spin models with Rydberg atoms [2], followed by a brief discussion of current limitations. Our work at Caltech now merges these directions with control of narrow and ultra-narrow optical transitions in alkaline earth atoms. I will give an overview of new opportunities in this context and recent results from our lab: 1) A record in imaging-fidelity for neutral atoms and demonstration of narrow-line cooling in tweezers [3,4]. 2) Rydberg excitation from a meta-stable clock state, which resulted in leading fidelities for entanglement of two neutral atoms [5]. 3) Demonstration of an optical clock with single-atom detection in tweezer arrays [6].

[1] Endres et al., Science 354, 1024 (2016).

[2] Bernien et al., Nature 551, 579, (2017).

[3] Covey et al., Phys. Rev. Lett. 122, 173201 (2019).

[4] Cooper et al., Phys. Rev. X 8, 041055 (2018).

[5] Madjarov*, Covey*, et al., Nat. Phys. (2020). https://doi.org/10.1038/s41567-020-0903-z

[6] Madjarov et al., Phys. Rev. X 9, 041052 (2019).

### Friday, July 31, 2020:

### Juggling with single atoms to observe many-body physics

In our quest to obtain a deeper understanding of strongly interacting many body quantum systems, we turn to finite systems that can be prepared at very low entropies. Single-atom sensitive detection allow for the determination of high-order correlation functions.

In our presentation we first show work with noninteracting spin polarized fermions and determine the correlations present solely due to indistinguishability, observing for the first time so-called Pauli crystals, geometric structures that form in the correlations between particles because of the antisymmetry of the wave function.

We will also report on a significant breakthrough observing a precursor of many-body physics in a finite 2D system that due to degeneracies in the harmonic potential exhibits a shell structure: Increasing the attraction between particles, we observe a non-monotonous behavior of the excitation spectrum when the attraction becomes larger than the spacing between shells. In the many-body limit this can be interpreted as a quantum phase transition from normal to superfluid and associated with a long-lived Higgs mode.

In the future we would like to use our newly established methods to measure correlations in (strongly) interacting systems. A first step could be to directly observe Cooper pairs by increasing the particle number.

### Friday, July 24, 2020:

### Emergent hydrodynamics in a strongly interacting dipolar spin ensemble

Even in the absence of a precise microscopic description, classical hydrodynamics provides a powerful framework for characterizing the macroscopic behavior of conserved densities, such as energy. Understanding whether and how it emerges in the late-time dynamics of many-body quantum systems has remained an enduringly hard question. In this talk, I will describe on-going experiments exploring the emergence of hydrodynamics in strongly interacting dipolar spin ensembles; in particular, we observe dynamics whose long-time limit is related to, but fundamentally distinct from ordinary diffusion. This distinction arises from the complex interplay between long-range interactions, positional disorder, and on-site random fields. Finally, time permitting, I will briefly describe a new class of many-body quantum teleportation circuits, which enable one to distinguish between various classes of quantum information dynamics as well as decoherence.

### Friday, July 17, 2020:

### Twists, gaps, dynamical phases, and superradiant emission on ultra-narrow optical transitions in strontium

I will describe superradiant pulses of light emitted from an optical transition that does not like to radiate light: the millihertz linewidth transition in strontium. We observe that the source is intrinsically up to a million times less sensitive to the thermal and technical vibration noise sources that limit laser frequency stability using traditional reference-cavity approaches. I will also describe the emergence of all-to-all spin-exchange interactions that produce a many-body energy gap and one-axis twisting dynamics. By coherently driving the system, we realize a Lipkin-Meshkov-Glick model, an iconic model in quantum magnetism, and observe the emergence of distinct dynamical phases. Finally, as time permits, I will briefly discuss the application of quantum nondemolition measurements to determine the strontium clock transition natural linewidth with 30 microhertz resolution, and to the creation of highly entangled states of rubidium atoms (directly observed 60 times or 18 dB surpassing of the Standard Quantum Limit) and our current efforts to apply these states in a proof-of-principle matter wave interferometer

### Friday, July 10, 2020:

### Using molecules to explore discrete symmetry violations in nuclear and particle physics

The close spacing of opposite-parity levels in molecules enhances the effects of parity-violating fundamental interactions. I will discuss results from two recent experiments probing such interactions. The first, ACME II, sets the tightest bound on the electron’s electric dipole moment. This result yields information on new CP-violating physics at energy scales far above the reach of the Large Hadron Collider. The second experiment, ZOMBIES, is designed to measure anapole moments of nuclei. A nuclear anapole moment arises from electroweak interactions within the nucleus, which are heavily modified in the presence of strong interactions and remain poorly understood. As a proof of principle, ZOMBIES recently demonstrated sensitivity roughly 7 times better than the best prior anapole moment measurements. Finally, I will discuss ongoing efforts and future prospects for dramatic improvements in sensitivity to these and other, related effects.

### Friday, July 3rd, 2020:

### July 4th holiday weekend: no seminar.

### Friday, June 26th, 2020:

### Scaling approaches for trapped-ion quantum computing

While error-prone quantum computers consisting of tens of qubits provide much excitement today, useful quantum computers are predicted to need more than 100,000 physical qubits. The primary cause of this scaling is the need for quantum error-correction to make these devices produce reliable outcomes. Scaling is likely to require the use of different technologies to those in use today, as well as benefitting from the exploration of new approaches. I will describe two areas which we are exploring at ETH Zürich in the context of trapped-ion quantum computing. On the technology side, we have demonstrated the use of chip traps fabricated in a commercial foundry to perform multi-qubit quantum logic using light delivered from the chip to the ion using integrated photonics, producing entangled Bell states with a fidelity of 99.3% [1]. We are also exploring the use of micro-Penning trap arrays to produce 2-dimensional ion systems for quantum computing and quantum simulation using only static trapping fields. This removes a number of constraints which stem from the use of strong radio-frequency fields for ion trapping, which is an attractive feature for scaling [2].

[1] K. Mehta et al. arXiv:2002.02258 (2020)

[2] S. Jain et al. arXiv:1812.06755v3 (2020)

TBA

### Friday, June 19th, 2020:

### Driven and self-driven quantum many-body systems

Applying a periodic drive to a quantum many-body system offers many prospects and a challenge. Whilst a wide range of non-trivial Hamiltonians can be engineered, Floquet-heating often spoils the party. I will first report on a scheme realizing the fundamental ingredient for a density-dependent gauge field by engineering non-trivial Peierls phases that depend on the site occupation of fermions in a Hubbard model [1]. I will further discuss experiments in which we demonstrated how to circumvent Floquet-heating by two-path quantum interference, effectively suppressing dissipative coupling to higher bands [2]. On a different note, an experiment will be discussed in which a dissipative coupling leads to a self-oscillating evolution between a spin wave and a density wave [3].

References:

[1]: F. Görg, K. Sandholzer, J. Minguzzi, R. Desbuquois, M. Messer, and T. Esslinger, arXiv:1812.05895, Nature Physics, 15, 1161 (2019).

[2]: N. Dogra, M. Landini, K. Kroeger, L. Hruby, T. Donner, and T. Esslinger, arXiv:1901.05974, Science, 366, 1496 (2019).

[3]: Konrad Viebahn; Joaquìn Minguzzi; Kilian Sandholzer; Anne-Sophie Walter; Frederik Görg; Tilman Esslinger, arXiv: 2003.05937

### Friday, June 12th, 2020:

### Long-lived spin squeezing in a metrologically relevant regime

Spin squeezing is a fascinating manifestation of many-particle entanglement as well as one of the most promising quantum technologies. By using quantum correlations to reduce the quantum projection noise in a collection of atomic spin-1/2's, spin squeezing removes a limitation that has already been reached in atomic fountain clocks and is expected to limit state-of-the-art lattice clocks and atomic sensors in the near future. Ground-breaking experiments have demonstrated methods to create spin-squeezed states and have even demonstrated squeezing enhancement in proof-of-principle clock and magnetometer measurements, though not yet at a metrologically relevant level. in all experiments so far, however, the coherence time of the atomic superpositions was short (typically below 10ms), while interrogation times in real clocks and sensors are often ten or hundred times longer. How squeezed states evolve on these timescales is a question that experiments have not yet been able to address. Besides its practical importance, the physics of this time evolution is an interesting question in itself. Long coherent evolution acts like a magnifying glass for atomic interactions, often leading to surprising effects. How do these interactions evolve when the initial state is no longer a product state of N independent atoms, but an entangled state with quantum correlations between all the atomic spins?

In a collaboration with the French time and frequency laboratory SYRTE (Observatoire de Paris), we have built an experiment combining a trapped-atom clock on an atom chip with a fiber Fabry-Perot microcavity to generate spin squeezing. This has enabled us to produce spin-squeezed states with with a lifetime on the order of a second. Observing the time evolution of the squeezed state over these long times reveals a surprising quantum phase magnification effect in the final measurement of the spin state. This effect results from a subtle interplay of the spin dynamics of interacting indistinguishable particles and the physics of cavity-based quantum non-demolition measurement of the collective spin.

### Friday, June 5th, 2020:

### Friday, May 29nd, 2020:

### Atomic physics in the solid-state with rare earth atoms

In this talk, I will discuss our group’s efforts to develop a novel solid-state platform for atomic physics based on rare earth ion defects in crystals. Solid-state “atoms” are attractive because they do not require laser cooling and trapping, can be arrayed at high density to enable strong interactions, and can be integrated with photonic and electronic devices for novel functionality. Rare earth ions are unique in that they maintain atom-like properties in solid hosts and can be incorporated into a wide range of materials and devices. However, owing to their weak photon emission, individual rare earth atoms have been challenging to observe. We overcome this obstacle by incorporating rare earth defects into nanophotonic circuits, enabling efficient detection of single erbium (Er3+) ions [1]. Furthermore, we exploit the atom-cavity coupling to engineer the optical selection rules to realize high-fidelity single-shot spin readout [2], despite the absence of a cycling transition in the bare ion. More recently, we have demonstrated a frequency-domain multiplexing technique enabling parallel, high-fidelity readout and control of many ions with deep sub-wavelength separations [3]. Together with ongoing efforts to controllably incorporate rare earth ions into novel materials offering improved coherence [4], this is a promising starting point towards numerous applications including telecom-wavelength quantum networks and arrays of strongly interacting spins with single-particle control.

[1] A. M. Dibos, M. Raha, C. M. Phenicie, and J. D. Thompson, Phys. Rev. Lett. 120, 243601 (2018).

[2] M. Raha, S. Chen, C. M. Phenicie, S. Ourari, A. M. Dibos, and J. D. Thompson, Nat. Commun. 11, 1605 (2020).

[3] S. Chen, M. Raha, C. M. Phenicie, S. Ourari, and J. D. Thompson, In Preparation (2020).

[4] C. M. Phenicie, P. Stevenson, S. Welinski, B. C. Rose, A. T. Asfaw, R. J. Cava, S. A. Lyon, N. P. de Leon, and J. D. Thompson, Nano Lett. 19, 8928 (2019).

### Friday, May 22nd, 2020:

### Floquet topological phases with ultracold atoms in periodically-driven lattices

Topological phases of matter exhibit remarkable electronic properties that offer unique possibilities for applications. A prominent example is the robust quantization of the Hall conductivity in quantum Hall insulators. A widespread technique for generating topological band structures in synthetic systems, such as ultracold atoms in optical lattices, is Floquet engineering [1]. This method relies on the periodic modulation of the system’s parameters to emulate the properties of a non-trivial static system and facilitated the realization of two paradigmatic topological lattice models: the Hofstadter and the Haldane model. Moreover, it inspired ideas for implementing complete lattice gauge theories [2].

The rich properties of Floquet systems, however, transcend those of their static counterparts [3]. The associated quasienergy spectrum can exhibit a non-trivial winding number, which leads to the appearance of anomalous chiral edge modes even in situations where the bulk bands have zero Chern numbers, hence, altering the well-known bulk-edge correspondence. A full classification of Floquet phases requires a new set of topological invariants. We have studied the rich Floquet phase diagram of a periodically-modulated honeycomb lattice using bosonic atoms. The novel properties of anomalous Floquet phases mentioned above open the door to exciting new non-equilibrium phases without any static analogue [4].

[1] A. Eckardt, Phys. Mod. Phys. 89, 311 (2017)

[2] C. Schweizer et al., Nat. Phys. (2019); L. Barbiero et al., Science Advances 5, eaav744 (2019)

[3] T. Kitagawa et al., Phys. Rev. B 82, 235114 (2010)

[4] K. Wintersperger et al., accepted in Nature Physics (arXiv:2002.09840)

### Friday, May 15th, 2020:

### Many-Body Physics with Random Circuits

The study of many-body quantum dynamics is a topical area of interest to a number of different subfields in physics. A unifying theme is the study of the dynamics of quantum information in systems ranging from electrons in solids to cold atomic gases to black holes. While many important developments have pushed this frontier forward in recent years, several foundational questions remain. These include the search for new dynamical universality classes (for example, classes of dynamics intermediate between thermalizing and many-body localized, or new out-of-equilibrium phases of matter such as time-crystals) and novel dynamical phase transitions between these different classes. While exact solutions are few and far between, much insight has been gained by studying the dynamics of tractable models of random circuits.

In this talk, I will discuss new universality classes of dynamics emerging from two families of random circuits. The first is family of `fractonic' unitary circuits which leads us to a new mechanism for localization via 'Hilbert space shattering'. The second is a family of non-unitary measurement-only circuits which display a novel entanglement phase transition driven by the "frustration" of the measurement ensemble. The latter connects to attempts to characterize noisy dynamics in NISQ devices.

### Friday, May 8th, 2020:

### Measurement of the fine structure constant as test of the Standard Model

Measuring the fine-structure constant α allows testing the consistency of theory and experiment across physics. Using the recoil frequency of cesium-133 atoms in a matter-wave interferometer, we recorded the most accurate measurement of the fine-structure constant to date: α = 1 / 137 . 035999046(27) at 2 .0 ×10-10 accuracy. Comparison with Penning trap measurements of the electron gyromagnetic anomaly ge - 2 via the Standard Model of particle physics is now limited by the uncertainty in ge - 2 . Implications for dark-sector candidates and electron substructure may be a sign of physics beyond the Standard Model that warrants further investigation. We will look at our current project to improve the accuracy further to the 10-part per trillion level.

### Friday, May 1st, 2020:

### Making Quantum Matter from Light

In this talk I will discuss ongoing efforts at UChicago to explore matter made of light. I will begin with a broad introduction to the challenges associated with making matter from photons, focusing specifically on (1) how to trap photons and imbue them with synthetic mass and charge; (2) how to induce photons to collide with one another; and (3) how to drive photons to order, by cooling or otherwise. I will then provide as examples two state-of-the-art photonic quantum matter platforms: microwave photons coupled to superconducting resonators and transmon qubits, and optical photons trapped in multimode optical cavities and made to interact through Rydberg-dressing. In each case I will describe a synthetic material created in that platform: a Mott insulator of microwave photons, stabilized by coupling to an engineered, non-Markovian reservoir, and a Laughlin molecule of optical photons prepared by scattering photons through the optical cavity. Building materials photon-by-photon will provide us with a unique opportunity to discover what all of the above words mean, and why they are important for quantum-materials science.

Trapped atomic ions are a leading platform for quantum computing and sensing experiments. These experiments require manipulation of the ions' quantum states and the creation of entanglement between ions, both of which are typically accomplished using laser beams. However, there are drawbacks to this approach, including errors due to photon scattering and the complexity of the required laser sources. Our group performs quantum state control using oscillating radiofrequency and microwave-frequency magnetic and electric fields, and their near-field gradients, instead of laser beams. A critical element is the use of a microfabricated surface-electrode ion trap, which holds the ions roughly 30 µm above the electrodes generating the control fields. We use low-power resonant laser beams for cooling, optical pumping, and readout. I will describe several of our recent results, including the creation of high-fidelity entangled states of two ions using only microwave and radio-frequency control fields, and the use of radio-frequency electric fields to generate squeezed states of ion motion for sensing applications and to amplify phonon-mediated ion-ion interactions.

### Friday, April 17th, 2020:

The development of microscopic detection of ensembles of neutral atoms has transformed our ability to study complex many-body systems. Techniques like quantum gas microscopy and optical tweezer arrays grant a unique single-particle-resolved perspective on solid-state analogs and idealized quantum spin models, as well as novel detection capabilities for quantities like entanglement. In this talk, I will describe our progress towards developing these tools for a new atomic species, strontium. In doing so, we establish new prospects enabled by the rich internal degrees-of-freedom associated with alkaline-earth atoms. I will report on our recent results in which we apply our platform to optical atomic clocks, a new application of optical tweezer arrays which indicates a number of strengths for metrology. In particular, I will describe our strategies for reaching arrays with hundreds of tweezers with sub-Hz atom-optical coherence, half-minute scale atomic coherence, and atomic stability on par with the state-of-the-art. I will then describe our parallel progress towards engineering entanglement on an optical clock transition, as well as new scaling strategies involving atom-by-atom assembly in optical lattice potentials.

### Friday, April 10th, 2020:

The normal state of high-temperature superconductors exhibits anomalous transport and spectral properties that are poorly understood. Cold atoms in optical lattices have been used to realize the celebrated Fermi-Hubbard model, widely believed to capture the essential physics of these materials. The recent development of fermionic quantum gas microscopes has enabled studying Hubbard systems with single-site resolution and extracting equilibrium charge and spin correlations. In this talk, I will report on using a quantum gas microscope to probe the transport and spectral properties of atomic Fermi-Hubbard systems. First, I will describe the development of a technique to measure microscopic charge diffusion, and hence resistivity, in doped Mott insulators. We have found that this resistivity exhibits a linear dependence on temperature and violates the Mott-Ioffe-Regel limit, two signatures of strange metallic behavior [1]. Next, I will discuss how we used the same technique to observe sub-diffusive charge transport in tilted Hubbard systems and present a hydrodynamic model that explains this observation in terms of an interplay of charge and heat transport, allowing the extraction of the infinite temperature heat diffusivity of the system [2]. Finally, I will describe the development of angle-resolved photoemission spectroscopy (ARPES) for Hubbard systems and its application to studying pseudogap physics in an attractive Hubbard system across the BEC-BCS crossover [3], setting the stage for future studies of the pseudogap regime in repulsive Hubbard systems.

[1] P. Brown et. al., Science 363, 379 (2019)

[2] E. Guardado-Sanchez et. al., PRX 10, 011043 (2020)

[3] P. Brown et. al., Nature Physics 16, 26 (2020)

### Friday, April 3rd, 2020:

Recently it has been proposed to search for particles outside the Standard Model (SM) in an intermediate mass range by means of precision isotope shift spectroscopy on narrow optical transitions. We perform such a measurement on two S → D transitions for five bosonic isotopes of Yb+ with an accuracy of ~ 300Hz, and observe a nonlinearity at the 3.3 σ level in the corresponding King plot. Such a nonlinearity can indicate physics beyond the SM, or higher-order effects within the SM. We identify the second-order field shift as the leading-order effect within the SM for Yb+. The observed nonlinearity pattern is consistent with both the second-order field shift, and with an unknown boson, but larger than expected from either source. In the future, more precise measurements on more transition available for Yb+ and Yb can be used to distinguish between effects within and outside the SM.