Previous seminars
May 7, 2024: Engineering quantum coherence and control in diluted spin systems
(Florida State University and the National High Magnetic Field Laboratory, Tallahassee)
Abstract
Recently we have demonstrated experimentally the implementation of a novel and universal method to increase the decoherence time of spins qubits [1] in systems with different anisotropies / symmetries / spin-orbit coupling and type of element. The method is based on Floquet engineering of spin qubits quasi-energies by adding a second microwave drive with a frequency commensurate to that of the main Rabi drive. Qualitatively, the increase in coherence time can be linked to dynamical sweet spots (level repulsion) in quasi-energy spectra. Quantitatively, we add insight using numerical simulations [2] aiming to clarify the actual physical processes that take place in the bath surrounding the qubit. We are also exploring the potential use of spin systems as quantum memories [3] and to that effect, we have performed spectroscopic and pulsed studies of S=7/2 Gd ions placed on a coplanar stripline superconducting resonator. In the weak coupling limit, continuous-wave spectroscopy of the cavity resonance perturbation allows us to detect the forbidden electro-nuclear transition of the 155,157Gd isotopes by applying a static field almost perpendicular to crystal c-axis [4]. By increasing the coupling of the spin ensemble to the resonator we observe spin-cavity dressed states with a large mode splitting of ~150 MHz. Numerical simulations based on Dicke model shows a strong hybridization of the first excited level in the presence of a photon and the second excited level with no photon as well as a strong perturbation of the spin ground state generated by photons.
[1] S. Bertaina, H. Vezin, H. De Raedt, and I. Chiorescu, Experimental protection of quantum coherence by using a phase-tunable image drive, Scientific Reports 10, 1 (2020)
[2] De Raedt, H.; Miyashita, S.; Michielsen, K.; Vezin, H.; Bertaina, S.U.; Chiorescu, I., Sustaining Rabi oscillations by using a phase-tunable image drive, European Physical Journal B, 95 (9), 158 (2022)
[3] M. Blencowe, Quantum computing: Quantum RAM, Nature 468, 44 (2010).
[4] Franco-Rivera, G.; Cochran, J.R.; Miyashita, S.; Bertaina, S.U.; Chiorescu, I., Strong Coupling of a Gd3+ Multilevel Spin System to an On-Chip Superconducting Resonator, Physical Review Applied, 19, 024067 (2023).
April 23, 2024: Quantum Optics with Giant Atoms
In quantum optics, atoms are usually approximated as point-like compared to the wavelength of the light they interact with. However, recent advances in experiments with artificial atoms built from superconducting circuits have shown that this assumption can be violated. Instead, these artificial atoms can couple to an electromagnetic field in a waveguide at multiple points, which are spaced wavelength distances apart. Such systems are called giant atoms. They have attracted increasing interest in the past few years (e.g., see the review in [1]), in particular because it turns out that the interference effects due to the multiple coupling points allow giant atoms to interact with each other through the waveguide without losing energy into the waveguide (theory in [2] and experiments in [3]).
This talk will review some of these developments [1-4]. Finally, we will also show how a giant atom coupled to a waveguide with varying impedance can give rise to chiral bound states [5].
[1] A.F. Kockum, Quantum optics with giant atoms -- the first five years, https://arxiv.org/abs/1912.13012
[2] A.F. Kockum, G. Johansson, F. Nori, Decoherence-Free Interaction between Giant Atoms in Waveguide Quantum Electrodynamics, Phys. Rev. Lett. 120, 140404 (2018).
[3] B. Kannan, et al., Waveguide quantum electrodynamics with superconducting artificial giant atoms, Nature 583, pp. 775 (2020).
[4] S. Terradas-Brianso, et al., Ultrastrong waveguide QED with giant atoms, Phys. Rev. A 106, 063717 (2022).
[5] X. Wang, T. Liu, A.F. Kockum, H.R. Li, F. Nori, Tunable Chiral Bound States with Giant Atoms, Phys. Rev. Lett. 126, 043602 (2021).
April 9, 2024: Experimental Quantum Electrodynamics
Traditional approaches to quantum optics are rooted in the reciprocal, frequency-momentum space. In this talk, I will discuss recent advances toward sub-cycle quantum optics, where, instead, quantum fields are accessed in a localized region of space-time [1-2]. Both regimes will be compared side-by-side to contrast the advantages of each approach, with a particular emphasis on quantum sensing proposals [3-5] in the mid-infrared frequency range. In the concluding part of the talk, I will summarize recent advances in producing few-cycle bright one- and two-mode squeezed vacuum states in a single few-cycle spatio-temporal mode with macroscopic photon occupation [6]. Such capabilities are poised to unlock a new era of (extreme) nonlinear quantum optics in the attosecond regime [7].
[1] C. Riek et al.; Science 350, 420-423 (2015)
[2] I-C. Benea-Chelmus et al.; Nature 568, 202-206 (2019)
[3] S. Virally, P. Cusson, DVS; Phys. Rev. Lett. 127, 270504 (2021)
[4] S. Gündoğdu et al; Laser Phot. Rev. 17, 2200706 (2023)
[5] S. Onoe, S. Virally, DVS; arXiv:2307.13088 (2023)
[6] P. Cusson, S. Virally, DVS; IRMMW-THz Conference, paper Th-PM2-5-7 (2023)
[7] 2023 Nobel Prize in Physics: Pierre Agostini, Ferenc Krausz and Anne L'Huillier. Experimental methods that generate attosecond pulses for studying electron dynamics in matter.
April 2, 2024: More-predictive density functionals, symmetry breaking, and strong correlation
Approximate density functionals constructed to satisfy known mathematical properties of the exact density functional for the exchange-correlation energy of a many-electron system can be predictive over a wide range of materials and molecules. The strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation [1] satisfies 17 exact constraints, and nicely describes some systems that were formerly thought to be beyond the reach of density functional theory, such as the cuprates [2]. Ground states that break the symmetry of a Coulomb-interacting Hamiltonian can be understood as dynamic density or spin-density fluctuations that drop to low or zero frequency [3,4] and so persist over long times. In many cases, symmetry breaking transforms the strong correlation in a symmetry-unbroken wavefunction into moderate correlation like that found in the uniform electron gas of high or valence-electron density (an “appropriate norm” for constraint-based approximations).
Supported by NSF DMR-1939528 and DE-SC0018331
[1] J. Sun, A. Ruzsinszky, and J.P. Perdew, Phys. Rev. Lett. 115, 036402 (2015)
[2] J.W. Furness, Y. Zhang, C. Lane, I.G. Buda, B. Barbiellini, R.S. Markiewicz, A. Bansil, and J. Sun, Commun. Phys. 1, 11 (2018)
[3] P.W. Anderson, Science 177, 393 (1972)
[4] J.P. Perdew, A. Ruzsinszky, J. Sun, N.K. Nepal, and A.D. Kaplan, Proc. Nat. Acad. Sci. USA 118, e2017850118 (2021)
March 12, 2024: Feedback loops and nuclear accidents: or the false promises of fast neutron reactors
Nuclear power plants, as history has shown, are capable of suffering severe accidents leading to the spread of radioactive contamination across large areas. This talk will explain why the possibility of future accidents can never be ruled out, focusing specifically on the case of fast neutron (breeder) reactors, a class of designs favoured by many people who propose these as the solution to the enduring problem of nuclear waste. It will combine insights from engineering and sociology to explain why nuclear energy is uniquely problematic.
February 20, 2024: Excitonic way to altermagnetism
Collinear magnetic order characterized by zero net magnetization but finite polarization in the reciprocal space - spin texture - has recently been identified in several materials. These so called altermagnets share a number of technologically interesting features with ferromagnets, while retaining the advantages of antiferromagnets. The idea of excitonic condensation and excitonic insulator has been reappearing since the pioneering work of Mott in the early 1960's, most recently in the context of 2D materials revolution. I will discuss condensation of spinful excitons in the minimal, two orbital, Hubbard model. I will show that doping of the excitonic condensate away from integer filling leads to formation of collinear spin textures. Unlike the textures found in the known altermagnetic materials, the spin textures due to excitonic condensates may exists without ordered atomic moments or even without a finite spin density. The theory will be supported by numerical simulations using dynamical mean-field theory.
February 6, 2024: Quantum trajectories, quantum potential, superoscillations: Madelung, de Broglie, Newton
The wave counterparts of the classical paths of material particles and the rays of geometrical optics are trajectories modified by a ‘quantum potential’. Wave interference corresponds to undulations in these trajectories, as envisaged by Isaac Newton in his attempts to understand what we now understand as diffraction. Trajectories are strongly influenced by phase singularities (aka wave vortices). The local quantum velocity (proportional to the phase gradient of the wavefunction), can be faster than the classically allowed speed. This is an example of superoscillations: variations in a bandlimited function that are faster than its largest Fourier frequency. Regions of superoscillation include the phase singularities, and are bounded by manifolds where the quantum potential is zero. The quantum potential suggests a generalisation of quantum mechanics, applicable to classical curl forces, which are not derivable from a potential.
January 23, 2024: Accelerating quantum chemistry with machine learning (ML) and artificial Intelligence (AI)
Deep learning is revolutionizing many areas of science and technology, particularly in natural language processing, speech recognition and computer vision. In this talk, we will provide an overview into latest developments of machine learning and AI methods and application to the problem of quantum chemistry at Isayev’s Lab at CMU. We identify several areas where existing methods have the potential to accelerate computational chemistry research and disrupt more traditional approaches.
First we will present a deep learning model that approximate solution of Schrodinger equation. Focusing on parametrization for drug-like organic molecules and proteins, we have developed a single ‘universal’ model which is highly accurate compared to reference quantum mechanical calculations at speeds 10^6 faster. Second, we propose an improved machine learning framework for simulating molecules in arbitrary spin and charge states.
January 9, 2024: When Fermi meets Bose: Strongly interacting few-body systems in one dimension
Duality between strongly interacting Bose systems and ideal Fermi gases in one dimension provides a rare opportunity of understanding quantum many-body problem in the limit of strong interactions. Recently, it motivated a number of experimental and theoretical works in cold-atom physics. I will review this progress with the focus on relevant few-body physics, which can shape our understanding of few-to-many-body transition in quantum physics. I will briefly explain how to find a solution to the corresponding wave equation, and propose experimental set-ups motivated by this solution.
December 19, 2023: Sub-wavelength lattices for ultracold atoms
Gediminas Juzeliūnas, Institute of Theoretical Physics and Astronomy, Vilnius University, Vilnius, Lithuania.
Abstract
Traditionally, optical lattices are created by interfering two or more light beams, so that atoms are trapped at minima or maxima of the emerging interference pattern depending on the sign of the atomic polarizability [1]. The characteristic distances over which such lattice potentials change are limited by diffraction and thus cannot be smaller than half of the optical wavelength λ. The diffraction limitation can be overcome and subwavelength lattices can be created using coherent coupling between atomic internal states [2-9]. In particular, recent experiments demonstrated deeply subwavelength lattices using atoms with N internal states Raman-coupled with lasers of wavelength λ [7]. The resulting unit cell was N times smaller compared to the usual λ/2 periodicity of an optical lattice.
In the present talk we will discuss various ways to produce subwavelength lattices and effects manifesting in these lattices. In particular, we will present our recent work on periodically driven subwavelength lattices [8], as well on two-dimensional subwavelength lattices affected by the synthetic magnetic flux [9]. Ongoing research on many-body effects in subwavelength lattices will also be discussed.
[1] I. Bloch, Nature Physics 1, 23 (2005).
[2] M. Łącki et al., Phys. Rev. Lett. 117, 233001 (2016).
[3] F. Jendrzejewski et al., Phys. Rev. A 94, 063422 (2016).
[4] Y. Wang et al, Phys. Rev. Lett. 120, 083601 (2018).
[5] E. Gvozdiovas, P. Račkauskas, G. Juzeliūnas, SciPost Phys. 11, 100 (2021).
[6] P. Kubala, J. Zakrzewski and M. Łącki, Phys. Rev. A 104, 053312 (2021).
[7] R. P. Anderson et al, Physical Review Research 2, 013149 (2020).
[8] D. Burba, M. Račiūnas, I. B. Spielman and G. Juzeliūnas, Phys. Rev. A 107, 023309 (2023).
[9] E. Gvozdiovas, I. B. Spielman and G. Juzeliūnas, Phys. Rev. A 107, 033328 (2023).
December 5, 2023: Laser Scanning Microscopy of Superconducting Microwave Devices – Collaborative Discoveries
Tomasz Dietl, International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland
Abstract
Inspired by experimental results for HgTe and (Hg,Mn)Te topological quantum wells accumulated in Würzburg and also by CETRERA/MagTop collaboration in Warsaw [1], theory of the quantum spin Hall effect was developed [2]. It was demonstrated that (i) the presence of dopants is necessary to pin the Fermi energy in the topological gap; (ii) scattering of edge electrons by acceptor holes explains quantitatively and without adjustable parameters a short magnitude of the topological protection length, provided that non-scalar terms in the electron-hole exchange and the Kondo and Luttinger effects are taken into account [2]. Realizing that also in the case of the quantum anomalous effects, hoping conductivity between impurity states deteriorates the quantization precision at above 0.1 K, we address here the question how to improve quantization accuracy in the case of those two quantum Hall effects. In this context the role of bound magnetic and lattice polarons, negative U centers, and co-doping will be discussed.
References
[1] I. Yahniuk et al., npj Quant. Mater. 4 (2019) 13; arXiv:2111.07581 (2021).
[2] T. Dietl, Phys. Rev. Lett. 130 (2023) 086202; Phys. Rev. B 107 (2023) 085421.
November 21, 2023: Laser Scanning Microscopy of Superconducting Microwave Devices – Collaborative Discoveries
I have been very fortunate to collaborate with Dr. A. P. Zhuravel of the Institute of Low Temperature Physics and Engineering (NASU) in Kharkiv, Ukraine on laser scanning microscopy of superconducting microwave devices. Together we have enjoyed a fruitful collaboration spanning parts of two centuries, and we have discovered many fascinating phenomena. We have developed and refined a microwave microscope that can image the distribution of RF currents in resonant microwave devices, and metamaterials. We have imaged current distributions at the edges of patterned thin films and observe large enhancements of current density at these edges. We have also imaged the anisotropic nonlinear Meissner effect in cuprate superconductors, providing a real-space image of the direction-dependent superconducting energy gap on the Fermi surface. Later, we demonstrated the power of this microscope by examining the microscopic properties of RF SQUID metamaterials. The SQUID metamaterials have a collective resonant response between 10 and 20 GHz, tuned by very small dc magnetic fields. Through RF current imaging we find that the SQUIDs do not all oscillate in phase while being tuned, but break in to domains with different resonant frequencies. Learning how to tame this disorder through machine learning and in-situ modification of the metamaterial is an ongoing effort, inspired by our collaboration.
Acknowledgement:
This work is funded by US Department of Energy through grant # DE-SC0017931, the US National Science Foundation through grant # DMR-2004386, and the Maryland Quantum Materials Center.
November 14, 2023: IBM’s experiment on quantum utility before fault tolerance and its implications
Full-fledged quantum computing will require extensive use of quantum error correction techniques. While these are believed to be within reach, in principle, practical implementations require numerous engineering advances and are thus a matter of distant future. In the meantime, investigations of the capabilities of so-called noisy intermediate-scale quantum (NISQ) devices are actively underway.
Last summer, an experiment from IBM Quantum [1] has shaken the community: simulation of a quantum system beyond the capabilities of classical calculations was claimed. I will review this experiment, as well as a number of follow-up theoretical works [2-6] in order to provide the insight into the current capabilities of NISQ devices.
[1] Y. Kim et al., “Evidence for the utility of quantum computing before fault tolerance,” Nature 618, 500 (2023).
[2] J. Tindall, M. Fishman, M. Stoudenmire, and D. Sels, “Efficient tensor network simulation of IBM’s kicked Ising experiment,” arXiv:2306.14887.
[3] K. Kechedzhi et al., “Effective quantum volume, fidelity and computational cost of noisy quantum processing experiments,” arXiv:2306.15970.
[4] T. Begušić and G. K.-L. Chan, “Fast classical simulation of evidence for the utility of quantum computing before fault tolerance,” arXiv:2306.16372.
[5] E. G. D. Torre and M. M. Roses, “Dissipative mean-field theory of IBM utility experiment,” arXiv:2308.01339.
[6] S. Patra, S. S. Jahromi, S. Singh, and R. Orus, “Efficient tensor network simulation of IBM’s largest quantum processors,” arXiv:2309.15642.
October 17, 2023: 'Smoking gun’ signatures of topological milestones in trivial materials by measurement fine-tuning and data postselection
Exploring the topology of electronic bands is a way to realize new states of matter with possible implications for information technology. Because bands cannot always be observed directly, a central question is how to tell that a topological regime has been achieved. Experiments are often guided by a prediction of a unique signal or a pattern, called "the smoking gun". Examples include peaks in conductivity, microwave resonances, and shifts in interference fringes. However, many condensed matter experiments are performed on relatively small, micron or nanometer-scale, specimens. These structures are in the so-called mesoscopic regime, between atomic and macroscopic physics, where phenomenology is particularly rich. In this paper, we demonstrate that the trivial effects of quantum confinement, quantum interference and charge dynamics in nanostructures can reproduce accepted smoking gun signatures of triplet supercurrents, Majorana modes, topological Josephson junctions and fractionalized particles. The examples we use correspond to milestones of topological quantum computing: qubit spectroscopy, fusion and braiding. None of the samples we use are in the topological regime. The smoking gun patterns are achieved by fine-tuning during data acquisition and by subsequent data selection to pick non-representative examples out of a fluid multitude of similar patterns that do not generally fit the "smoking gun" designation. Building on this insight, we discuss ways that experimentalists can rigorously delineate between topological and non-topological effects, and the effects of fine-tuning by deeper analysis of larger volumes of data.
For more details, please, see the post on Substack.
October 3, 2023: How to Derive a New Theory and Decoherence-Free Entropic Gravity: Model and Experimental Tests
We will provide an answer to the question: “What kind of observations and assumptions are minimally needed to formulate a physical model?” Our answer to this question leads to the new systematic approach of Operational Dynamical Modeling (ODM), which allows deducing equations of motions from time evolution of observables. Using ODM, we are not only able to re-derive well-known physical theories (such as the Schrödinger and Newton equations), but also solve open problems in quantum nonequilibrium statistical dynamics and formulate new theory of entropic gravity that has been experimentally tested.
Erik Verlinde's theory of entropic gravity, postulating that gravity is not a fundamental force but rather emerges thermodynamically, has garnered much attention as a possible resolution to the quantum gravity problem. Some have ruled this theory out on grounds that entropic forces are by nature noisy and entropic gravity would therefore display far more decoherence than is observed in ultra-cold neutron experiments. We address this criticism by modeling linear gravity acting on small objects as an open quantum system. We show that the proposed master equation is fully compatible with the bounce experiment for ultra-cold neutrons. In addition, comparing our mode of entropic gravity to the Diosi-Penrose model for gravity induced decoherence indicates that the two theories are incompatible.