Workshop on Quantum Information Science with Cold Atoms

Thursday 10:00-10:40

Speaker: Aaron Young (JILA/University of Colorado Boulder)

Title: Programmable quantum sensors with alkaline earth atom arrays

Abstract: The narrow-line optical clock transition present in alkaline earth atoms forms the basis of some of the most precise measurements of time. By introducing additional microscopic controls to large ensembles of these atoms, we are able to engineer both the states of these atoms and their environment to perform ever more precise measurements. Specifically, we combine the nice properties of these atoms with optical tweezers, which provide programmable control of the atom positions, and optical lattices, which provide a route towards scalability beyond what can currently be achieved with tweezer arrays. These tools allow us to prepare large ensembles of atoms in states with very low entropy, and engineer the environment experienced by these atoms to preserve their coherence. We further combine the programmable control provided by optical tweezers with Rydberg-mediated interactions to prepare entangled states that can be used to perform measurements with a precision beyond the standard quantum limit.

Thursday 10:40-11:20

Speaker: Joonhee Choi (Stanford/CALTECH)

Title: Benchmarking a large-scale quantum simulator beyond the exact classical simulation threshold

Abstract: Today’s quantum computers and simulators are currently capable of running quantum operation on a few tens to hundreds of qubits. However, they are still noisy in a sense that their performance on a global level is far from ideal due to environmental noise and control imperfections. The recently developed, efficient benchmarking protocols can in principle be considered to measure the global fidelity of a quantum device, but all these methods require the preparation of a reference state obtained by a classical computer. Since an exact classical representation of the targeted states becomes infeasible as the size of a quantum system increases, characterizing the fidelity of large-scale quantum systems remains elusive. In this talk, we present a systematic fidelity estimation in such a large system regime, based on computationally efficient but approximate classical algorithms. In particular, the limited accuracy of classical algorithms raises a question of whether a classical or quantum device is a better realization of ideal, large-scale quantum dynamics. Using a Rydberg atom-array quantum simulator with up to 60 atoms, we demonstrate and compare three different benchmarking approaches, including extrapolation, split-array benchmarking, and ratio benchmarking, to efficiently learn the many-body fidelity of a large-scale quantum simulator. Then, we systematically estimate the required resources for the classical algorithms to reach the same experimental fidelity as a function of system size, which enlightens how close we are to achieving the ultimate goal of quantum advantage.

Thursday 11:20-12:00

Speaker: Jaewook Ahn (KAIST)

Title: Quantum computing with Rydberg atom graphs

Abstract: One after another, scalable quantum computers are being developed. While their computational capabilities are limited, whether they can be used for specific computational problems, especially the problems in which digital algorithms are inefficient, is of great interest. Problems whose computational complexity is classified as NP (non-deterministic polynomial) cannot be efficiently calculated by digital computing algorithms. Therefore, attention is focused on whether quantum computers can be used to compute NP-problems, especially NP-complete problems with polynomial time reducibility to all other NP-problems. In this presentation, we introduce our experiments of using Rydberg atom qubit graphs to solve NP-complete problems, which include the maximum independent set problem, 3-SAT problem, and prime number factorization problem. We then discuss performance requirements to challenge the computational limits of digital computers.

Thursday 14:00-14:40

Speaker: Akio Kawasaki (AIST)

Title: Searches for New Physics with Atomic Clocks

Abstract: Extremely high precision measurement of frequency ratio realized by optical atomic clocks enables us to perform not only accurate time-keeping but also searches for new physics. One of the earliest works on this is the search for time-variation of fundamental constants, such as the proton-to-electron mass ratio and the fine structure constant. Recent development in the theory of ultralight dark matter, dark matter candidates with a mass less than 1 eV or much smaller, opened a way to search for dark matter with atomic clock comparisons. In this talk, I first discuss recent results on the ultralight dark matter search by comparing optical and microwave atomic clocks. I also briefly mention the ongoing search for a new narrow-linewidth transition in ytterbium that has high sensitivity to the variation of the fine structure constant.

Thursday 14:40-15:20

Speaker: Kyungtae Kim (JILA)

Title: JILA 1D Wannier-Stark optical lattice clock

Abstract: In this talk, we introduce JILA 1D optical lattice clock that utilizes Wannier-Stark (WS) states in a shallow vertical lattice. The experiment features T2 coherence over 30 seconds, resulting in frequency resolving power at the level of redshift below 1 mm throughout the sample [1]. The central parts of the system are the microscopic frequency map based on in-situ imaging and the engineering of the atomic interaction with shallow lattice [2]. With the partially delocalized WS state, we can balance on-site p-wave interaction and off-site s-wave interaction at "magic lattice depth." The low lattice intensity reduces many systematics, including the light shift, one of the largest contributions to the uncertainty, and Raman scattering. We present the recent evaluation of the lattice light shift at the 5E-19 fractional frequency uncertainty based on the precise control and understanding of the motional effects of the atoms [3]. Furthermore, we describe ongoing efforts to reduce the uncertainty from black-body radiation.

[1] T. Bothwell et al., Nature. 602, 420–424 (2022).

[2] A. Aeppli et al., Science Advances. 8, eadc9242 (2022).

[3] K. Kim, A. Aeppli, T. Bothwell, J. Ye, arXiv:2210.16374 (2022).

This talk will be also given in the offline (KAIST, Physics department seminar room #1323)

Thursday 16:00-16:40

Speaker: Yoshiro Takahashi (Kyoto University)

Title: Ultracold ytterbium atoms in an optical lattice - from dissipative Hubbard model to new physics search-

Abstract: In this talk, I will report our recent experiments using ultracold two-electron atoms of ytterbium (Yb) loaded into an optical lattice. I will present our study of an SU(N=6) Fermi- Hubbard model by working with 173Yb. This enlarged spin symmetry of SU(N) is a powerful tool to lower the temperature of atoms in an optical lattice, known as a Pomeranchuk cooling effect. The detailed comparison between theory and experiment allows us to realize a lowest temperature of cold-atom Fermi-Hubbard model [1]. More recently, we have studied the quantum magnetism in an open dissipative SU(6) Fermi-Hubbard system, reveling the dynamical change of the spin correlations from antiferrimagntic to ferromagnetic ones [2]. Other efforts in our lab will be also reported, including the precision measurement of the isotope shifts with the part-per-billion precision, allowing us to obtain a bound of the coupling of a new hypothetical particle beyond Standard Model [3].

[1] S. Taie, et al., “Observation of antiferromagnetic correlations in an ultracold SU(N) Hubbard model”, Nature Physics, (2022).

[2] K. Honda et al., “Observation of the Sign Reversal of the Magnetic Correlation in a Driven-Dissipative Fermi-Hubbard System”, arXiv:2205.13162.

[3] K. Ono, et al., “Observation of non-linearity of generalized King plot in the search for new boson”, Phys. Rev. X 12, 021033 (2022).

Thursday 16:40-17:20

Speaker: Zhen-Sheng Yuan (USTC)

Title: Efficiently Extracting Multi-Point Correlations of a Floquet Thermalized System

Abstract: Featuring excellent coherence properties and ultra-low entropy achieved in large-scale atom arrays, ultracold atoms in optical lattices are a promising platform for demonstrating practical quantum computational advantage applied in simulating many-body dynamics. In this work, we experimentally characterized the thermalized phase of a driven many-body system via entanglement entropy and multi-point correlations, which is beyond the capability of classical computations. Leveraging dedicated precise manipulations and atom-number-resolved detection through a quantum gas microscope with bichromatic spin-dependent superlattices, we implemented such a driven Hubbard chain involving up to 20 atoms in 32 sites, corresponding to a Hilbert space of dimension 10^14. We validated the samples via Bayesian hypothesis tests in the classical verifiable regime. In the advantage regime, it would take at least 2,500 seconds to generate a single sample for the Frontier supercomputer, the most powerful supercomputer worldwide, with the currently known best algorithms and supposing its random access memory (RAM) is sufficient. For comparison, our quantum simulator takes only 500 seconds to perform the same task. Employing superlattices to perform Hong-Ou-Mandel-like interferences, we measured the Renyi entropy of the driving system and observed the volume law and chaotic dynamics of the entanglement entropy in the thermalized phase. Multi-point correlations of up to 14th-order extracted from experimental samples offer clear distinctions between the thermalized and many-body-localized phases.

Thursday 17:20-18:00

Speaker: Eun-Ah Kim (Cornell University)

Title: Non-abelian anyons and graph gauge theory on a superconducting processor

Abstract: Topological quantum computation requires creating and braiding non-Abelian anyons. I will describe a simple and systematic approach to constructing effective unitary protocols for braiding, manipulation, and readout of non-Abelian anyons and preparing their entangled states on a NISQ device. Our approach is based on our new graph gauge theory on a planar qubit graph with vertices of degree 2, 3, and 4. I will discuss experimental observations guided by our theory.

Friday 10:00-10:40

Speaker: Huanqian Loh (NUS)

Title: Scaling up atom arrays

Abstract: Neutral atom arrays are a promising platform for the bottom-up control of individual qubits in a scalable and programmable way. These atom arrays host tunable interactions and can be arranged to form arbitrary geometries, making them attractive candidates for quantum computation and simulation. Scaling up atom arrays while maintaining high fidelity control would allow one to simulate larger and more complex quantum many-body systems. In this talk, I will discuss the limits of scaling up atom arrays and present two solutions to overcome these limits: the first involving a new class of "magic" wavelengths for optically trapping atoms, and the second involving a novel parallel rearrangement algorithm to rapidly assemble defect-free arrays of hundreds of singly-trapped atoms in real time. I will also discuss different routes for inducing programmable interactions between the singly-trapped particles, with an eye towards future quantum simulation studies.

Friday 10:40-11:20

Speaker: Bo Yan (Zhejiang University)

Title: Progress on laser cooling of BaF molecule

Abstract: Extremely high precision measurement of frequency ratio realized by optical atomic clocks enables us to perform not only accurate time-keeping but also searches for new physics. One of the earliest works on this is the search for time-variation of fundamental constants, such as the proton-to-electron mass ratio and the fine structure constant. Recent development in the theory of ultralight dark matter, dark matter candidates with a mass less than 1 eV or much smaller, opened a way to search for dark matter with atomic clock comparisons. In this talk, I first discuss recent results on the ultralight dark matter search by comparing optical and microwave atomic clocks. I also briefly mention the ongoing search for a new narrow-linewidth transition in ytterbium that has high sensitivity to the variation of the fine structure constant.

Friday 11:20-12:00

Speaker: Shiqian Ding (Tsinghua university)

Title: Laser-cooled ultracold molecules in an optical lattice.

Abstract: Dense ultracold molecular samples offer new platforms for quantum chemistry, quantum information science, strongly correlated many-body physics and precision tests of fundamental physics. In this talk, I will present our work in Prof. Jun Ye's group at JILA on achieving a dense ultracold molecular sample by loading more than 10^3 laser-cooled YO molecules in an optical lattice. We demonstrate a robust cooling in the lattice to 1 μK temperature, the lowest for laser-cooled molecules.

Friday 13:30-14:10

Speaker: Tyler Neely (University of Queensland)

Title: The equilibrium phases of two-dimensional vortex matter

Abstract: The maximum entropy principle of statistical mechanics is one of the most powerful probabilistic tools in theoretical physics, and its application towards understanding the behaviour of turbulent flows has a long history [1]. However, directly applying the maximum entropy approach to real fluids has only resulted in qualitative agreement; classical fluids are complicated by the presence of viscosity and a lack of sufficiently ergodic mixing amongst the large number of active degrees of freedom. Here we focus on a chiral (same circulation direction) system of superfluid vortices, injected into a disc-shaped Bose-Einstein condensate. I will describe our experimental tests of the maximum entropy principle for predicting the equilibrium states of the vortex system. By stirring the superfluid, we controllably inject clusters of vortices and arrange their initial positions. By observing their dynamics, we find that relaxation to equilibrium occurs within experimental timescales and explore wide ranges of the predicted equilibrium phases [2]. Furthermore, exploration of the low-energy phases of the system has revealed a range of strongly correlated vortex liquid states that are relevant towards emulating the behaviour of the 2D electron gas. These states of vortex matter have gained prominence in the theory of the fractional quantum hall effect, where the 2D electron gas moves analogous to vortices in an incompressible fluid, and the vortex density maps to the density of the quantum hall droplet. While facilitating the exploration of low-energy vortex configurations, this work also demonstrates an adaptable experimental technique for creating arbitrary arrangements of vortices, paving the way for experiments in more complex trap geometries.

[1] L. Onsager, Nuovo Cimento 6, 279 (1949).

[2] M. T. Reeves, K. Goddard-Lee, G. Gauthier, O. R. Stockdale, H. Salman, T. Edmonds, X. Yu, A. S. Bradley, M. Baker, H. Rubinsztein-Dunlop, M. J. Davis, T. W. Neely, Physical Review X, 12, 011031 (2022).

Friday 14:10-14:50

Speaker: Gyu-boong Jo (HKUST)

Title: Non-Hermitian skin effect in a two-dimensional topological system without non-reciprocity

Abstract: Non-Hermitian concept has generalized the notion of band topology with associated exceptional points (EPs), also known as the parity-time symmetry breaking point, leading to the counter- intuitive phenomena. Non-Hermitian skin effect, involving the accumulation of particles at the boundary, is one such circumstance, but its experimental realization has been limited so far to low-dimensional non-reciprocal systems. Here, we report on the reailzation of a two-dimensional non-Hermitian topological band with ultracold atoms by combining spin-orbit-coupled optical lattices with tunable dissipation.In this platform, a pair of EPs are created in the band structures without non-reciprocity, connected to each other by an open-ended bulk Fermi arc, in contrast to the contours with closed loops in Hermitian systems. The associated EPs emerges and shifts with increasing dissipation, leading to the formation of Fermi arc. Furthermore, evidence from the direct measurement of spectral topology in the complex energy plane indicates the existence of skin effect in 2D. Our work would shed a light on the connection between two distinct phenomena that only exist in non-Hermitian systems, i.e., the exceptional degeneracies and the non-Hermitian skin effect in high dimensions.

Friday 14:50-15:30

Speaker: Tenzin Rabga (SNU)

Title: Experimental Study of the Inhomogeneous Kibble-Zurek Mechanism in a Bose Gas of Rubidium

Abstract: Inhomogeneous Kibble-Zurek Mechanism (IKZM) provides the framework for understanding the non-equilibrium defect formation dynamics in an inhomogeneous system as it undergoes a continuous thermal phase transition. Degenerate quantum gases provide an ideal platform for studying such critical phenomena. Although the IKZM theory correctly predicts the qualitative dependence of the defect number density on the thermal quench rate, our recent studies in Bose gases show significant quantitative deviations in the predicted power-law scaling, as well as the observation of defect number saturation at rapid quench rates. This saturation of the defect number can be attributed to an early-time coarsening effect. In the study presented here, we extend our investigations into the IKZM using a Bose gas of rubidium and examine the effect of the optical trapping potential geometry on certain key parameters of the defect nucleation dynamics. Our observations indicate a strong dependence of the KZ scaling exponent on the underlying trap geometry, while the early coarsening effect is insensitive to the trap geometry. Moreover, the trend in the power-law scaling exponent as a function of the underlying trap configuration currently cannot be explained within the IKZM framework. We also observe the important role causality plays in the phase transition dynamics in such inhomogeneous systems. We present the relevant results and outline scopes for future studies to better understand such non-equilibrium phase transition dynamics.