Workshop on Quantum Information Science with Cold Atoms

Thursday 13:00-13:40

Speaker: Joonhee Choi (CALTECH)

Title: Quantum Device Benchmarking from Many-body Quantum Chaos

Abstract: Here we present a simple and efficient benchmarking protocol to estimate the many-body fidelity between a target state and the actual state obtained from experimental evolution. Our protocol relies only on time evolution of a quantum system undergoing chaotic dynamics, followed by projective measurements in a fixed local basis without any local control; this is in stark contrast to many existing methods which require fine-tuned spatiotemporal control and substantial experimental resources that scale exponentially with system size. Fundamentally, this simplification stems from a universal phenomenon in many-body quantum chaos: the emergence of universal random statistics in a local region. We demonstrate our benchmarking protocol numerically for random unitary circuits, and experimentally using a Rydberg quantum simulator.

Thursday 13:40-14:20

Speaker: Soonwon Choi (MIT)

Title: Emergent quantum randomness and its application for quantum device benchmarking

Abstract: In this talk, we describe a novel, universal phenomenon that occurs in strongly interacting many-body quantum dynamics beyond the conventional thermalization. The observed universality leads to the development of a novel benchmarking method applicable for a wide variety of near-term quantum devices. More specifically, we point out that a single many-body wavefunction can encode an ensemble of a large number of pure states defined on a subsystem. For a wide class of many-body wavefunctions, we show that the ensembles encoded in them display universal statistical properties by using a notion in quantum information theory, called quantum state k-designs. The special case (k=1) reduces to the conventional quantum thermalization. The universality is corroborated by two theorems, solvable models, extensive numerical simulations of Hamiltonian dynamics, and recent experimental observations based on a Rydberg quantum simulator. Our results offer a new approach for studying quantum chaos and provide a practical method for sampling pseudorandom quantum states. As an example of practical utility, I will explain how our results allow us to develop a novel sample-efficient benchmarking protocol, which has been already demonstrated in an experiment.

Thursday 14:20-15:00

Speaker: Jaewook Ahn (KAIST)

Title: Rydberg-atom programmable quantum simulator

Abstract: We present experimental results of quantum annealing and quantum approximate optimization algorithms used to obtain the solutions of a few exemplary NP-complete problems, such as maximal-independent-set and max-cut problems, mapped on to Rydberg-atom planar and nonplanar graph arrangements.

Thursday 15:15-15:55

Speaker: Gyu-Boong Jo (HKUST)

Title: Non-Hermitian spin-orbit-coupled quantum gases

Abstract: Spin-orbit coupling (SOC), an essential mechanism underlying intriguing quantum phenomena from the spin Hall effect to topological insulators, has been mostly investigated in the Hermitian regime without dissipation. In this talk, I will report the realization of the non-Hermitian spin-orbit-coupled quantum gases. First, I will describe how such SOC mechanism can be affected by dissipation in the context of non-Hermitian physics. Such dissipation enables the energy gap to be engineered and closed at the critical dissipation value, the so-called exceptional point (EP), accompanied by the Parity-Time symmetry-breaking transition. The ability to dynamically engineer non-Hermitian spin-orbit-coupled energy band allows us to investigate the topological structure around the EP by dynamically encircling it in a parameter space and show chiral spin transfer around the EP. Our work highlights the full controllability of non-Hermitian quantum systems and sets the stage for realization of non-Hermitian topological states with SOC.

Thursday 15:55-16:35

Speaker: Yong-il Shin (SNU)

Title: Saturation of defects in a rapidly quenched Bose gas

Abstract: When a system undergoes a symmetry-breaking phase transition, spatial domains of the ordered phase randomly develop, and topological defects are possibly formed at their interfaces. The Kibble–Zurek mechanism (KZM) provides a universal framework for the defect formation, predicting the power-law dependence of the defect density on the quench rate. However, recently, a noticeable deviation from KZM predictions was observed in atomic gas experiments, in which a trapped sample was quenched into a superfluid phase, generating quantum vortices, which became saturated for rapid quenches. In this talk, I will present our experimental study on the defect saturation with a weakly interacting Bose gas. We observe that the number of quantum vortices exhibits a Poissonian distribution not only for a slow quench in the Kibble–Zurek scaling regime but also for a fast quench in the saturation regime. This indicates that the defect saturation is not caused by destructive vortex collisions after their formation, but by the early-time coarsening in an emerging condensate. I will discuss further experimental evidence on the early-time coarsening effect in defect formation.

Thursday 16:35-17:15

Speaker: Woo Jin Kwon (LENS)

Title: Programmable quantum vortex collider revealing sound emission and annihilation

Abstract: Vortices are at the heart of numerous fluid phenomena. Dissipation of the energy of quantized vortices, whose topological nature protects them against diffusing their vorticity, constitutes a central concept in describing quantum hydrodynamics, such as superfluid turbulence and its decay. Interestingly, a vortex can dissipate its kinetic energy via phonon emissions due to vortex-sound interaction. However, the scarcity of convincing experimental demonstrations of sound radiation from decaying quantum vortices has limited our deep understanding. Here, to unveil the nature of vortex dissipation, we realize a deterministic, programmable quantum vortex collider in homogeneous, planar atomic Fermi superfluids and directly observe sound-mediated dissipation and its ultimate form, i.e., vortex annihilation. We present vortex collision measurement across the BEC-BCS crossover and find a higher vortex energy dissipation crossing to BCS superfluid regime, highlighting the significance of fermionic quasiparticle states filling up a vortex core. Our experiment provides the direct evidence of sound emissions from quantum vortex decay and a starting point for understanding vortex collisions that underlie quantum turbulence.

Friday 13:30-14:10

Speaker: Yoshiro Takahashi (Kyoto University)

Title: Disorder-induced Thouless pumping of ultracold atoms in an optical lattice

Abstract: Robustness against perturbations lies at the heart of topological phenomena. If, however, a perturbation such as disorder becomes dominant, it may cause a topological phase transition between topologically non-trivial and trivial phases. Here we experimentally reveal the competition and interplay between topology and disorder in a Thouless pump realized with ultracold atoms in an optical lattice [1], by creating a quasi-periodic potential from weak to strong regimes in a controllable manner. We demonstrate a disorder-induced pumping in which the presence of disorder can induce a non-trivial pump for a specific pumping sequence, while no pump is observed in the clean limit [2]. Our highly controllable system could be a unique platform for studying various disorder-related novel effects in a wide range of topological quantum phenomena.

[1] S. Nakajima, et al. Topological thouless pumping of ultracold fermions. Nature Physics 12, 296 (2016).

[2] S. Nakajima, et al. Competition and interplay between topology and quasi-periodic disorder in Thouless pumping of ultracold atoms. Nature Physics, (2021).

Friday 14:10-14:50

Speaker: Ippei Danshita (Kindai University)

Title: Comparison between optical-lattice quantum simulation and numerical simulators in quench dynamics of the Bose-bubbard model

Abstract: Recent technological advances in quantum simulations using cold atoms allow for studying nonequilibrium dynamics of quantum many-body systems even in regimes that numerical simulations using classical computers cannot access. Focusing on spreading of correlations induced by a quantum quench, we use outputs of optical-lattice quantum simulators for the Bose-Hubbard model in order to examine the performance of numerical methods, namely the SU(3) truncated Wigner approximation (TWA) and the projected entangled pair states (PEPS) algorithm. We show that while the SU(3) TWA can quantitatively simulate the dynamics to some extent, the validity time scale is not long enough for capturing the propagation velocity of the correlation. On the other hand, we find that there are parameter regions where PEPS can capture the propagation velocity. The success of PEPS leads us to also compute correlation spreading dynamics of the Ising model, which can be realized with Rydberg atoms in an optical tweezer array. We expect that the outputs from PEPS will be useful benchmarks for future experiments.

Friday 15:00-15:40

Speaker: Takeshi Fukuhara (RIKEN)

Title: Single-site-resolved imaging of ultracold atoms in a triangular lattice

Abstract: Ultracold atoms in optical lattices provide an excellent platform to study many-body quantum systems. Especially, a quantum gas microscope, which enables us to observe and control atoms at the single-site level, is a powerful tool for such studies. Our main target is a quantum simulation of frustrated systems, which are expected to exhibit various phenomena and non-trivial quantum states such as quantum spin liquids. The simplest example of frustrated systems can be realized with a triangular lattice. In this talk, I will present single-site-resolved fluorescence imaging of ultracold rubidium-87 atoms in a triangular optical lattice [1]. Experimental parameters for the fluorescence imaging have been automatically optimized by using a machine learning technique. I will also introduce experimental results [1,2] of automatic optimization based on the Bayesian optimization.

[1] R. Yamamoto et al., “Single-site-resolved imaging of ultracold atoms in a triangular optical lattice,” New Journal of Physics 22, 123028 (2020).

[2] I. Nakamura et al., “Non-standard trajectories found by machine learning for evaporative cooling of 87Rb atoms,” Optics Express 27, 20435 (2019).

Friday 15:40-16:20

Speaker: Bing Yang (IQOQI)

Title: Many-body dynamics in an extended Bose-Hubbard quantum simulator

Abstract: Quantum many-body system can exhibit collective phenomena that enable the realization of novel quantum phases and have the potential to exponentially speed up information processing. Ultracold atoms in optical lattices provide a clean and controllable platform for studying complex many-body problems. Recently, we have cooled such a Bose-Hubbard quantum simulator with 10000 atoms by immersing it into removable superfluid reservoirs. We further demonstrate a new type of two-qubit gate for entangling atom pairs, achieving a fidelity of 0.993(1). By adiabatically connecting the atomic spins with the superexchange interaction, we have established a Heisenberg antiferromagnet in a 70-site extended spin chain. Our results offer a setting for exploring low-energy many-body phases with the Bose-Hubbard quantum simulator.