Session S06: Quantum Simulation of Many-Body Systems

Unconventional many-body scarring in a Bose--Hubbard quantum simulator

Quantum many-body scarring presents a novel mechanism that delays the onset of thermal equilibrium in non-integrable models. Using a large-scale Bose--Hubbard quantum simulator, we realize many-body scarring by emulating the PXP model with the tilted optical lattice and subsequently extending the scarring phenomenon to an unconventional regime in the unity-filling state. We demonstrate the combination of detuning and periodic driving deters the scrambling of initial state information by measuring the quantum fidelity of single-site subsystems with many-body interference in a superlattice. The interference protocol also helps read out single-site entanglement entropy, which further illustrates the trapping of the many-body system in a low-entropy subspace. Our work paves the way for investigating many-body scars in ultracold-atom experiments and exploring its relation to lattice gauge theories, Hilbert space fragmentation, and disorder-free localization.   

Catching Bethe phantoms and quantum many-body scars: Long-lived spin-helix states in Heisenberg magnets

Exact solutions for quantum many-body systems are rare and provide valuable insight to universal phenomena. Here we show experimentally in anisotropic Heisenberg chains that special helical spin patterns can have long lifetimes. This finding confirms the recent prediction of phantom Bethe states, exact many-body eigenstates carrying finite momenta yet no energy. We theoretically find analogous stable spin helices even in higher dimensions and in other non-integrable systems, where they now imply non-thermalizing dynamics associated with quantum many-body scars. We use phantom spin helices to directly measure the interaction anisotropy which has a major contribution from short-range off-site interactions that have not been observed before. Phantom helix states open new opportunities for quantum simulations of spin physics and studies of many-body dynamics.   

Quantum Matter Synthesizer: Seeing and arranging individual atoms in an optical lattice

The capability to detect and rearrange individual atoms in a quantum gas promises a new research frontier to synthesize generic quantum matter and realize universal quantum simulation of many-body physics. Toward this goal, we present the construction and characterization of a new platform called Quantum Matter Synthesizer (QMS), where we integrate quantum gas microscopy of atoms in a two-dimensional lattice with an optical tweezer array generated by Digital Micromirror Devices (DMD). We demonstrate site-resolved imaging with a diffraction-limited resolution of 630 nm. We also develop fast computation algorithm to perform real-time control of the DMD optical potential at >2kHz, which will form moving optical tweezers to re-arrange atoms in the triangular lattice. The technical implementation and prospective research topics will be detailed in this talk.   

Tuning and probing local thermalization of a Floquet-engineered dipolar ensemble

We experimentally study the many-body out-of-equilibrium dynamics of a three-dimensional, dipolar-interacting spin system with tunable XYZ Heisenberg anisotropy. We utilize advanced Hamiltonian engineering techniques and leverage the inherent disorder in the system to probe global and local spin autocorrelation functions for various XYZ Hamiltonians. The out-of-equilibrium dynamics of global and local observables display striking differences in the vicinity of SU(2)-symmetric Heisenberg Hamiltonians, where global observables are conserved, but local thermalization nonetheless proceeds. The decay shapes of local correlators further exhibit a stretched exponential controlled by the form of the Hamiltonian, which are explained by a model that explicitly realizes the system acting as its own bath. In particular, tuning the Hamiltonian modifies the correlation times and effective fields driving dynamics, thus leading to different thermalization behavior. Our results provide detailed microscopic mechanisms of the relaxation of closed, interacting quantum many-body systems as they approach local thermal equilibrium.   

Quantum simulation of many-body non-equilibrium dynamics in tilted Fermi-Hubbard models

Identifying applications of state-of-the art quantum computers and quantum simulators within their limitations is a problem of contemporary interest. Problems in quantum many-body physics often lie at the intersection of computational hardness and physical pertinence. Here we show that a quantum simulator can be used to develop new, efficient classical algorithms to study the dynamics of many-body systems. We first consider a localized 1D Fermi-Hubbard system where the exact techniques of computing the time dynamics are inefficient, and develop an efficient  approximate theory. Our approximate theory does not have an error estimate and therefore, we use a quantum simulator to benchmark its performance in terms of accuracy of its outcome. We use the approximate theory to further study the interacting features of localized systems [1]. Moreover, we extend this approximation to the 2D Fermi-Hubbard model, where the reach of existing numerical techniques is very limited. We show that our method, together with sequence extrapolation techniques can be used to study some 2D Fermi-Hubbard models.   

Exploring Dynamical Phases in a Multilevel Atom-Cavity System

Out-of-equilibrium dynamics of many-body systems can exhibit rich behavior not found in equilibrium systems. For example, certain non-dissipative, interacting systems can experience distinct dynamical phases when externally driven or quenched out of equilibrium. These “dynamical phases” have associated phase transitions which become sharp in the thermodynamic limit. Cavity-mediated interactions between cold atoms trapped in an optical cavity can act as a powerful tool to study such systems. Previously, we observed a dynamical phase transition using an ensemble of ~10^{6 88}Sr atoms coupled to a cavity along the narrow-linewidth ¹S₀-³P₁ transition [1]. We have also proposed a scheme to simulate dynamics of a low-energy BCS superconductor, which exhibits three dynamical phases [2]. In this talk, we present progress towards observing dynamical phases in a multilevel system composed of the ¹S₀ ground state and the m = ±1 Zeeman states of the ³P₁ manifold in ⁸⁸Sr. By varying the Zeeman splitting between the excited states and introducing controlled inhomogeneity in the atomic transition frequency across the ensemble, we expect to map out a full phase diagram and measure the distinct dynamics in different phases of the system.

[1] Nature 580, 602-607 (2020)

[2] PRL 126, 173601 (2021)   

Towards trapped-ion simulations of open quantum systems

Quantum systems have a finite degree of interaction with their environments. Therefore, understanding the dynamics of open quantum systems is of great importance for both real-life technological applications as well as for the fundamental understanding of the quantum many-body systems.

Here we present our progress in building a quantum simulator composed of a chain of Yb⁺ ions trapped in a uniquely designed experimental setup featuring a linear 3D Paul trap with high uniformity along the ion chain and high NA optical access from multiple directions to allow individual ion readout and addressing.

We intend to use this setup to study dissipative processes in both electronic (spin) and motional degrees of freedom.  The flexibility of our system will allow to introduce both local optical pumping and detection operations and individual sympathetic cooling. The former will be used to study the competition of unitary spin evolution of the system and non-unitary processes which leads to non-equilibrium phenomena such as dissipative ferromagnetic-paramagnetic phase transitions and measurement induced entanglement phase transitions. Ion-resolved cooling will be used to study the evolution of a spin system in contact with an engineered reservoir.   

Quantum simulations of time-reversal broken interacting systems in a trapped ion quantum computer

Interacting quantum many-body systems in which time-reversal symmetry is broken give rise to a variety of rich collective behaviors, and are therefore a major target of research in modern physics. Quantum simulators can potentially be used to explore and understand such systems, which are often beyond the computational reach of classical simulation. Quantum simulators which are embedded in universal quantum computers are especially advantageous since simulation results can be analyzed using tools such as quantum-classical variational optimization, measurement of topological string operators as well as tomography.

Using a method developed in our group [1], we experimentally realize quantum simulations of interacting, time-reversal broken and 2d quantum systems in a universal trapped-ion quantum processor [2]. Specifically, we realize a quantum ring threaded by magnetic flux. We adiabatically prepare and measure the ring’s ground state. We also quench the ring and measure flux dependent chiral currents. Next we realize a triangular spin ladder threaded by a staggered magnetic flux. We show that this 2d model exhibits non-trivial interactions between its excitations.

 

References:

[1] Manovitz et al. PRXQ 1, 020303 (2020). 

[2] Manovitz et al. arXiv:2111.04155 (2021).   

Quantum simulation with two-dimensional ion crystal

Quantum simulation can provide a solution of a complex problem that can be intractable in classical means. In particular, it is challenging to numerically simulate many-body quantum system in 2-dimension, which reveals geometric frustration and topological phase. A trapped-ion system has been used to realize quantum simulation of spin-models, but existing experiments were mostly limited to 1-dimensional ion-chain. Here, we present the quantum simulation with 2-dimensional ion-crystal that experimentally study quantum magnets on triangular lattice, and adiabatically prepare their ground-state for the first time. The 2D crystals are confined in a monolithic ion-trap, which eliminates disturbance of micromotions for quantum operations. Spin-dependent-force is applied to generate the Ising Hamiltonian, and ramp the strength of transverse field to adiabatically reach the ground state of the Ising Hamiltonian. We experimentally study the ferromagnetic and the frustrated ground states with ion numbers up to 10 ions. Our work paves the way to simulate 2-dimensional materials with ion-trap platform.