Entanglement growth and correlation spreading with variable-range interactions in spin and fermionic tunneling models

We investigate the dynamics following a global parameter quench for two one-dimensional models with variable-range power-law interactions: a long-range transverse Ising model, which has recently been realized in chains of trapped ions, and a long-range lattice model for spinless fermions with long-range tunneling. For the transverse Ising model, the spreading of correlations and growth of entanglement are computed using numerical matrix product state techniques, and are compared with exact solutions for the fermionic tunneling model. We identify transitions between regimes with and without an apparent linear light cone for correlations, which correspond closely between the two models. For long-range interactions, we find that despite the lack of a light cone, correlations grow slowly as a power law at short times, and that—depending on the structure of the initial state—the growth of entanglement can also be sublinear. These results are understood through analytical calculations, and should be measurable in experiments with trapped ions

Enhanced superexchange in a tilted mott insulator

In an optical lattice, entropy and mass transport by first-order tunneling are much faster than spin transport via superexchange. Here we show that adding a constant force (tilt) suppresses first-order tunneling, but not spin transport, realizing new features for spin Hamiltonians. Suppression of the superfluid transition can stabilize larger systems with faster spin dynamics. For the first time in a many-body spin system, we vary superexchange rates by over a factor of 100 and tune spin-spin interactions via the tilt. In a tilted lattice, defects are immobile and pure spin dynamics can be studied

Spin-models, dynamics and criticality with atoms in tilted optical superlattices

Funding: Strathclyde: UK EPSRC Programme Grant DesOEQ (EP/P009565/1), the European Union Horizon 2020 collaborative project QuProCS — QuantumProbes for Complex Systems (Grant Agreement No. 641277), and by the EOARD via AFOSR Grant No. FA9550-18-1-0064.We show that atoms in tilted optical superlattices provide a platform for exploring coupled spin chains of forms that are not present in other systems. In particular, using a period-2 superlattice in one dimension, we show that coupled Ising spin chains with XZ and ZZ spin coupling terms can be engineered. We use optimized tensor network techniques to explore the criticality and nonequilibrium dynamics in these models, finding a tricritical Ising point in regimes that are accessible in current experiments. These setups are ideal for studying low-entropy physics, as initial entropy is “frozen-out” in realizing the spin models, and provide an example of the complex critical behavior that can arise from interaction-projected models.Publisher PDFPeer reviewe

Resonant two-site tunnelling dynamics of bosons in a tilted optical superlattice

Funding: Strathclyde: UK EPSRC Program Grant DesOEQ (No.EP/P009565/1), the European Union Horizon 2020 collaborative project Quantum Probes for Complex Systems (Grant Agreement No. 641277), and by the EOARD via AFOSR Grant No. FA9550-18-1-0064.We study the nonequilibrium dynamics of a one-dimensional Bose-Hubbard model in a gradient potential and a superlattice, beginning from a deep Mott insulator regime with an average filling of one particle per site. Studying a quench that is near resonance to tunneling of the particles over two lattice sites, we show how a spin model emerges consisting of two coupled Ising chains that are coupled by interaction terms in a staggered geometry. We compare and contrast the behavior in this case with that in a previously studied case where the resonant tunneling was over a single site. Using optimized tensor network techniques to calculate finite-temperature behavior of the model, as well as finite-size scaling for the ground state, we conclude that the universality class of the phase transition for the coupled chains is that of a tricritical Ising point. We also investigate the out-of-equilibrium dynamics after the quench in the vicinity of the resonance and compare dynamics with recent experiments realized without the superlattice geometry. This model is directly realizable in current experiments and reflects a general way to realize spin models with ultracold atoms in optical lattices.Publisher PDFPeer reviewe

Treelike interactions and fast scrambling with cold atoms

We propose an experimentally realizable quantum spin model that exhibits fast scrambling, based on nonlocal interactions that couple sites whose separation is a power of 2. By controlling the relative strengths of deterministic, nonrandom couplings, we can continuously tune from the linear geometry of a nearest-neighbor spin chain to an ultrametric geometry in which the effective distance between spins is governed by their positions on a tree graph. The transition in geometry can be observed in quench dynamics, and is furthermore manifest in calculations of the entanglement entropy. Between the linear and treelike regimes, we find a peak in entanglement and exponentially fast spreading of quantum information across the system. Our proposed implementation, harnessing photon-mediated interactions among cold atoms in an optical cavity, offers a test case for experimentally observing the emergent geometry of a quantum many-body system

Dynamics of quantum many-body systems with long-range interactions

Constantly increasing experimental possibilities with strongly correlated systems of ultracold atoms in optical lattices and trapped ions make them one of the most promising candidates for quantum simulation and quantum computation in the near future, and open new opportunities for study many-body physics. Out-of-equilibrium properties of such complex systems present truly fascinating and rich physics, which is yet to be fully understood. This thesis studies many-body dynamics of quantum systems with long-range interactions and addresses a few distinct issues. The first one is related to a growing interest in the use of ultracold atoms in optical lattices to simulate condensed matter systems, in particular to understand their magnetic properties. In our project on tilted optical lattices we map the dynamics of bosonic particles with resonantly enhanced long-range tunnelings onto a spin chain with peculiar interaction terms. We study the novel properties of this system in and out of equilibrium. The second main topic is the dynamical growth of entanglement and spread of correlations between system partitions in quench experiments. Our investigation is based on current experiments with trapped ions, where the range of interactions can be tuned dynamically from almost neighboring to all-to-all. We analyze the role of this interaction range in non-equilibrium dynamics. The third topic we address is a new method of quantum state estimation, certified Matrix Product State (MPS) tomography, which has potential applications in regimes unreachable by full quantum state tomography. The investigation of quantum many-body systems often goes beyond analytically solvable models; that is where numerical simulations become vital. The majority of results in this thesis were obtained via the Density Matrix Renormalization Group (DMRG) methods in the context of the MPS and Matrix Product Operator(MPO) formalism. Further developing and optimizing these methods made it possible to obtain eigenstates and thermal states as well as to calculate the time dependent dynamics in quenches for experimentally relevant regimes.Constantly increasing experimental possibilities with strongly correlated systems of ultracold atoms in optical lattices and trapped ions make them one of the most promising candidates for quantum simulation and quantum computation in the near future, and open new opportunities for study many-body physics. Out-of-equilibrium properties of such complex systems present truly fascinating and rich physics, which is yet to be fully understood. This thesis studies many-body dynamics of quantum systems with long-range interactions and addresses a few distinct issues. The first one is related to a growing interest in the use of ultracold atoms in optical lattices to simulate condensed matter systems, in particular to understand their magnetic properties. In our project on tilted optical lattices we map the dynamics of bosonic particles with resonantly enhanced long-range tunnelings onto a spin chain with peculiar interaction terms. We study the novel properties of this system in and out of equilibrium. The second main topic is the dynamical growth of entanglement and spread of correlations between system partitions in quench experiments. Our investigation is based on current experiments with trapped ions, where the range of interactions can be tuned dynamically from almost neighboring to all-to-all. We analyze the role of this interaction range in non-equilibrium dynamics. The third topic we address is a new method of quantum state estimation, certified Matrix Product State (MPS) tomography, which has potential applications in regimes unreachable by full quantum state tomography. The investigation of quantum many-body systems often goes beyond analytically solvable models; that is where numerical simulations become vital. The majority of results in this thesis were obtained via the Density Matrix Renormalization Group (DMRG) methods in the context of the MPS and Matrix Product Operator(MPO) formalism. Further developing and optimizing these methods made it possible to obtain eigenstates and thermal states as well as to calculate the time dependent dynamics in quenches for experimentally relevant regimes

Density matrix renormalization group for continuous quantum systems

We introduce a versatile and practical framework for applying matrix product state techniques to continuous quantum systems. We divide space into multiple segments and generate continuous basis functions for the many-body state in each segment. By combining this mapping with existing numerical density matrix renormalization group routines, we show how one can accurately obtain the ground-state wave function, spatial correlations, and spatial entanglement entropy directly in the continuum. For a prototypical mesoscopic system of strongly interacting bosons we demonstrate faster convergence than standard grid-based discretization. We illustrate the power of our approach by studying a superfluid-insulator transition in an external potential. We outline how one can directly apply or generalize this technique to a wide variety of experimentally relevant problems across condensed matter physics and quantum field theory