358 research outputs found
Resummation for Nonequilibrium Perturbation Theory and Application to Open Quantum Lattices
Lattice models of fermions, bosons, and spins have long served to elucidate
the essential physics of quantum phase transitions in a variety of systems.
Generalizing such models to incorporate driving and dissipation has opened new
vistas to investigate nonequilibrium phenomena and dissipative phase
transitions in interacting many-body systems. We present a framework for the
treatment of such open quantum lattices based on a resummation scheme for the
Lindblad perturbation series. Employing a convenient diagrammatic
representation, we utilize this method to obtain relevant observables for the
open Jaynes-Cummings lattice, a model of special interest for open-system
quantum simulation. We demonstrate that the resummation framework allows us to
reliably predict observables for both finite and infinite Jaynes-Cummings
lattices with different lattice geometries. The resummation of the Lindblad
perturbation series can thus serve as a valuable tool in validating open
quantum simulators, such as circuit-QED lattices, currently being investigated
experimentally.Comment: 15 pages, 9 figure
Observation of a dissipative phase transition in a one-dimensional circuit QED lattice
Condensed matter physics has been driven forward by significant experimental
and theoretical progress in the study and understanding of equilibrium phase
transitions based on symmetry and topology. However, nonequilibrium phase
transitions have remained a challenge, in part due to their complexity in
theoretical descriptions and the additional experimental difficulties in
systematically controlling systems out of equilibrium. Here, we study a
one-dimensional chain of 72 microwave cavities, each coupled to a
superconducting qubit, and coherently drive the system into a nonequilibrium
steady state. We find experimental evidence for a dissipative phase transition
in the system in which the steady state changes dramatically as the mean photon
number is increased. Near the boundary between the two observed phases, the
system demonstrates bistability, with characteristic switching times as long as
60 ms -- far longer than any of the intrinsic rates known for the system. This
experiment demonstrates the power of circuit QED systems for studying
nonequilibrium condensed matter physics and paves the way for future
experiments exploring nonequilbrium physics with many-body quantum optics
Imaging Photon Lattice States by Scanning Defect Microscopy
Microwave photons inside lattices of coupled resonators and superconducting
qubits can exhibit surprising matter-like behavior. Realizing such open-system
quantum simulators presents an experimental challenge and requires new tools
and measurement techniques. Here, we introduce Scanning Defect Microscopy as
one such tool and illustrate its use in mapping the normal-mode structure of
microwave photons inside a 49-site Kagome lattice of coplanar waveguide
resonators. Scanning is accomplished by moving a probe equipped with a sapphire
tip across the lattice. This locally perturbs resonator frequencies and induces
shifts of the lattice resonance frequencies which we determine by measuring the
transmission spectrum. From the magnitude of mode shifts we can reconstruct
photon field amplitudes at each lattice site and thus create spatial images of
the photon-lattice normal modes
Preparing quantum many-body scar states on quantum computers
Quantum many-body scar states are highly excited eigenstates of many-body
systems that exhibit atypical entanglement and correlation properties relative
to typical eigenstates at the same energy density. Scar states also give rise
to infinitely long-lived coherent dynamics when the system is prepared in a
special initial state having finite overlap with them. Many models with exact
scar states have been constructed, but the fate of scarred eigenstates and
dynamics when these models are perturbed is difficult to study with classical
computational techniques. In this work, we propose state preparation protocols
that enable the use of quantum computers to study this question. We present
protocols both for individual scar states in a particular model, as well as
superpositions of them that give rise to coherent dynamics. For superpositions
of scar states, we present both a system-size-linear depth unitary and a
finite-depth nonunitary state preparation protocol, the latter of which uses
measurement and postselection to reduce the circuit depth. For individual
scarred eigenstates, we formulate an exact state preparation approach based on
matrix product states that yields quasipolynomial-depth circuits, as well as a
variational approach with a polynomial-depth ansatz circuit. We also provide
proof of principle state-preparation demonstrations on superconducting quantum
hardware.Comment: 20 Pages, 15 Figures, 2 Tables. V2: corrected typo
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