35 research outputs found
Many-Body Quantum Spin Dynamics with Monte Carlo Trajectories on a Discrete Phase Space
Interacting spin systems are of fundamental relevance in different areas of
physics, as well as in quantum information science, and biology. These spin
models represent the simplest, yet not fully understood, manifestation of
quantum many-body systems. An important outstanding problem is the efficient
numerical computation of dynamics in large spin systems. Here we propose a new
semiclassical method to study many-body spin dynamics in generic spin lattice
models. The method is based on a discrete Monte Carlo sampling in phase-space
in the framework of the so-called truncated Wigner approximation. Comparisons
with analytical and numerically exact calculations demonstrate the power of the
technique. They show that it correctly reproduces the dynamics of one- and
two-point correlations and spin squeezing at short times, thus capturing
entanglement. Our results open the possibility to study the quantum dynamics
accessible to recent experiments in regimes where other numerical methods are
inapplicable.Comment: 8 pages, 6 figure
A generalized phase space approach for solving quantum spin dynamics
Numerical techniques to efficiently model out-of-equilibrium dynamics in
interacting quantum many-body systems are key for advancing our capability to
harness and understand complex quantum matter. Here we propose a new numerical
approach which we refer to as GDTWA. It is based on a discrete semi-classical
phase-space sampling and allows to investigate quantum dynamics in lattice spin
systems with arbitrary . We show that the GDTWA can accurately
simulate dynamics of large ensembles in arbitrary dimensions. We apply it for
spin-models with dipolar long-range interactions, a scenario arising in
recent experiments with magnetic atoms. We show that the method can capture
beyond mean-field effects, not only at short times, but it also correctly
reproduces long time quantum-thermalization dynamics. We benchmark the method
with exact diagonalization in small systems, with perturbation theory for short
times, and with analytical predictions made for closed system which feature
quantum-thermalization at long times. By computing the Renyi entropy, currently
an experimentally accessible quantifier of entanglement, we reveal that large
systems can feature larger entanglement than corresponding systems.
Our analyses demonstrate that the GDTWA can be a powerful tool for modeling
complex spin dynamics in regimes where other state-of-the art numerical methods
fail
Cavity enhanced transport of excitons
We show that exciton-type transport in certain materials can be dramatically
modified by their inclusion in an optical cavity: the modification of the
electromagnetic vacuum mode structure introduced by the cavity leads to
transport via delocalized polariton modes rather than through tunneling
processes in the material itself. This can help overcome exponential
suppression of transmission properties as a function of the system size in the
case of disorder and other imperfections. We exemplify massive improvement of
transmission for excitonic wave-packets through a cavity, as well as
enhancement of steady-state exciton currents under incoherent pumping. These
results may have implications for experiments of exciton transport in
disordered organic materials. We propose that the basic phenomena can be
observed in quantum simulators made of Rydberg atoms, cold molecules in optical
lattices, as well as in experiments with trapped ions.Comment: 10 pages, 7 figures, [v2]: Updated reference to complementary work
arXiv:1409.2514, [v3]: Update to version accepted for publicatio
Thermalization of strongly interacting bosons after spontaneous emissions in optical lattices
We study the out-of-equilibrium dynamics of bosonic atoms in a 1D optical
lattice, after the ground-state is excited by a single spontaneous emission
event, i.e. after an absorption and re-emission of a lattice photon. This is an
important fundamental source of decoherence for current experiments, and
understanding the resulting dynamics and changes in the many-body state is
important for controlling heating in quantum simulators. Previously it was
found that in the superfluid regime, simple observables relax to values that
can be described by a thermal distribution on experimental time-scales, and
that this breaks down for strong interactions (in the Mott insulator regime).
Here we expand on this result, investigating the relaxation of the momentum
distribution as a function of time, and discussing the relationship to
eigenstate thermalization. For the strongly interacting limit, we provide an
analytical analysis for the behavior of the system, based on an effective
low-energy Hamiltonian in which the dynamics can be understood based on
correlated doublon-holon pairs.Comment: 8 pages, 5 figure
Light scattering and dissipative dynamics of many fermionic atoms in an optical lattice
We investigate the many-body dissipative dynamics of fermionic atoms in an
optical lattice in the presence of incoherent light scattering. Deriving and
solving a master equation to describe this process microscopically for many
particles, we observe contrasting behaviour in terms of the robustness against
this type of heating for different many-body states. In particular, we find
that the magnetic correlations exhibited by a two-component gas in the Mott
insulating phase should be particularly robust against decoherence from light
scattering, because the decoherence in the lowest band is suppressed by a
larger factor than the timescales for effective superexchange interactions that
drive coherent dynamics. Furthermore, the derived formalism naturally
generalizes to analogous states with SU(N) symmetry. In contrast, for typical
atomic and laser parameters, two-particle correlation functions describing
bound dimers for strong attractive interactions exhibit superradiant effects
due to the indistinguishability of off-resonant photons scattered by atoms in
different internal states. This leads to rapid decay of correlations describing
off-diagonal long-range order for these states. Our predictions should be
directly measurable in ongoing experiments, providing a basis for
characterising and controlling heating processes in quantum simulation with
fermions.Comment: 18 pages, 7 figure
Local density of states on a vibrational quantum dot out of equilibrium
We calculate the nonequilibrium local density of states on a vibrational
quantum dot coupled to two electrodes at T=0 using a numerically exact
diagrammatic Monte Carlo method. Our focus is on the interplay between the
electron-phonon interaction strength and the bias voltage. We find that the
spectral density exhibits a significant voltage dependence if the voltage
window includes one or more phonon sidebands. A comparison with
well-established approximate approaches indicates that this effect could be
attributed to the nonequilibrium distribution of the phonons. Moreover, we
discuss the long transient dynamics caused by the electron-phonon coupling.Comment: 9 pages, 11 figure
Dressed, noise- or disorder- resilient optical lattices
External noise is inherent in any quantum system, and can have especially
strong effects for systems exhibiting sensitive many-body phenomena. We show
how a dressed lattice scheme can provide control over certain types of noise
for atomic quantum gases in the lowest band of an optical lattice, removing the
effects of lattice amplitude noise to first order for particular choices of the
dressing field parameters. We investigate the non-equilibrium many-body
dynamics for bosons and fermions induced by noise away from this parameter
regime, and also show how the same technique can be used to reduce spatial
disorder in projected lattice potentials.Comment: 4+ Pages, 4 Figure
Cavity-assisted mesoscopic transport of fermions: Coherent and dissipative dynamics
We study the interplay between charge transport and light-matter interactions
in a confined geometry, by considering an open, mesoscopic chain of two-orbital
systems resonantly coupled to a single bosonic mode close to its vacuum state.
We introduce and benchmark different methods based on self-consistent solutions
of non-equilibrium Green's functions and numerical simulations of the quantum
master equation, and derive both analytical and numerical results. It is shown
that in the dissipative regime where the cavity photon decay rate is the
largest parameter, the light-matter coupling is responsible for a steady-state
current enhancement scaling with the cooperativity parameter. We further
identify different regimes of interest depending on the ratio between the
cavity decay rate and the electronic bandwidth. Considering the situation where
the lower band has a vanishing bandwidth, we show that for a high-finesse
cavity, the properties of the resonant Bloch state in the upper band are
transfered to the lower one, giving rise to a delocalized state along the
chain. Conversely, in the dissipative regime with low cavity quality factors,
we find that the current enhancement is due to a collective decay of
populations from the upper to the lower band.Comment: 52 pages, 11 figure