52 research outputs found
Dissipative Dicke Model with Collective Atomic Decay: Bistability, Noise-Driven Activation and Non-Thermal First Order Superradiance Transition
The Dicke model describes the coherent interaction of a laser-driven ensemble
of two level atoms with a quantized light field. It is realized within cavity
QED experiments, which in addition to the coherent Dicke dynamics feature
dissipation due to e.g. atomic spontaneous emission and cavity photon loss.
Spontaneous emission supports the uncorrelated decay of individual atomic
excitations as well as the enhanced, collective decay of an excitation that is
shared by atoms and whose strength is determined by the cavity geometry. We
derive a many-body master equation for the dissipative Dicke model including
both spontaneous emission channels and analyze its dynamics on the basis of
Heisenberg-Langevin and stochastic Bloch equations. We find that the collective
loss channel leads to a region of bistability between the empty and the
superradiant state. Transitions between these states are driven by non-thermal,
markovian noise. The interplay between dissipative and coherent elements leads
to a genuine non-equilibrium dynamics in the bistable regime, which is
expressed via a non-conservative force and a multiplicative noise kernel
appearing in the stochastic Bloch equations. We present a semiclassical
approach, based on stochastic nonlinear optical Bloch equations, which for the
infinite-range Dicke Model become exact in the large--limit. The absence of
an effective free energy functional, however, necessitates to include
fluctuation corrections with for finite to locate
the non-thermal first-order phase transition between the superradiant and the
empty cavity.Comment: as published in Physical Review
Effects of Smooth Boundaries on Topological Edge Modes in Optical Lattices
Since the experimental realization of synthetic gauge fields for neutral
atoms, the simulation of topologically non-trivial phases of matter with
ultracold atoms has become a major focus of cold atom experiments. However,
several obvious differences exist between cold atom and solid state systems,
for instance the finite size of the atomic cloud and the smooth confining
potential. In this article we show that sharp boundaries are not required to
realize quantum Hall or quantum spin Hall physics in optical lattices and, on
the contrary, that edge states which belong to a smooth confinement exhibit
additional interesting properties, such as spatially resolved splitting and
merging of bulk bands and the emergence of robust auxiliary states in bulk gaps
to preserve the topological quantum numbers. In addition, we numerically
validate that these states are robust against disorder. Finally, we analyze
possible detection methods, with a focus on Bragg spectroscopy, to demonstrate
that the edge states can be detected and that Bragg spectroscopy can reveal how
topological edge states are connected to the different bulk bands.Comment: 12 pages, 10 figures, updated figures and minor text correction
Many-Body Quantum Optics with Decaying Atomic Spin States: (, ) Dicke model
We provide a theory for quantum-optical realizations of the open Dicke model
with internal, atomic spin states subject to spontaneous emission with rate
. This introduces a second decay channel for excitations to
irreversibly dissipate into the environment, in addition to the photon loss
with rate , which is composed of individual atomic decay processes and
a collective atomic decay mechanism. The strength of the latter is determined
by the cavity geometry. We compute the mean-field non-equilibrium steady states
for spin and photon observables in the long-time limit, .
Although does not conserve the total angular momentum of the spin
array, we argue that our solution is exact in the thermodynamic limit, for the
number of atoms . In light of recent and upcoming
experiments realizing superradiant phase transitions using internal atomic
states with pinned atoms in optical lattices, our work lays the foundation for
the pursuit of a new class of open quantum magnets coupled to quantum light.Comment: 17 pages, 6 figures; added appendix for the derivation of a
collective atomic decay mechanism in a Lindblad formalism; version as
published in Physical Review
Vanishing density of states in weakly disordered Weyl semimetals
The Brillouin zone of the clean Weyl semimetal contains points at which the
density of states (DoS) vanishes. Previous work suggested that below a certain
critical concentration of impurities this features is preserved including in
the presence of disorder. This result got criticized for its neglect of rare
disorder fluctuations which might bind quantum states and hence generate a
finite DoS. We here show that in spite of their existence these states are so
fragile that their contribution effectively vanishes when averaged over
continuous disorder distributions. This means that the integrity of the nodal
points remains protected for weak disorder.Comment: 4 page
Majorana Loop Models for Measurement-Only Quantum Circuits
Projective measurements in random quantum circuits lead to a rich breadth of
entanglement phases and extend the realm of non-unitary quantum dynamics. Here
we explore the connection between measurement-only quantum circuits in one
spatial dimension and the statistical mechanics of loop models in two
dimensions. While Gaussian Majorana circuits admit a microscopic mapping to
loop models, for non-Gaussian, i.e., generic Clifford, circuits a corresponding
mapping may emerge only on a coarse grained scale. We then focus on a
fundamental symmetry of loop models: the orientability of world lines. We
discuss how orientability enters in the measurement framework, acting as a
separatrix for the universal long-wavelength behavior in a circuit. When
orientability is broken, the circuit falls into the universality class of
closely packed loops with crossings (CPLC) and features a Goldstone phase with
a peculiar, universal -scaling of the entanglement entropy. In turn,
when orientability is preserved, the long-wavelength behavior of the circuit
mimics that of (coupled) two-dimensional Potts models. We demonstrate the
strength of the loop model approach by numerically simulating a variety of
measurement-only Clifford circuits. Upon varying the set of measured operators,
a rich circuit dynamics is observed, ranging from CPLC to the -state Potts
model (percolation), the -state Potts model (Ising) and coupled Potts models
(BKT) universality class. Loop models thus provide a handle to access a large
class of measurement-only circuits and yield a blueprint on how to realize
desired entanglement phases by measurement
Controlling excitation avalanches in driven Rydberg gases
Recent experiments with strongly interacting, driven Rydberg ensembles have introduced a promising setup for the study of self-organized criticality (SOC) in cold atom systems. Based on this setup, we theoretically propose a control mechanism for the paradigmatic avalanche dynamics of SOC in the form of a time-dependent drive amplitude. This gives access to a variety of avalanche dominated, self-organization scenarios, prominently including self-organized criticality, as well as sub- and supercritical dynamics. We analyze the dependence of the dynamics on external scales and spatial dimensionality. It demonstrates the potential of driven Rydberg systems as a playground for the exploration of an extended SOC phenomenology and their relation to other common scenarios of SOC, such as, e.g., in neural networks and on graphs
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