31 research outputs found
Vortex formation and dynamics in two-dimensional driven-dissipative condensates
We investigate the real-time evolution of lattice bosons in two spatial
dimensions whose dynamics is governed by a Markovian quantum master equation.
We employ the Wigner-Weyl phase space quantization and derive the functional
integral for open quantum many-body systems that governs the time evolution of
the Wigner function. Using the truncated Wigner approximation, in which quantum
fluctuations are only taken into account in the initial state whereas the
dynamics is governed by classical evolution equations, we study the buildup of
long-range correlations due to the action of non-Hermitean quantum jump
operators that constitute a mechanism for dissipative cooling. Starting from an
initially disordered state corresponding to a vortex condensate, the
dissipative process results in the annihilation of vortex-antivortex pairs and
the establishment of quasi long-range order at late times. We observe that a
finite vortex density survives the cooling process which disagrees with the
analytically constructed vortex-free Bose-Einstein condensate at asymptotic
times. This indicates that quantum fluctuations beyond the truncated Wigner
approximation need to be included to fully capture the physics of dissipative
Bose-Einstein condensation.Comment: 11 pages, 3 figures. Revised version: Derivation and discussion
extended, accepted for publication in PR
Schwinger effect in inhomogeneous electric fields
The vacuum of quantum electrodynamics is unstable against the formation of
many-body states in the presence of an external electric field, manifesting
itself as the creation of electron-positron pairs (Schwinger effect). This
effect has been a long-standing but still unobserved prediction as the
generation of the required field strengths has not been feasible so far.
However, due to the advent of a new generation of high-intensity laser systems
such as the European XFEL or the Extreme Light Infrastructure (ELI), this
effect might eventually become observable within the next decades. Based on the
equal-time Wigner formalism, various aspects of the Schwinger effect in
electric fields showing both temporal and spatial variations are investigated.
Regarding the Schwinger effect in time-dependent electric fields, analytic
expressions for the equal-time Wigner function in the presence of a static as
well as a pulsed electric field are derived. Moreover, the pair creation
process in the presence of a pulsed electric field with sub-cycle structure,
which acts as a model for a realistic laser pulse, is examined. Finally, an ab
initio simulation of the Schwinger effect in a simple space- and time-dependent
electric field is performed for the first time, allowing for the calculation of
the time evolution of various observables like the charge density, the particle
number density or the number of created particles.Comment: PhD thesis, 121 page
Anomaly-induced dynamical refringence in strong-field QED
We investigate the impact of the Adler-Bell-Jackiw anomaly on the
nonequilibrium evolution of strong-field quantum electrodynamics (QED) using
real-time lattice gauge theory techniques. For field strengths exceeding the
Schwinger limit for pair production, we encounter a highly absorptive medium
with anomaly-induced dynamical refractive properties. In contrast to earlier
expectations based on equilibrium properties, where net anomalous effects
vanish because of the trivial vacuum structure, we find that out-of-equilibrium
conditions can have dramatic consequences for the presence of quantum currents
with distinctive macroscopic signatures. We observe an intriguing tracking
behavior, where the system spends longest times near collinear field
configurations with maximum anomalous current. Apart from the potential
relevance of our findings for future laser experiments, similar phenomena
related to the chiral magnetic effect are expected to play an important role
for strong QED fields during initial stages of heavy-ion collision experiments.Comment: 5 pages, 4 figures, references adde
Pulse shape optimization for electron-positron production in rotating fields
We optimize the pulse shape and polarization of time-dependent electric
fields to maximize the production of electron-positron pairs via strong field
quantum electrodynamics processes. The pulse is parametrized in Fourier space
by a B-spline polynomial basis, which results in a relatively low-dimensional
parameter space while still allowing for a large number of electric field
modes. The optimization is performed by using a parallel implementation of the
differential evolution, one of the most efficient metaheuristic algorithms. The
computational performance of the numerical method and the results on pair
production are compared with a local multistart optimization algorithm. These
techniques allow us to determine the pulse shape and field polarization that
maximize the number of produced pairs in computationally accessible regimes.Comment: 16 pages, 10 figure
Dissipative Bose-Einstein condensation in contact with a thermal reservoir
We investigate the real-time dynamics of open quantum spin- or hardcore
boson systems on a spatial lattice, which are governed by a Markovian quantum
master equation. We derive general conditions under which the hierarchy of
correlation functions closes such that their time evolution can be computed
semi-analytically. Expanding our previous work [Phys. Rev. A 93, 021602 (2016)]
we demonstrate the universality of a purely dissipative quantum Markov process
that drives the system of spin- particles into a totally symmetric
superposition state, corresponding to a Bose-Einstein condensate of hardcore
bosons. In particular, we show that the finite-size scaling behavior of the
dissipative gap is independent of the chosen boundary conditions and the
underlying lattice structure. In addition, we consider the effect of a uniform
magnetic field as well as a coupling to a thermal bath to investigate the
susceptibility of the engineered dissipative process to unitary and nonunitary
perturbations. We establish the nonequilibrium steady-state phase diagram as a
function of temperature and dissipative coupling strength. For a small number
of particles , we identify a parameter region in which the engineered
symmetrizing dissipative process performs robustly, while in the thermodynamic
limit , the coupling to the thermal bath destroys any
long-range order.Comment: 30 pages, 8 figures; Revised version: Minor changes and references
adde
Real-time simulation of non-equilibrium transport of magnetization in large open quantum spin systems driven by dissipation
Using quantum Monte Carlo, we study the non-equilibrium transport of
magnetization in large open strongly correlated quantum spin
systems driven by purely dissipative processes that conserve the uniform or
staggered magnetization. We prepare both a low-temperature Heisenberg
ferromagnet and an antiferromagnet in two parts of the system that are
initially isolated from each other. We then bring the two subsystems in contact
and study their real-time dissipative dynamics for different geometries. The
flow of the uniform or staggered magnetization from one part of the system to
the other is described by a diffusion equation that can be derived
analytically.Comment: 6 pages, 5 figures. Revised version: Discussion extended and
references adde
Real-time simulation of the Schwinger effect with Matrix Product States
Matrix Product States (MPS) are used for the simulation of the real-time
dynamics induced by an electric quench on the vacuum state of the massive
Schwinger model. For small quenches it is found that the obtained oscillatory
behavior of local observables can be explained from the single-particle
excitations of the quenched Hamiltonian. For large quenches damped oscillations
are found and comparison of the late time behavior with the appropriate Gibbs
states seems to give some evidence for the onset of thermalization. Finally,
the MPS real-time simulations are explicitly compared with the semi-classical
approach and, as expected, agreement is found in the limit of large quenches.Comment: Small changes, matching its published versio