14 research outputs found
Nonequilibrium dynamics of spin-boson models from phase space methods
An accurate description of the nonequilibrium dynamics of systems with
coupled spin and bosonic degrees of freedom remains theoretically challenging,
especially for large system sizes and in higher than one dimension. Phase space
methods such as the Truncated Wigner Approximation (TWA) have the advantage of
being easily scalable and applicable to arbitrary dimensions. In this work we
adapt the TWA to generic spin-boson models by making use of recently developed
algorithms for discrete phase spaces [Schachenmayer, PRX 5, 011022 (2015)].
Furthermore we go beyond the standard TWA approximation by applying a scheme
based on the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy of equations
[Pucci, PRB 93, 174302 (2016)] to our coupled spin-boson model. This allows in
principle to study how systematically adding higher order corrections improves
the convergence of the method. To test various levels of approximation we study
an exactly solvable spin-boson model which is particularly relevant for
trapped-ion arrays. Using TWA and its BBGKY extension we accurately reproduce
the time evolution of a number of one- and two-point correlation functions in
several dimensions and for arbitrary number of bosonic modes.Comment: 10+5 pages, 5 figure
Non-equilibrium Quantum Spin Dynamics from 2PI Functional Integral Techniques in the Schwinger Boson Representation
We present a non-equilibrium quantum field theory approach to the
initial-state dynamics of spin models based on two-particle irreducible (2PI)
functional integral techniques. It employs a mapping of spins to Schwinger
bosons for arbitrary spin interactions and spin lengths. At next-to-leading
order (NLO) in an expansion in the number of field components, a wide range of
non-perturbative dynamical phenomena are shown to be captured, including
relaxation of magnetizations in a 3D long-range interacting system with
quenched disorder, different relaxation behaviour on both sides of a quantum
phase transition and the crossover from relaxation to arrest of dynamics in a
disordered spin chain previously shown to exhibit many-body-localization. Where
applicable, we employ alternative state-of-the-art techniques and find rather
good agreement with our 2PI NLO results. As our method can handle large system
sizes and converges relatively quickly to its thermodynamic limit, it opens the
possibility to study those phenomena in higher dimensions in regimes in which
no other efficient methods exist. Furthermore, the approach to classical
dynamics can be investigated as the spin length is increased
Emergent dark states from superradiant dynamics in multilevel atoms in a cavity
We investigate the collective decay dynamics of atoms with a generic
multilevel structure (angular momenta ) coupled to two
light modes of different polarization inside a cavity. In contrast to two-level
atoms, we find that multilevel atoms can harbour eigenstates that are perfectly
dark to cavity decay even within the subspace of permutationally symmetric
states (collective Dicke manifold). The dark states arise from destructive
interference between different internal transitions and are shown to be
entangled. Remarkably, the superradiant decay of multilevel atoms can end up
stuck in one of these dark states, where a macroscopic fraction of the atoms
remains excited. This opens the door to the preparation of entangled dark
states of matter through collective dissipation useful for quantum sensing and
quantum simulation. Our predictions should be readily observable in current
optical cavity experiments with alkaline-earth atoms or Raman-dressed
transitions.Comment: 22+9 pages, 14+4 figures (like published version, except for minor
editorial corrections