1,414 research outputs found
Generalized mean-field approach to simulate large dissipative spin ensembles with long range interactions
We simulate the collective dynamics in spin lattices with long range
interactions and collective decay in one, two and three dimensions. Starting
from a dynamical mean-field approach derived by local factorization of the
density operator we improve the numerical approximation of the full master
equation by including pair correlations at any distance. This truncations
enable us to drastically increase the number of spins in our numerical
simulations from about ten spins in case of the full quantum model to several
ten-thousands in the mean-field approximation and a few hundreds if pair
correlations are included. Extensive numerical tests help us identify
interaction strengths and geometric configurations where these approximations
perform well and allow us to state fairly simple error estimates. By simulating
systems of increasing size we show that in one and two dimensions we can
include as many spins as needed to capture the properties of infinite size
systems with high accuracy, while in 3D the method does not converge to desired
accuracy within the system sizes we can currently implement. Our approach is
well suited to give error estimates of magic wavelength optical lattices for
atomic clock applications and corresponding super radiant lasers
Emergent Quasicrystalline Symmetry in Light-Induced Quantum Phase Transitions
The discovery of quasicrystals with crystallographically forbidden rotational symmetries has changed the notion of the ordering in materials, yet little is known about the dynamical emergence of such exotic forms of order. Here we theoretically study a nonequilibrium cavity-QED setup realizing a zero-temperature quantum phase transition from a homogeneous Bose-Einstein condensate to a quasicrystalline phase via collective superradiant light scattering. Across the superradiant phase transition, collective light scattering creates a dynamical, quasicrystalline optical potential for the atoms. Remarkably, the quasicrystalline potential is " emergent" as its eightfold rotational symmetry is not present in the Hamiltonian of the system, rather appears solely in the low-energy states. For sufficiently strong two-body contact interactions between atoms, a quasicrystalline order is stabilized in the system, while for weakly interacting atoms the condensate is localized in one or few of the deepest minima of the quasicrystalline potential
Temperature gradient driven lasing and stimulated cooling
A laser can be understood as thermodynamic engine converting heat to a
coherent single mode field close to Carnot efficiency. From this perspective
spectral shaping of the excitation light generates a higher effective
temperature on the pump than on the gain transition. Here, using a toy model of
a quantum well structure with two suitably designed tunnel-coupled wells kept
at different temperature, we study a laser operated on an actual spatial
temperature gradient between pump and gain region. We predict gain and narrow
band laser emission for a sufficient temperature gradient and resonator
quality. Lasing appears concurrent with amplified heat flow and points to a new
form of stimulated solid state cooling. Such a mechanism could raise the
operating temperature limit of quantum cascade lasers by substituting phonon
emission driven injection, which generates intrinsic heat, by an extended model
with phonon absorption steps
Atomic selfordering in a ring cavity with counterpropagating pump
The collective dynamics of mobile scatterers and light in optical resonators
generates complex behaviour. For strong transverse illumination a phase
transition from homogeneous to crystalline particle order appears. In contrast,
a gas inside a single-side pumped ring cavity exhibits an instability towards
bunching and collective acceleration called collective atomic recoil lasing
(CARL). We demonstrate that by driving two orthogonally polarized counter
propagating modes of a ring resonator one realises both cases within one
system. The corresponding phase diagram depending on the two pump intensities
exhibits regions in which either a generalized form of self-ordering towards a
travelling density wave with constant centre of mass velocity or a CARL
instability is formed. Controlling the cavity driving then allows to accelerate
or slow down and trap a sufficiently dense beam of linearly polarizable
particles.Comment: 5 page
A realization of a quasi-random walk for atoms in time-dependent optical potentials
We consider the time dependent dynamics of an atom in a two-color pumped
cavity, longitudinally through a side mirror and transversally via direct
driving of the atomic dipole. The beating of the two driving frequencies leads
to a time dependent effective optical potential that forces the atom into a
non-trivial motion, strongly resembling a discrete random walk behavior between
lattice sites. We provide both numerical and analytical analysis of such a
quasi-random walk behavior
Bright and dark excitons in an atom--pair filled optical lattice within a cavity
We study electronic excitations of a degenerate gas of atoms trapped in pairs
in an optical lattice. Local dipole-dipole interactions produce a long lived
antisymmetric and a short lived symmetric superposition of individual atomic
excitations as the lowest internal on-site excitations. Due to the much larger
dipole moment the symmetric states couple efficiently to neighbouring lattice
sites and can be well represented by Frenkel excitons, while the antisymmetric
dark states stay localized. Within a cavity only symmetric states couple to
cavity photons inducing long range interactions to form polaritons. We
calculate their dispersion curves as well as cavity transmission and reflection
spectra to observe them. For a lattice with aspherical sites bright and dark
states get mixed and their relative excitation energies depend on photon
polarizations. The system should allow to study new types of solid state
phenomena in atom filled optical lattices
Optomechanics with molecules in a strongly pumped ring cavity
Cavity cooling of an atom works best on a cyclic optical transition in the
strong coupling regime near resonance, where small cavity photon numbers
suffice for trapping and cooling. Due to the absence of closed transitions a
straightforward application to molecules fails: optical pumping can lead the
particle into uncoupled states. An alternative operation in the far
off-resonant regime generates only very slow cooling due to the reduced
field-molecule coupling. We predict to overcome this by using a strongly driven
ring-cavity operated in the sideband cooling regime. As in the optomechanical
setups one takes advantage of a collectively enhanced field-molecule coupling
strength using a large photon number. A linearized analytical treatment
confirmed by full numerical quantum simulations predicts fast cooling despite
the off-resonant small single molecule - single photon coupling. Even ground
state cooling can be obtained by tuning the cavity field close to the
Anti-stokes sideband for sufficiently high trapping frequency. Numerical
simulations show quantum jumps of the molecules between the lowest two trapping
levels, which can be be directly and continuously monitored via scattered light
intensity detection
Observation of decoherence with a movable mirror
Recently it has been proposed to use parity as a measure of the mechanism
behind decoherence or the transformation from quantum to classical. Here, we
show that the proposed experiment is more feasible than previously thought, as
even an initial thermal state would exhibit the hypothesized symmetry breaking.Comment: Proceedings of the Lake Garda "quantum puzzles" conferenc
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