1,079 research outputs found
Critical relaxation with overdamped quasiparticles in open quantum systems
We study the late-time relaxation following a quench in a driven-dissipative
quantum many-body system. We consider the open Dicke model, describing the
infinite-range interactions between atoms and a single, lossy
electromagnetic mode. We show that the dynamical phase transition at a critical
atom-light coupling is characterized by the interplay between reservoir-driven
and intrinsic relaxation processes. Above the critical coupling, small
fluctuations in the occupation of the dominant quasiparticle-mode start to grow
in time while the quasiparticle lifetime remains finite due to losses. Near the
critical interaction strength we observe a crossover between exponential and
power-law relaxation, the latter driven by collisions between
quasiparticles. For a quench exactly to the critical coupling, the power-law
relaxation extends to infinite times, but the finite lifetime of quasiparticles
prevents ageing to appear. We predict our results to be accessible to quench
experiments with ultracold bosons in optical resonators.Comment: 3+4 Figure
Self-organised Limit-Cycles, Chaos and Phase-Slippage with a Superfluid inside an Optical Resonator
We study dynamical phases of a driven Bose-Einstein condensate coupled to the
light field of a high-Q optical cavity. For high field seeking atoms at red
detuning the system is known to show a transition from a spatially homogeneous
steady-state to a self-organized regular lattice exhibiting super-radiant
scattering into the cavity. For blue atom pump detuning the particles are
repelled from the maxima of the light-induced optical potential suppressing
scattering. We show that this generates a new dynamical instability of the
self-ordered phase, leading to the appearance of self-ordered stable
limit-cycles characterized by large amplitude self-sustained oscillations of
both the condensate density and cavity field. The limit-cycles evolve into
chaotic behavior by period doubling. Large amplitude oscillations of the
condensate are accompanied by phase-slippage through soliton nucleation at a
rate which increases by orders of magnitude in the chaotic regime. Different
from a superfluid in a closed setup, this driven dissipative superfluid is not
destroyed by the proliferation of solitons since kinetic energy is removed
through cavity losses.Comment: 4 figure
Umklapp Superradiance from a Collisionless Quantum Degenerate Fermi Gas
The quantum dynamics of the electromagnetic light mode of an optical cavity
filled with a coherently driven Fermi gas of ultracold atoms strongly depends
on geometry of the Fermi surface. Superradiant light generation and
self-organization of the atoms can be achieved at low pumping threshold due to
resonant atom-photon Umklapp processes, where the fermions are scattered from
one side of the Fermi surface to the other by exchanging photon momenta. The
cavity spectrum exhibits sidebands, that, despite strong atom-light coupling
and cavity decay, retain narrow linewidth, due to absorptionless transparency
windows outside the atomic particle-hole continuum and the suppression of
inhomogeneous broadening and thermal fluctuations in the collisionless Fermi
gas.Comment: Revised version, as accepted to Physical Review Letter
Topological soliton-polaritons in 1D systems of light and fermionic matter
Quantum nonlinear optics is a quickly growing field with large technological
promise, at the same time involving complex and novel many-body phenomena. In
the usual scenario, optical nonlinearities originate from the interactions
between polaritons, which are hybrid quasi-particles mixing matter and light
degrees of freedom. Here we introduce a type of polariton which is
intrinsically nonlinear and emerges as the natural quasi-particle in presence
quantum degenerate fermionic matter. It is a composite object made of a fermion
trapped inside an optical soliton forming a topological defect in a
spontaneously formed crystalline structure. Each of these soliton-polaritons
carries a topological quantum number, as they create a domain
wall between two crystalline regions with opposite dimerization so that the
fermion is trapped in an interphase state. These composite objects are formally
equivalent to those appearing in the Su-Schrieffer-Heeger (SSH) model for
electrons coupled to lattice phonons.Comment: Edited version. 6+7 pages, 3 figure
Spontaneous crystallization of light and ultracold atoms
Coherent scattering of light from ultracold atoms involves an exchange of
energy and momentum introducing a wealth of non-linear dynamical phenomena. As
a prominent example particles can spontaneously form stationary periodic
configurations which simultaneously maximize the light scattering and minimize
the atomic potential energy in the emerging optical lattice. Such self-ordering
effects resulting in periodic lattices via bimodal symmetry breaking have been
experimentally observed with cold gases and Bose-Einstein condensates (BECs)
inside an optical resonator. Here we study a new regime of periodic pattern
formation for an atomic BEC in free space, driven by far off-resonant
counterpropagating and non-interfering lasers of orthogonal polarization. In
contrast to previous works, no spatial light modes are preselected by any
boundary conditions and the transition from homogeneous to periodic order
amounts to a crystallization of both light and ultracold atoms breaking a
continuous translational symmetry. In the crystallized state the BEC acquires a
phase similar to a supersolid with an emergent intrinsic length scale whereas
the light-field forms an optical lattice allowing phononic excitations via
collective back scattering, which are gapped due to the infinte-range
interactions. The studied system constitutes a novel configuration allowing the
simulation of synthetic solid state systems with ultracold atoms including
long-range phonon dynamics
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