32 research outputs found
Bistability effect in the extreme strong coupling regime of the Jaynes-Cummings model
We study the nonlinear response of a driven cavity QED system in the extreme
strong coupling regime where the saturation photon number is below one by many
orders of magnitude. In this regime, multi-photon resonances within the
Jaynes--Cummings spectrum up to high order can be resolved. We identify an
intensity and frequency range of the external coherent drive for which the
system exhibits bistability instead of resonant multi-photon transitions. The
cavity field evolves into a mixture of the vacuum and another quasi-classical
state well separated in phase space. The corresponding time evolution of the
outgoing intensity is a telegraph signal alternating between two attractors
Depolarization shift of the superradiant phase transition
We investigate the possibility of a Dicke-type superradiant phase transition
of an atomic gas with an extended model which takes into account the
short-range depolarizing interactions between atoms approaching each other as
close as the atomic size scale, which interaction appears in a regularized
electric-dipole picture of the QED of atoms. By using a mean field model, we
find that a critical density does indeed exist, though the atom-atom contact
interaction shifts it to a higher value than it can be obtained from the bare
Dicke-model. We argue that the system, at the critical density, transitions to
the condensed rather than the "superradiant" phase.Comment: 7 pages, 1 figur
Cavity nonlinear optics with few photons and ultracold quantum particles
The light force on particles trapped in the field of a high-Q cavity mode
depends on the quantum state of field and particle. Different photon numbers
generate different optical potentials anddifferent motional states induce
different field evolution. Even for weak saturation and linear polarizability
the induced particle motion leads to nonlinear field dynamics. We derive a
corresponding effective field Hamiltonian containing all the powers of the
photon number operator, which predicts nonlinear phase shifts and squeezing
even at the few-photon level. Wave-function simulations of the full
particle-field dynamics confirm this and show significant particle-field
entanglement in addition.Comment: 5 pages, 5 figure
Microscopic physics of quantum self-organisation of optical lattices in cavities
We study quantum particles at zero temperature in an optical lattice coupled
to a resonant cavity mode. The cavity field substantially modifies the particle
dynamics in the lattice, and for strong particle-field coupling leads to a
quantum phase with only every second site occupied. We study the growth of this
new order out of a homogeneous initial distribution for few particles as the
microscopic physics underlying a quantum phase transition. Simulations reveal
that the growth dynamics crucially depends on the initial quantum many-body
state of the particles and can be monitored via the cavity fluorescence.
Studying the relaxation time of the ordering reveals inhibited tunnelling,
which indicates that the effective mass of the particles is increased by the
interaction with the cavity field. However, the relaxation becomes very quick
for large coupling.Comment: 14 pages 6 figure
Adequacy of the Dicke model in cavity QED: a counter-"no-go" statement
The long-standing debate whether the phase transition in the Dicke model can
be realized with dipoles in electromagnetic fields is yet an unsettled one. The
well-known statement often referred to as the "no-go theorem", asserts that the
so-called A-square term, just in the vicinity of the critical point, becomes
relevant enough to prevent the system from undergoing a phase transition. At
variance with this common belief, in this paper we prove that the Dicke model
does give a consistent description of the interaction of light field with the
internal excitation of atoms, but in the dipole gauge of quantum
electrodynamics. The phase transition cannot be excluded by principle and a
spontaneous transverse-electric mean field may appear. We point out that the
single-mode approximation is crucial: the proper treatment has to be based on
cavity QED, wherefore we present a systematic derivation of the dipole gauge
inside a perfect Fabry-P\'erot cavity from first principles. Besides the impact
on the debate around the Dicke phase transition, such a cleanup of the
theoretical ground of cavity QED is important because currently there are many
emerging experimental approaches to reach strong or even ultrastrong coupling
between dipoles and photons, which demand a correct treatment of the Dicke
model parameters
Infinitesimal multi-mode Bargmann-state representation
We construct a representation of the Hilbert-space of a light mode (harmonic oscillator) in which an arbitrary state vector is expanded using Bargmann states \Bket{\alpha} with real parameters which are in an infinitesimal vicinity of zero. The complete Hilbert-space structure is represented in the one- and multimode case as well, making the representation able to deal with problems of continuous-variable quantum information processing