13 research outputs found
The Dicke model phase transition in the quantum motion of a Bose-Einstein condensate in an optical cavity
We show that the motion of a laser-driven Bose-Einstein condensate in a
high-finesse optical cavity realizes the spin-boson Dicke-model. The quantum
phase transition of the Dicke-model from the normal to the superradiant phase
corresponds to the self-organization of atoms from the homogeneous into a
periodically patterned distribution above a critical driving strength. The
fragility of the ground state due to photon measurement induced back action is
calculated.Comment: 5 pages, 2 figure
Photonic tuning of quasi-particle decay in a superfluid
We show that the damping rate of elementary excitations of hybrid systems
close to a phase transition can undergo a remarkable resonance like enhancement
before mode softening takes place. In particular, we consider the friction of a
collective density wave in a homogeneous superfluid of weakly interacting
bosonic atoms coupled to the electromagnetic field of a single mode optical
resonator. Here the Beliaev damping can thus be controlled by an external laser
drive and be enhanced by several orders of magnitude
Cavity optomechanics with a trapped, interacting Bose-Einstein condensate
The dispersive interaction of a Bose-Einstein condensate with a single mode
of a high-finesse optical cavity realizes the radiation pressure coupling
Hamiltonian. In this system the role of the mechanical oscillator is played by
a single condensate excitation mode that is selected by the cavity mode
function. We study the effect of atomic s-wave collisions and show that it
merely renormalizes parameters of the usual optomechanical interaction.
Moreover, we show that even in the case of strong harmonic confinement---which
invalidates the use of Bloch states---a single excitation mode of the
Bose-Einstein condensate couples significantly to the light field, that is the
simplified picture of a single "mechanical" oscillator mode remains valid
Quantum noise in a transversely pumped cavity Bose--Hubbard model
We investigate the quantum measurement noise effects on the dynamics of an
atomic Bose lattice gas inside an optical resonator. We describe the dynamics
by means of a hybrid model consisting of a Bose--Hubbard Hamiltonian for the
atoms and a Heisenberg--Langevin equation for the lossy cavity field mode. We
assume that the atoms are prepared initially in the ground state of the lattice
Hamiltonian and then start to interact with the cavity mode. We show that the
cavity field fluctuations originating from the dissipative outcoupling of
photons from the resonator lead to vastly different effects in the different
possible ground state phases, i.e., the superfluid, the supersolid, the Mott-
and the charge-density-wave phases. In the former two phases with the presence
of a superfluid wavefunction, the quantum measurement noise appears as a
driving term leading to excess noise depletion of the ground state. The time
scale for the system to leave the ground scale is determined analytically. For
the latter two incompressible phases, the quantum noise results in the
fluctuation of the chemical potential. We derive an analytical expression for
the corresponding broadening of the quasiparticle resonances
On the critical exponent of a quantum noise driven phase transition: the open system Dicke-model
The quantum phase transition of the Dicke-model has been observed recently in
a system formed by motional excitations of a laser-driven Bose--Einstein
condensate coupled to an optical cavity [1]. The cavity-based system is
intrinsically open: photons can leak out of the cavity where they are detected.
Even at zero temperature, the continuous weak measurement of the photon number
leads to an irreversible dynamics towards a steady-state which exhibits a
dynamical quantum phase transition. However, whereas the critical point and the
mean field is only slightly modified with respect to the phase transition in
the ground state, the entanglement and the critical exponents of the singular
quantum correlations are significantly different in the two cases.Comment: 7 page
Self-organization of a Bose-Einstein condensate in an optical cavity
The spatial self-organization of a Bose-Einstein condensate (BEC) in a
high-finesse linear optical cavity is discussed. The condensate atoms are
laser-driven from the side and scatter photons into the cavity. Above a
critical pump intensity the homogeneous condensate evolves into a stable
pattern bound by the cavity field. The transition point is determined
analytically from a mean-field theory. We calculate the lowest lying Bogoliubov
excitations of the coupled BEC-cavity system and the quantum depletion due to
the atom-field coupling.Comment: 11 pages, final version. Accepted for publication in EPJ
Excitations of optically driven atomic condensate in a cavity: theory of photodetection measurements
Recent experiments have demonstrated an open system realization of the Dicke
quantum phase transition in the motional degrees of freedom of an optically
driven Bose-Einstein condensate in a cavity. Relevant collective excitations of
this light-matter system are polaritonic in nature, allowing access to the
quantum critical behavior of the Dicke model through light leaking out of the
cavity. This opens the path to using photodetection based quantum optical
techniques to study the dynamics and excitations of this elementary quantum
critical system. We first discuss the photon flux observed at the cavity face
and find that it displays a different scaling law near criticality than that
obtained from the mean field theory for the equivalent closed system. Next, we
study the second order correlation measurements of photons leaking out of the
cavity. Finally, we discuss a modulation technique that directly captures the
softening of polaritonic excitations. Our analysis takes into account the
effect of the finite size of the system which may result in an effective
symmetry breaking term.Comment: 18 pages, 5 figure
Multimode mean-field model for the quantum phase transition of a Bose-Einstein condensate in an optical resonator
We develop a mean-field model describing the Hamiltonian interaction of
ultracold atoms and the optical field in a cavity. The Bose-Einstein condensate
is properly defined by means of a grand-canonical approach. The model is
efficient because only the relevant excitation modes are taken into account.
However, the model goes beyond the two-mode subspace necessary to describe the
self-organization quantum phase transition observed recently. We calculate all
the second-order correlations of the coupled atom field and radiation field
hybrid bosonic system, including the entanglement between the two types of
fields.Comment: 10 page