533 research outputs found
Interaction induced Landau-Zener transitions
By considering a quantum critical Lipkin-Meshkov-Glick model we analyze a new
type of Landau-Zener transitions where the population transfer is mediated by
interaction rather than from a direct diabatic coupling. For this scenario, at
a mean-field level the dynamics is greatly influenced by quantum interferences.
In particular, regardless of how slow the Landau-Zener sweep is, for certain
parameters almost no population transfer occurs, which is in stark contrast to
the regular Landau-Zener model. For moderate system sizes, this
counterintuitive mean-field behaviour is not duplicated in the quantum case.
This can be attributed quantum fluctuations and the fact that multi-level
Landau-Zener-St\"uckelberg interferences have a `dephasing' effect on the above
mentioned phenomenon. We also find a discrepancy between the quantum and
mean-field models in terms of how the transfer probabilities scale with the
sweep velocity.Comment: 6 pages, 3 figure
Cavity assisted generation of sustainable macroscopic entanglement of ultracold gases
Prospects for reaching persistent entanglement between two spatially
separated atomic Bose-Einstein condensates are outlined. The system set-up
comprises of two condensates loaded in an optical lattice, which, in return, is
confined within a high-Q optical resonator. The system is driven by an external
laser that illuminates the atoms such that photons can scatter into the cavity.
In the superradiant phase a cavity field is established and we show that the
emerging cavity mediated interactions between the two condensates is capable of
entangling them despite photon losses. This macroscopic atomic entanglement is
sustained throughout the time-evolution apart from occasions of sudden
deaths/births. Using an auxiliary photon mode and coupling it to a collective
quadrature of the two condensates we demonstrate that the auxiliary mode's
squeezing is proportional to the atomic entanglement and as such it can serve
as a probe field of the macroscopic entanglement.Comment: Invited submission to ATOMS in special edition on "Cavity QED with
Ultracold Atoms
Some remarks on 'superradiant' phase transitions in light-matter systems
In this paper we analyze properties of the phase transition that appears in a
set of quantum optical models; Dicke, Tavis-Cummings, quantum Rabi, and finally
the Jaynes-Cummings model. As the light-matter coupling is increased into the
deep strong coupling regime, the ground state turns from vacuum to become a
superradiant state characterized by both atomic and photonic excitations. It is
pointed out that all four transitions are of the mean-field type, that quantum
fluctuations are negligible, and hence these fluctuations cannot be responsible
for the corresponding vacuum instability. In this respect, these are not
quantum phase transitions. In the case of the Tavis-Cummings and
Jaynes-Cummings models, the continuous symmetry of these models implies that
quantum fluctuations are not only negligible, but strictly zero. However, all
models possess a non-analyticity in the ground state in agreement with a
continuous quantum phase transition. As such, it is a matter of taste whether
the transitions should be termed quantum or not. In addition, we also consider
the modifications of the transitions when photon losses are present. For the
Dicke and Rabi models these non-equilibrium steady states remain critical,
while the criticality for the open Tavis-Cummings and Jaynes-Cummings models is
completely lost, i.e. in realistic settings one cannot expect a true critical
behaviour for the two last models.Comment: 25 pages (single column), 6 figure
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