7 research outputs found
How to Maximize the Capacity of General Quantum Noisy Channels
A general quantum noisy channel is analyzed, wherein the transmitted qubits
may experience symmetry-breaking decoherence, along with memory effects. We
find the optimal basis not to be fully entangled, but a combination of
factorized and partially-entangled states in the presence of memory, asymmetry
and the state-bias of the noise. Capacity-maximization is shown to be
achievable by combining temporal shaping of the transmitted qubits and optimal
basis selection.Comment: 4 pages, 3 figure
Electromagnetic induced transparency and slow light in interacting quantum degenerate atomic gases
We systematically develop the full quantum theory for the electromagnetic
induced transparency (EIT) and slow light properties in ultracold Bose and
Fermi gases. It shows a very different property from the classical theory which
assumes frozen atomic motion. For example, the speed of light inside the atomic
gases can be changed dramatically near the Bose-Einstein condensation
temperature, while the presence of the Fermi sea can destroy the EIT effect
even at zero temperature. From experimental point of view, such quantum EIT
property is mostly manifested in the counter-propagating excitation schemes in
either the low-lying Rydberg transition with a narrow line width or in the D2
transitions with a very weak coupling field. We further investigate the
interaction effects on the EIT for a weakly interacting Bose-Einstein
condensate, showing an inhomogeneous broadening of the EIT profile and
nontrivial change of the light speed due to the quantum many-body effects
beyond mean field energy shifts.Comment: 7 figure
Collisionally inhomogeneous Bose-Einstein condensates in double-well potentials
In this work, we consider quasi-one-dimensional Bose-Einstein condensates
(BECs), with spatially varying collisional interactions, trapped in double well
potentials. In particular, we study a setup in which such a 'collisionally
inhomogeneous' BEC has the same (attractive-attractive or repulsive-repulsive)
or different (attractive-repulsive) type of interparticle interactions. Our
analysis is based on the continuation of the symmetric ground state and
anti-symmetric first excited state of the noninteracting (linear) limit into
their nonlinear counterparts. The collisional inhomogeneity produces a
saddle-node bifurcation scenario between two additional solution branches; as
the inhomogeneity becomes stronger, the turning point of the saddle-node tends
to infinity and eventually only the two original branches remain present, which
is completely different from the standard double-well phenomenology. Finally,
one of these branches changes its monotonicity as a function of the chemical
potential, a feature especially prominent, when the sign of the nonlinearity
changes between the two wells. Our theoretical predictions, are in excellent
agreement with the numerical results.Comment: 14 pages, 12 figures, Physica D, in pres
TheqQuantum acousto optic effect in Bose-Einstein condensate
We investigate the interaction between a single mode light field and an
elongated cigar shaped Bose-Einstein condensate (BEC), subject to a temporal
modulation of the trap frequency in the tight confinement direction. Under
appropriate conditions, the longitudinal sound like waves (Faraday waves) in
the direction of weak confinement acts as a dynamic diffraction grating for the
incident light field analogous to the acousto-optic effect in classical optics.
The change in the refractive index due to the periodic modulation of the BEC
density is responsible for the acousto-optic effect. The dynamics is
characterised by Bragg scattering of light fom the matter wave Faraday grating
and simultaneous Bragg scattering of the condensate atoms from the optical
grating formed due to the interference between the incident light and the
diffracted light fields. Varying the intensity of the incident laser beam we
observe the transition from the acousto-optic effect regime to the atomic Bragg
scattering regime, where Rabi oscillations between two momentum levels of the
atoms are observed. We show that the acousto-optic effect is reduced as the
atomic interaction is increased