361 research outputs found
Unconditional two-mode squeezing of separated atomic ensembles
We propose schemes for the unconditional preparation of a two-mode squeezed
state of effective bosonic modes realized in a pair of atomic ensembles
interacting collectively with optical cavity and laser fields. The scheme uses
Raman transitions between stable atomic ground states and under ideal
conditions produces pure entangled states in the steady state. The scheme works
both for ensembles confined within a single cavity and for ensembles confined
in separate, cascaded cavities.Comment: 4 pages, 2 figure
Coupling of effective one-dimensional two-level atoms to squeezed light
A cavity QED system is analyzed which duplicates the dynamics of a two-level
atom in free space interacting exclusively with broadband squeezed light. We
consider atoms in a three or four-level Lambda-configuration coupled to a
high-finesse optical cavity which is driven by a squeezed light field. Raman
transitions are induced between a pair of stable atomic ground states via the
squeezed cavity mode and coherent driving fields. An analysis of the reduced
master equation for the atomic ground states shows that a three-level atomic
system has insufficient parameter flexibility to act as an effective two-level
atom interacting exclusively with a squeezed reservoir. However, the inclusion
of a fourth atomic level, coupled dispersively to one of the two ground states
by an auxiliary laser field, introduces an extra degree of freedom and enables
the desired interaction to be realised. As a means of detecting the reduced
quadrature decay rate of the effective two-level system, we examine the
transmission spectrum of a weak coherent probe field incident upon the cavity
Mimicking a Squeezed Bath Interaction: Quantum Reservoir Engineering with Atoms
The interaction of an atomic two-level system and a squeezed vacuum leads to
interesting novel effects in atomic dynamics, including line narrowing in
resonance fluorescence and absorption spectra, and a suppressed (enhanced)
decay of the in-phase and out-of phase component of the atomic polarization. On
the experimental side these predictions have so far eluded observation,
essentially due to the difficulty of embedding atoms in a 4 pi squeezed vacuum.
In this paper we show how to ``engineer'' a squeezed-bath-type interaction for
an effective two-level system. In the simplest example, our two-level atom is
represented by the two ground levels of an atom with angular momentum J=1/2 ->
J=1/2 transition (a four level system) which is driven by (weak) laser fields
and coupled to the vacuum reservoir of radiation modes. Interference between
the spontaneous emission channels in optical pumping leads to a squeezed bath
type coupling, and thus to symmetry breaking of decay on the Bloch sphere. With
this system it should be possible to observe the effects predicted in the
context of squeezed bath - atom interactions. The laser parameters allow one to
choose properties of the squeezed bath interaction, such as the (effective)
photon number expectation number N and the squeezing phase phi. We present
results of a detailed analytical and numerical study.Comment: 24 pages, 8 figure
Implementation of quantum gates and preparation of entangled states in cavity QED with cold trapped ions
We propose a scheme to perform basic gates of quantum computing and prepare
entangled states in a system with cold trapped ions located in a single mode
optical cavity. General quantum computing can be made with both motional state
of the trapped ion and cavity state being qubits. We can also generate
different kinds of entangled states in such a system without state reduction,
and can transfer quantum states from the ion in one trap to the ion in another
trap. Experimental requirement for achieving our scheme is discussed.Comment: To appear in J. Opt.
Multipartite Entanglement and Quantum State Exchange
We investigate multipartite entanglement in relation to the theoretical
process of quantum state exchange. In particular, we consider such entanglement
for a certain pure state involving two groups of N trapped atoms. The state,
which can be produced via quantum state exchange, is analogous to the
steady-state intracavity state of the subthreshold optical nondegenerate
parametric amplifier. We show that, first, it possesses some 2N-way
entanglement. Second, we place a lower bound on the amount of such entanglement
in the state using a novel measure called the entanglement of minimum bipartite
entropy.Comment: 12 pages, 4 figure
Thermal Properties of Interacting Bose Fields and Imaginary-Time Stochastic Differential Equations
Matsubara Green's functions for interacting bosons are expressed as classical
statistical averages corresponding to a linear imaginary-time stochastic
differential equation. This makes direct numerical simulations applicable to
the study of equilibrium quantum properties of bosons in the non-perturbative
regime. To verify our results we discuss an oscillator with quartic
anharmonicity as a prototype model for an interacting Bose gas. An analytic
expression for the characteristic function in a thermal state is derived and a
Higgs-type phase transition discussed, which occurs when the oscillator
frequency becomes negative.Comment: Published versio
Laser cooling of trapped ions: the influence of micromotion
Laser cooling of a single trapped ion in a Paul trap is discussed theoretically in the Lamb-Dicke limit, with full consideration of the time dependence of the trapping potential. Resulting mean kinetic energies are defined as time averages over one period of the micromotion and are compared with final temperatures expected from the laser cooling treatment with harmonic traps. For laser-atom detunings close to the micromotion frequency the results differ significantly from those expected for a harmonic trap potential. A physical interpretation is given and simple formulas are derived for the strong confinement case
Analysis of dynamical tunnelling experiments with a Bose-Einstein condensate
Dynamical tunnelling is a quantum phenomenon where a classically forbidden
process occurs, that is prohibited not by energy but by another constant of
motion. The phenomenon of dynamical tunnelling has been recently observed in a
sodium Bose-Einstein condensate. We present a detailed analysis of these
experiments using numerical solutions of the three dimensional Gross-Pitaevskii
equation and the corresponding Floquet theory. We explore the parameter
dependency of the tunnelling oscillations and we move the quantum system
towards the classical limit in the experimentally accessible regime.Comment: accepted for publication in Physical Review
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