31 research outputs found
Superluminal Optical Phase Conjugation: Pulse Reshaping and Instability
We theoretically investigate the response of optical phase conjugators to
incident probe pulses. In the stable (sub-threshold) operating regime of an
optical phase conjugator it is possible to transmit probe pulses with a
superluminally advanced peak, whereas conjugate reflection is always
subluminal. In the unstable (above-threshold) regime, superluminal response
occurs both in reflection and in transmission, at times preceding the onset of
exponential growth due to the instability.Comment: 9 pages, 6 figures, RevTex, to appear in Phys. Rev.
Braggoriton--Excitation in Photonic Crystal Infiltrated with Polarizable Medium
Light propagation in a photonic crystal infiltrated with polarizable
molecules is considered. We demonstrate that the interplay between the spatial
dispersion caused by Bragg diffraction and polaritonic frequency dispersion
gives rise to novel propagating excitations, or braggoritons, with intragap
frequencies. We derive the braggoriton dispersion relation and show that it is
governed by two parameters, namely, the strength of light-matter interaction
and detuning between the Bragg frequency and that of the infiltrated molecules.
We also study defect-induced states when the photonic band gap is divided into
two subgaps by the braggoritonic branches and find that each defect creates two
intragap localized states inside each subgap.Comment: LaTeX, 8 pages, 5 figure
Entanglement and Extreme Spin Squeezing
For any mean value of a cartesian component of a spin vector we identify the
smallest possible uncertainty in any of the orthogonal components. The
corresponding states are optimal for spectroscopy and atomic clocks. We show
that the results for different spin J can be used to identify entanglement and
to quantity the depth of entanglement in systems with many particles. With the
procedure developed in this letter, collective spin measurements on an ensemble
of particles can be used as an experimental proof of multi-particle
entanglementComment: 4 pages, 2 figures, minor changes in the presentatio
Atomic Quantum State Teleportation and Swapping
A set of protocols for atomic quantum state teleportation and swapping
utilizing Einstein-Podolsky-Rosen light is proposed. The protocols are suitable
for collective spin states of a macroscopic sample of atoms, i.e. for
continuous atomic variables. Feasibility of experimental realization for
teleportation of a gas sample of atoms is analyzed.Comment: 4 pages, 1 figur
Stationary two-atom entanglement induced by nonclassical two-photon correlations
A system of two two-level atoms interacting with a squeezed vacuum field can
exhibit stationary entanglement associated with nonclassical two-photon
correlations characteristic of the squeezed vacuum field. The amount of
entanglement present in the system is quantified by the well known measure of
entanglement called concurrence. We find analytical formulas describing the
concurrence for two identical and nonidentical atoms and show that it is
possible to obtain a large degree of steady-state entanglement in the system.
Necessary conditions for the entanglement are nonclassical two-photon
correlations and nonzero collective decay. It is shown that nonidentical atoms
are a better source of stationary entanglement than identical atoms. We discuss
the optimal physical conditions for creating entanglement in the system, in
particular, it is shown that there is an optimal and rather small value of the
mean photon number required for creating entanglement.Comment: 17 pages, 5 figure
Experimental demonstration of quantum memory for light
The information carrier of today's communications, a weak pulse of light, is
an intrinsically quantum object. As a consequence, complete information about
the pulse cannot, even in principle, be perfectly recorded in a classical
memory. In the field of quantum information this has led to a long standing
challenge: how to achieve a high-fidelity transfer of an independently prepared
quantum state of light onto the atomic quantum state? Here we propose and
experimentally demonstrate a protocol for such quantum memory based on atomic
ensembles. We demonstrate for the first time a recording of an externally
provided quantum state of light onto the atomic quantum memory with a fidelity
up to 70%, significantly higher than that for the classical recording. Quantum
storage of light is achieved in three steps: an interaction of light with
atoms, the subsequent measurement on the transmitted light, and the feedback
onto the atoms conditioned on the measurement result. Density of recorded
states 33% higher than that for the best classical recording of light on atoms
is achieved. A quantum memory lifetime of up to 4 msec is demonstrated.Comment: 22 pages (double line spacing) incl. supplementary information, 4
figures, accepted for publication in Natur