75 research outputs found

### Multiple scattering of light by atoms with internal degeneracy

An analytical microscopic theory for the resonant multiple scattering of
light by cold atoms with arbitrary internal degeneracy is presented. It permits
to calculate the average amplitude and the average intensity for one-photon
states of the full transverse electromagnetic field in a dilute medium of
unpolarized atoms. Special emphasis is laid upon an analysis in terms of
irreducible representations of the rotation group. It allows to sum explicitly
the ladder and maximally crossed diagrams, giving the average intensity in the
Boltzmann approximation and the interference corrections responsible for weak
localization and coherent backscattering. The exact decomposition into field
modes shows that the atomic internal degeneracy contributes to the
depolarization of the average intensity and suppresses the interference
corrections. Static as well as dynamic quantities like the transport velocity,
diffusion constants and relaxation times for all field modes and all atomic
transitions are derived.Comment: Corrected minor errors. Slightly extended version of the article
appeared in prin

### Comment on ``Intensity correlations and mesoscopic fluctuations of diffusing photons in cold atoms''

In a recent Letter (Phys. Rev. Lett. \textbf{98}, 083601 (2007),
arXiv:cond-mat/0610804), O. Assaf and E. Akkermans claim that the angular
correlations of the light intensity scattered by a cloud of cold atoms with
internal degeneracy (Zeeman sublevels) of the ground state overcome the usual
Rayleigh law. More precisely, they found that they become exponentially large
with the size of the sample. In what follows, we will explain why their results
are wrong and, in contrary, why the internal degeneracy leads to lower
intensity correlations.Comment: 1 page. Comment submitted to PR

### Coherent Backscattering with Nonlinear Atomic Scatterers

We study coherent backscattering of a quasi-monochromatic laser by a dilute
gas of cold two-level atoms. We consider the perturbative regime of weak
intensities, where nonlinear effects arise from {\em inelastic} two-photon
scattering processes. Here, coherent backscattering can be formed by
interference between {\em three} different scattering amplitudes. Consequently,
if elastically scattered photons are filtered out from the photodetection
signal by means of suitable frequency-selective detection, we find the
nonlinear backscattering enhancement factor to exceed the linear barrier two.Comment: 4 pages, 3 figure

### Mesoscopic scattering of spin s particles

Quantum effects in weakly disordered systems are governed by the properties
of the elementary interaction between propagating particles and impurities.
Long range mesoscopic effects due to multiple scattering are derived by
iterating the single scattering vertex, which has to be appropriately
diagonalized. In the present contribution, we present a systematic and detailed
diagonalisation of the diffuson and cooperon vertices responsible for weak
localisation effects. We obtain general expressions for eigenvalues and
projectors onto eigenmodes, for any spin and arbitrary elementary interaction
with impurities. This description provides a common frame for a unified theory
of mesoscopic spin physics for electrons, photons, and other quantum particles.
We treat in detail the case of spin-flip scattering of electrons by freely
orientable magnetic impurities and briefly review the case of photon scattering
from degenerate dipole transitions in cold atomic gases.Comment: published version, with a new figure and new section

### Entanglement detection from interference fringes in atom-photon systems

A measurement scheme of atomic qubits pinned at given positions is studied by
analyzing the interference pattern obtained when they emit photons
spontaneously. In the case of two qubits, a well-known relation is revisited,
in which the interference visibility is equal to the concurrence of the state
in the infinite spatial separation limit of the qubits. By taking into account
the super-radiant and sub-radiant effects, it is shown that a state tomography
is possible when the qubit spatial separation is comparable to the wavelength
of the atomic transition. In the case of three qubits, the relations between
various entanglement measures and the interference visibility are studied,
where the visibility is defined from the two-qubit case. A qualitative
correspondence among these entanglement relations is discussed. In particular,
it is shown that the interference visibility is directly related to the maximal
bipartite negativity.Comment: 12 pages, 2 figures, published versio

### Coherent Backscattering of Light with Nonlinear Atomic Scatterers

We study coherent backscattering of a monochromatic laser by a dilute gas of
cold two-level atoms in the weakly nonlinear regime. The nonlinear response of
the atoms results in a modification of both the average field propagation
(nonlinear refractive index) and the scattering events. Using a perturbative
approach, the nonlinear effects arise from inelastic two-photon scattering
processes. We present a detailed diagrammatic derivation of the elastic and
inelastic components of the backscattering signal both for scalar and vectorial
photons. Especially, we show that the coherent backscattering phenomenon
originates in some cases from the interference between three different
scattering amplitudes. This is in marked contrast with the linear regime where
it is due to the interference between two different scattering amplitudes. In
particular we show that, if elastically scattered photons are filtered out from
the photo-detection signal, the nonlinear backscattering enhancement factor
exceeds the linear barrier two, consistently with a three-amplitude
interference effect.Comment: 18 pages, 13 figures, submitted to Phys. Rev.

### Light transport in cold atoms and thermal decoherence

By using the coherent backscattering interference effect, we investigate
experimentally and theoretically how coherent transport of light inside a cold
atomic vapour is affected by the residual motion of atomic scatterers. As the
temperature of the atomic cloud increases, the interference contrast
dramatically decreases emphazising the role of motion-induced decoherence for
resonant scatterers even in the sub-Doppler regime of temperature. We derive
analytical expressions for the corresponding coherence time.Comment: 4 pages - submitted to Physical Review Letter

### Coherent light transport in a cold Strontium cloud

We study light coherent transport in the weak localization regime using
magneto-optically cooled strontium atoms. The coherent backscattering cone is
measured in the four polarization channels using light resonant with a J=0 to
J=1 transition of the Strontium atom. We find an enhancement factor close to 2
in the helicity preserving channel, in agreement with theoretical predictions.
This observation confirms the effect of internal structure as the key mechanism
for the contrast reduction observed with an Rubidium cold cloud (see: Labeyrie
et al., PRL 83, 5266 (1999)). Experimental results are in good agreement with
Monte-Carlo simulations taking into account geometry effects.Comment: 4 pages, 2 figure

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