Wireless communications with diffuse waves

Diffuse, multiple-scattered waves can be very efficient for information transfer through disordered media, provided that antenna arrays are used for both transmission and reception of signals. Information capacity C of a communication channel between two identical linear arrays of n equally-spaced antennas, placed in a disordered medium with diffuse scattering, grows linearly with n and can attain considerable values, if antenna spacing a > lambda/2, where lambda is the wavelength. Decrease of a below lambda/2 makes the signals received by different antennas partially correlated, thus introducing redundancy and reducing capacity of the communication system. When the size of antenna arrays is well below lambda/2, the scaling of C with n becomes logarithmic and capacity is low.Comment: RevTEX 4, 3 pages, 2 figure

Finite-size scaling analysis of localization transition for scalar waves in a 3D ensemble of resonant point scatterers

We use the random Green's matrix model to study the scaling properties of the localization transition for scalar waves in a three-dimensional (3D) ensemble of resonant point scatterers. We show that the probability density $p(g)$ of normalized decay rates of quasi-modes $g$ is very broad at the transition and in the localized regime and that it does not obey a single-parameter scaling law for finite system sizes that we can access. The single-parameter scaling law holds, however, for the small-$g$ part of $p(g)$ which we exploit to estimate the critical exponent $\nu$ of the localization transition. Finite-size scaling analysis of small-$q$ percentiles $g_q$ of $p(g)$ yields an estimate $\nu \simeq 1.55 \pm 0.07$. This value is consistent with previous results for Anderson transition in the 3D orthogonal universality class and suggests that the localization transition under study belongs to the same class.Comment: 11 pages, 10 figures, revised manuscrip

Spatio-temporal speckle correlations for imaging in turbid media

We discuss the far-field spatio-temporal cross-correlations of waves multiple-scattered in a turbid medium in which is embedded a hidden heterogeneous region (inclusion) characterized by a distinct scatterer dynamics (as compared to the rest of the medium). We show that the spatio-temporal correlation is affected by the inclusion which suggests a new method of imaging in turbid media. Our results allow qualitative interpretation in terms of diffraction theory: the cross-correlation of scattered waves behaves similarly to the intensity of a wave diffracted by an aperture.Comment: RevTeX, 4 pages, a modified version is published in "Waves and Imaging Through Complex Media", ed. by P. Sebbah (Kluwer Academic Publishers, Dordrecht, 2001

Quantum versus classical effects in two-photon speckle patterns

We discuss quantum and classical aspects of two-photon interference in light transmission through disordered media. We show that disorder is the main factor that suppresses the interference, whatever the quantum state of the incident light. Secondarily, the two-photon interference is affected by the quantum nature of light (i.e., the well-defined number of photons in the two-photon entangled and Fock states as compared to the coherent state). And finally, entanglement is a resource that allows to prepare two-photon states with special symmetries with respect to the interchange of the photons and, in particular, the states with bosonic and fermionic symmetries. The two-photon interference is more robust for the latter states and its sign can be inverted for the fermionic state.Comment: 13 pages, 10 figures, revised tex

Localization transition for light scattering by cold atoms in an external magnetic field

We establish a localization phase diagram for light in a random three-dimensional (3D) ensemble of motionless two-level atoms with a three-fold degenerate upper level, in a strong static magnetic field. Localized modes appear in a narrow spectral band when the number density of atoms $\rho$ exceeds a critical value $\rho_c \simeq 0.1 k_0^3$, where $k_0$ is the wave number of light in the free space. A critical exponent of the localization transition taking place upon varying the frequency of light at a constant $\rho > \rho_c$ is estimated to be $\nu = 1.57 \pm 0.07$. This classifies the transition as an Anderson localization transition of 3D orthogonal universality class.Comment: 5 pages, 3 figures + supplemental materia

Time-dependent reflection at the localization transition

A short quasi-monochromatic wave packet incident on a semi-infinite disordered medium gives rise to a reflected wave. The intensity of the latter decays as a power law $1/t^{\alpha}$ in the long-time limit. Using the one-dimensional Aubry-Andr\'{e} model, we show that in the vicinity of the critical point of Anderson localization transition, the decay slows down and the power-law exponent $\alpha$ becomes smaller than both $\alpha = 2$ found in the Anderson localization regime and $\alpha = 3/2$ expected for a one-dimensional random walk of classical particles.Comment: 9 pages, 6 figures. Revised tex

Eigenvalue distributions of large Euclidean random matrices for waves in random media

We study probability distributions of eigenvalues of Hermitian and non-Hermitian Euclidean random matrices that are typically encountered in the problems of wave propagation in random media.Comment: 29 pages, 10 figure

Long-range correlations of density in a Bose-Einstein condensate expanding in a random potential

We study correlations of atomic density in a weakly interacting Bose-Einstein condensate, expanding diffusively in a random potential. We show that these correlations are long-range and that they are strongly enhanced at long times. Density at distant points exhibits negative correlations.Comment: 4 pages, 5 figure

Photon noise in a random laser amplifier with fluctuating properties

We study fluctuations of the number of photocounts measured by an ideal photodetector illuminated by light scattered in an amplifying disordered medium, below the threshold for random lasing. We show that the variance of fluctuations and their correlation function carry information about fluctuating properties of the medium. A direct link is established between the fluctuations of the number of photocounts due to the amplified spontaneous emission (ASE) and the dimensionless conductance g of the medium. Our results suggest a possibility of probing amplifying disordered media by analyzing statistics of their ASE, without illuminating them from outside by a probe beam.Comment: 14 pages, 9 figure