Complex quantum systems

The presence of a disordered potential can give rise to the localization of matter quantum waves, known as Anderson localization. The precise determination of the mobility edge between localized and delocalized states, as well as the associated critical exponents, as a function of the dimension and nature of disorder remains a major unsolved challenge of physics, both theoretical and experimental. We recently demonstrated that the percolation threshold of the localization landscape corresponds to the position of this mobility edge. We are collaborating with Alain Aspect’s team at IOGS to achieve a complete understanding of this phenomenon in cold atom systems, notably in the correlated disorder realized in laser speckles [1].

Regarding light waves, our main focus in recent years has been on controlling the propagation of classical waves. However, the quantum character of a wave can strongly alter its propagation. For example, when photons occupy compressed or entangled Fock states, the two-photon coincidence rate may exhibit correlations that cannot be explained in a classical formalism. Very recently, in collaboration with Yaron Bromberg’s team (Racah Institute of Physics, Israel), we demonstrated the existence of a correlation between entangled photons that holds in a multiply scattering medium, despite the dynamics of the medium [2]. This effect is the quantum analogue of the famous coherent backscatter cone.

Figure 1 Measurement of the coincidence rate of entangled photons reflected by a scattering medium, as a function of the collection angle.