Quantum Optics in Multiple Scattering Random Media – University of Copenhagen

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02 November 2011

Quantum Optics in Multiple Scattering Random Media

Noise is inevitable in all measurements and limits the performance of optical devices like amplifiers and lasers [1]. At the most fundamental level, optical noise is limited by quantum fluctuations of light associated with Heisenberg’s uncertainty relation. Remarkably, these quantum fluctuations can be manipulated, which gives rise to a whole zoo of different quantum states of light that in recent years have been realized experimentally in the laboratory.

Scattering is a fundamental process where the propagation direction of light is altered due to changes in the refractive index. In a random distribution of scatterers, where each scatterer has dimensions only a fraction of the optical wavelength, light will be multiple scattered. A characteristic of multiple scattering is the intensity speckle pattern (Fig. 1) that can be observed by a transmission measurement on a random medium and is due to interference between different propagation paths through the sample.

Left panel: Multiply scattered light waves are totally randomized in their direction while they propagate through disordered media of length L. The transport mean free path l characterizes the average distance between two scattering events. Right panel: Measured spatial intensity distribution of light transmitted through a multiple scattering medium displaying a volume speckle pattern.

Multiple scattering of light is an extremely active and interdisciplinary research field that so far has focused mainly on classical optics. Only very recently it was predicted and demonstrated that novel effects are to be expected in quantum optics description of multiple scattering [2-6]. When light is scattered many times, the propagation direction is randomized, i.e. the transmitted light becomes uncorrelated. Surprisingly, quantum states of light can be used to induce strong correlations between different propagation directions [4,6] that have no classical analogue.

Measured spatial quantum correlation function versus power of the incident light beam. For classical (blue points) and nonclassical (red points) photon fluctuations, positive and negative spatial correlations are observed, respectively. Every data point represents an ensemble average over three different positions of the sample. The curves are theoretical predictions and the dashed line represents the uncorrelated case.

We study experimentally and theoretically the transport of quantum noise through multiple scattering media. Fig. 2 demonstrates spatial quantum correlations that are induced by multiple scattering of light [6]. The quantum correlation is observed between photons propagating along two different light paths through a random medium. As nonclassical light source we used squeezed light where we can continuously tune the photon fluctuations relative to the average number of photons. We thereby generate classical and nonclassical photon fluctuations (ratio of 4.6 and 0.52), respectively. For nonclassical incident photon fluctuations negative spatial correlations are observed and for classical incident photon fluctuations positive spatial correlations are observed. The experimental data are very well explained by the results of a full quantum theory for multiple light scattering.