Vladan Vuletic, “Quantum physics: Entangled quartet,” News & Views, Nature 468: 384–385, 18 November 2010, summarizes the paper K. S. Choi, A. Goban, S. B. Papp, S. J. van Enk & H. J. Kimble, “Entanglement of spin waves among four quantum memories,” Nature 468: 412-416, 18 November 2010.
“Single photons can be stored in atomic gases. Choi et al. investigate what happens to interference when light is stored simultaneously in as many as four spatially distinct atomic clouds. The authors demonstrate quantum correlations (entanglement) in this composite matter–light system, and study how entanglement ultimately fades away to leave only classical correlations.
Choi et al. have measured quantum entanglement in a composite matter–light system by combining results from particle-type and wave-type measurements. In the particle-type set-up, a photon stored in one box can reach only one detector (D1, D2, D3 or D4). In the wave-type measurement, the photon is placed simultaneously in all four boxes and the light emerging from the boxes is combined through an arrangement of partially reflecting and totally reflecting mirrors such that light from any box can reach any detector.
In a classical world, something is either a particle or a wave, so a physical system will exhibit correlations either in the particle-type or wave-type detection set-up — but not in both. However, in the quantum world that we live in, it is possible to place, for example, a single photon simultaneously in all boxes such that correlations are observed in both detection set-ups. And this is exactly what Choi et al. have done in their experiment.
The authors measured correlations between the different boxes, either in the particle-type detection set-up or in the wave-type set-up. From the combination of these measurements, they extracted the degree of entanglement of the light shared between the four boxes. Using a method previously developed for a single photon travelling simultaneously along four possible paths, they identified quantitative criteria, involving combinations of particle-type and wave-type detection results, that allowed them to distinguish among entanglement between all four boxes, or three, or just two of them. In the presence of noise and other imperfections, they observed a gradual transition from four-party entanglement to no entanglement.”