Quantum physics is known for its counter-intuitive principles. One such principle — that a single photon can be in as many as four places at the same time — has now been demonstrated. See Letter p.412
When light is shone through two closely separated slits and onto a distant screen, a periodic light pattern emerges as a result of interference between the light waves emanating from the two slits. Where quantum physics is concerned, some of the deepest mysteries — or, in the opinion of the iconic Richard Feynman, the only mystery — arise when that experiment is performed not with strong classical light waves but with a single particle. Although indivisible, a single particle also produces an interference pattern, so it must have passed simultaneously through both slits.
Building on recent advances1 enabling the storage of single photons in atomic gases, Choi et al.2 (page 412 of this issue) 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.
Classical correlations can arise in situations in which there is limited knowledge of a system. For instance, if we know only that one coin (or photon) has been hidden in one of four boxes, then detecting the coin in one box would instantaneously tell us that the other three boxes are empty — even if they were separated from each other by light years. It is hardly surprising that such 'particle-type' detection (Fig. 1a) can reveal classical correlations between the numbers of coins found in the different boxes if the total number of coins is known a priori.
Classical correlations can also arise between multiple light waves that are combined on partially reflecting mirrors before detection, such that the origin of the detected light is unknown (wave-type detection). For example, if identical light waves have been stored simultaneously in all four boxes — or, for that matter, coins sufficiently small to display quantum, wave-like character — and the light emerging from the boxes is combined through a series of partially reflecting and totally reflective mirrors before reaching four detectors (Fig. 1b), then the outputs of the detectors would vary as the path length between each box and the corresponding first mirror is varied.
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.2 have done in their experiment.
Choi and colleagues used four atomic ensembles as the storage boxes. Such systems not only can hold the photon, but also can act as highly directional light emitters that can be triggered on demand through the application of a laser pulse1,3. The authors measured correlations between the different boxes, either in the particle-type detection set-up (Fig. 1a) or in the wave-type set-up (Fig. 1b). From the combination of these measurements, they extracted the degree of entanglement of the light shared between the four boxes. Using a method previously developed4 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.
Although entanglement among more than four parties has been observed (the current record is for a system of 14 ions5, and entanglement has been inferred among 100 atoms6), Choi and colleagues' system2 is special because the entanglement can be efficiently mapped on demand from a material system onto a light field. Atomic ensembles such as those used by the authors have already reached light-storage times of milliseconds at the single-photon level7,8. If those storage times can be extended to seconds, and some other technical performance parameters improved, such sources will have a variety of potential applications in secure quantum communication over long distances1. The ensembles could then be used to build quantum networks over which quantum information can be distributed.
The astute reader may wonder how it is that quantum correlations can be observed with a single photon given that any correlation requires more than one system. The controversy about this issue can be resolved9 by viewing the four boxes as the systems that exhibit correlations (in photon number), rather than considering a single photon with qualms about its parent box.
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