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One photon stored in four places at once
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Vuletic, Vladan. “Quantum physics: Entangled quartet.” Nature
468.7322 (2010): 384-385.
As Published
Nature Publishing Group
Author's final manuscript
Thu May 26 04:55:20 EDT 2016
Citable Link
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Detailed Terms
One photon stored in four places at once
Quantum mechanics allows a particle to exist in different places at the same time. Now a single
photon has been stored simultaneously in four locations while maintaining its wave character.
When light passes through two slits to hit a distant screen, a periodic light pattern emerges that is
associated with the interference of the waves emanating from the two sources. Some of quantum
physics’ deepest mysteries – or, according to the iconic Richard Feynman, its only mystery – arise when
that observation is made with a single particle that, although indivisible, must have passed
simultaneously through both slits. Recent advances in the storage of single photons in atomic gases [1]
have now enabled a tour-de-force experiment that investigates interference with light stored
simultaneously in four spatially distinct atom clouds, as reported on p. xxx of this issue. Chou et al.
demonstrate stronger-than-classical correlations (entanglement) in this composite matter-light system,
and study how the entanglement gives way to weaker and weaker, and ultimately only classical,
Classical correlations can arise in situations of limited knowledge about a system. For instance, if we
know that one coin (or photon) has been hidden in one of four boxes, then discovering 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 not surprising that “particle-type” detection (Fig. 1a) reveals
correlations in the number of coins found in the different boxes if the total number of coins is known.
If, on the other hand, identical light has been stored simultaneously in all four boxes – or, for that
matter, coins sufficiently small to display quantum wave character – then in an alternative measurement
where the light is combined in an interferometer before detection (Fig. 1b), we would find that the
detection probabilities vary periodically with the interferometer path length differences. This “wavetype” detection reveals correlations in the phases of the stored waves, and full correlations require all
boxes to initially contain light.
In a classical world the system can be initially prepared to exhibit either particle-type or wave-type
correlations, but not both. The system will exhibit particle-type correlations if a single photon is placed
in one and only one of the boxes, in which case there will be no interference in Fig. 1b. Alternatively, if
all boxes are filled simultaneously with identical classical fields, the system will exhibit full wave-type
correlations. However, these classical fields necessarily contain many photons, in which case one would
expect no correlations in the particle-type detection setup of Fig. 1a. Nevertheless, in the quantum
world we live in, it is possible to prepare non-locally stored single photon such that full correlations are
observed, no matter which detection setup (Fig. 1a or Fig. 1b) is chosen. Quantum correlations are thus
stronger than classical correlations in that different types of correlations can coexist in one and the
same initial state.
Chou et al. use four atomic ensembles as the storage boxes. Such systems not only hold the photon, but
also act as highly directional emitters that can be triggered on demand by the application of a laser pulse
[1,2]. The Caltech team then measures correlations between the different boxes either in the particle-
type detection setup (Fig. 1a) or in the wave-type setup (Fig. 1b), and from the combination of those
measurements extracts the degree of entanglement. Using a method previously developed for a single
photon traveling simultaneously along four possible paths [3], they identify boundaries between
entanglement that necessarily involves all four boxes, three, or just two of the boxes, and observe the
gradual transition from fourfold to no entanglement in the presence of noise and other imperfections.
While quantum-correlated states with more parts have been observed (the current records stand at
observed entanglement for 14 ions [4], and inferred entanglement of over 100 atoms [5]), the present
system is special in that the entanglement can be efficiently mapped on demand from a material system
onto a light field. Such systems, that have already reached light storage times measured in milliseconds
[6,7], have a variety of potential applications in quantum-protected communication over long distances
[1] or overcoming quantum limits in precision measurements.
P.S. The astute reader may wonder how it is that quantum correlations can be tested with a single
photon as any correlation requires more than one system. The controversy about this issue can be
resolved [8] 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.
Figure caption
Quantum mechanics allows a single particle to exist simultaneously in multiple locations. Here a single
photon is simultaneously stored in four boxes (atomic ensembles). The single-photon character of the
stored light can be detected in a particle-type measurement (a), where only one of the four detectors
D1, D2, D3, D4 will register a photon. Alternatively, the wave character of the single photon can be tested
via its ability to interfere with itself in a wave-type measurement setup (b). In a classical world,
correlations in the detection setups (a), (b) are mutually exclusive, and hence the combination of both
measurements can probe quantum correlations between the four boxes.
1. L.-M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, “Long-distance quantum communication with
atomic ensembles and linear optics”, Nature 414, 413 (2001).
2. V. Vuletic, “When Superatoms Talk Photons”, Nature Physics News & Views 2, 801 (2006).
3. S. B. Papp, K.S. Choi, H. Deng, P. Lougovski, S. J. van Enk, and H. J. Kimble, "Characterization of
Multipartite Entanglement for One Photon Shared Among Four Optical Modes", Science 324,
764 (2009).
4. Thomas Monz et al., “Coherence of large-scale entanglement”, arXiv:1009.6126 (2010).
5. C. Gross, T. Zibold, E. Nicklas, J. Estève, M. K. Oberthaler, “Nonlinear atom interferometer
surpasses classical precision limit”, Nature 464, 1165 (2010).
6. R. Zhao et al., "Long-Lived Quantum Memory", Nature Physics 5, 100 (2009).
7. B. Zhao et al, “A Millisecond Quantum Memory for Scalable Quantum Networks”, Nature Physics
5, 95-99 (2009).
8. S. J. van Enk, “Single-particle entanglement”, Phys. Rev. A 72, 064306 (2005), and references