Vacuum-induced transparency

Nearly 20 years ago, Stanford University theorist John Field made a bold prediction: Given a cloud of two-level atoms that are opaque to a light beam, simply placing the cloud between two closely spaced mirrors can make it transparent to the same beam. A group led by MIT’s Vladan Vuletić has now experimentally demonstrated that bit of quantum weirdness—even on the single-photon level. Their method is a variant of electromagnetically induced transparency—the use of a control laser to render atoms in a solid or gas transparent to a second probe beam that would otherwise be absorbed (see the article by Stephen Harris, Physics Today, July 1997, page 36 ). The MIT researchers shine a probe beam on roughly 10⁵ cold cesium atoms (red) in an optical cavity, as illustrated here. Thanks to the cavity’s ultra-reflective mirrors, which amplify the electric field of any photons in it, the atoms absorb and then re-emit photons into a single cavity mode. The concerted interaction of the probe with the atoms and reflections of its scattered self alters the atoms’ transparency to the probe beam; no control beam is required. Vuletić says that the concept may form the basis for devices such as photon-number filters: Because different numbers of photons in the cavity experience different time delays during nonlinear interactions with the atoms, one can discern the number of photons in the transmitted probe beam. (H. Tanji-Suzuki et al., Science 333, 1266, 2011 .)—R. Mark Wilson