Nearly 20 years ago, while working on atomic systems that can lase without inversion, Stanford University doctoral student John Field made a bold prediction. Given a cloud of three-level atoms that are opaque to a light beam, he argued, simply placing the atoms between two closely spaced mirrors can make it transparent to the same beam.1 A group led by MIT’s Vladan Vuletić has now experimentally demonstrated the unusual effect using an exceedingly weak light beam—pulses containing a few or even single photons—focused into an ensemble of about 105 cesium atoms.2 The effect is not small: They see a 40% reduction in absorption probability that appears to emerge out of the quantum blue—induced by the electromagnetic vacuum field in the empty space of an optical cavity. Additional photons injected into the cavity reduce the absorption further still.
The achievement is part of a larger effort over the...
