Photons are good carriers of quantum information. They’re relatively easy to entangle, in amplitude and phase as well as in polarization, and they can traverse long distances quickly. But quantum communication protocols often involve storing qubits and saving them for later—to be measured jointly with other qubits, for example—and no method yet exists for storing light for arbitrarily long times without unacceptably corrupting its quantum state.
Beginning in 1999 several research groups have used electromagnetically induced transparency to slow and stop light (see Physics Today, March 2001, page 17). Those experiments used relatively intense signals whose quantum properties were overwhelmed by classical noise. In 2004 Eugene Polzik, of the Danish Center for Quantum Optics in Copenhagen, and colleagues reported the first successful demonstration of storing the information in faint pulses more faithfully than could be done with classical measurements. 1 They did it by mapping part of the...
