Single-photon generation from stored excitation in an atomic ensemble

Single photons are generated from an ensemble of cold Cs atoms via the protocol of Duan et al. [Nature (London) 414, 413 (2001)]. Conditioned upon an initial detection from field 1 at 852 nm, a photon in field 2 at 894 nm is produced in a controlled fashion from excitation stored within the atomic ensemble. The single-quantum character of field 2 is demonstrated by the violation of a Cauchy-Schwarz inequality, namely w(1(2),1(2)\ to1(1))=0.24+/-0.05not greater than or equal to1, where w(1(2),1(2)\1(1)) describes the detection of two events (1(2),1(2)) conditioned upon an initial detection 1(1), with w-->0 for single photons.