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 $w(1_2,1_2|1_1)=0.24\pm 0.05\ngeqq 1$, 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 $w\to 0$ for single photons. DOI: http://dx.doi.org/10.1103/PhysRevLett.92.213601 © 2004 The American Physical Society