Atomic filtering for hybrid continuous-variable/discrete-variable   quantum optics
  We demonstrate atomic filtering of frequency-degenerate photon pairs from a sub-threshold optical parametric oscillator (OPO). The filter, a modified Faraday anomalous dispersion optical filter (FADOF), achieves 70% peak transmission simultaneous with 57 dB out-of-band rejection and a 445 MHz transmission bandwidth. When applied to the OPO output, only the degenerate mode, containing one-mode squeezed vacuum, falls in the filter pass-band; all other modes are strongly suppressed. The high transmission preserves non-classical continuous-variable features, e.g. squeezing or non-gaussianity, while the high spectral purity allows reliable discrete-variable detection and heralding. Correlation and atomic absorption measurements indicate a spectral purity of 96% for the individual photons, and 98% for the photon pairs. These capabilities will enable generation of atom-resonant hybrid states, e.g. "Schr\"odinger kittens" obtained by photon subtraction from squeezed vacuum, making these exotic states available for quantum networking and atomic quantum metrology applications. 
Joanna A. Zieli\'nska, Federica A. Beduini, Vito Giovanni Lucivero and   Morgan W. Mitchell
quant-ph