Atomic filtering for hybrid continuous-variable/discrete-variable quantum optics

Abstract

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. "Schrödinger kittens" obtained by photon subtraction from squeezed vacuum, making these exotic states available for quantum networking and atomic quantum metrology applications.

Interferometric Measurement of the Biphoton Wave Function

“…These show a sinusoidal behaviour (dashed line, A + B cos 2φ fit to the data) revealing two-photon interference as predicted by (7). Insets show g beams by a calcite beam displacer and passed through a narrowband (445 MHz) atomic filter [25,26], in order to isolate the squeezed vacuum and block with high efficiency the hundreds of non-degenerate frequency modes generated by the OPO. The maximum transmission frequency of this filter is located at 2.7 GHz to the red of the center of the rubidium D 1 line, and the laser frequency is stabilised at this particular frequency by using an integrated electro-optic modulator to add sidebands to the laser prior to the saturated absorption spectroscopy.…”

“…The observation of a dip in the correlation function is especially interesting, because it clearly signals destructive interference of two-photon amplitudes from the coherent and the squeezed vacuum states. The interference visibility is limited by accidental coincidence counts, which are mainly due to the residual OPO locking beam and to non-degenerate modes passing through the filter [26]. However, these do not affect the wavefunction reconstruction: the accidentals add a term independent from τ to the g (2) , which is canceled by the subtractions in Eq.…”

ElecSus: Extension to arbitrary geometry magneto-optics

“…One can then use these optimised filters in other applications, e.g. using the filter to make an intrinsically frequencystable laser system [9], creating a dichroic beam splitter for Raman light [18] or filtering frequency-degenerate photon pairs from an optical parametric oscillator [19].…”