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Journal of the Korean Physical Society

Generation of nonclassical narrowband photon pairs from a cold rubidium cloud

Young‐Wook Cho, Kwang-Kyoon Park, Jong Chan Lee, Yoon-Ho Kim

Published Aug 1, 2013• 8 citations
Publisher

Abstract

We report the generation of nonclassical narrowband photon pairs via the spontaneous four-wave mixing process in a cold Rubidium ( 87 Rb) cloud prepared in a magneto-optical trap. The generated photon pairs exhibit strong violation of the Cauchy-Schwarz inequality by a factor of 625 ± 45, confirming nonclassical correlation between the photons. The biphoton wave packets, measured for several different experimental conditions, are in good agreement with the theoretical results.

DOI
10.3938/jkps.63.943

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We report the generation of nonclassical narrowband photon pairs via the spontaneous four-wave mixing process in a cold Rubidium ( 87 Rb) cloud prepared in a magneto-optical trap. The generated photon pairs exhibit strong violation of the Cauchy-Schwarz inequality by a factor of 625 ± 45, confirming nonclassical correlation between the photons. The biphoton wave packets, measured for several different experimental conditions, are in good agreement with the theoretical results.
We present our recently-built experimental setup designed to generate near-infrared and narrow-band correlated photon pairs by inducingfour-wave mixing in a cold gas of 87Rb atoms confined in a magneto-optical trap. The experimental setup and its automation and control approach are described in detail. A characterization of the optical density of the atomic ensemble as well as the basic statistical measurements of the generated light are reported. The non-classical nature of the photons pairs is confirmed by observing a violation of Cauchy-Schwarz inequality by a factor of 5.6 × 10 5 in a Hanbury Brown – Twiss interferometer. A 1/e coherence time for the heralded, idler photons of 4.4 ± 0.1 ns is estimated from our observations. We are able to achieve a value of 104 s−1pair-detection-rate, which results in a spectral brightness of 280 (MHz s)−1. The combination of high brightness and narrow-band spectrum makes this photon-pair source a viable tool in fundamental studies of quantum states and opens the door to use them in quantum technologies.
Non-classical paired photons are generated by a four-wave mixing process in a far-detuning three-level system with cold atoms. A violation of the Cauchy-Schwartz inequality of a factor of 310 is observed. This phenomenon shows that paired photons have a non-classical correlation. The experimental results are compared with theoretical results obtained using perturbation theory. The oscillation frequencies of the two-photon intensity correlation functions are in reasonable agreement with the effective Rabi frequencies of the coupling laser. However, we find that the dephasing rates (or decay rates) observed are far larger than the theoretical values.
We report the experimental verification of nonclassical correlations for an unfiltered spontaneous four-wave mixing process in an ensemble of cold two-level atoms, confirming theoretical predictions by Du et al. in 2007 for the violation of a Cauchy-Schwarz inequality in the system, and obtaining R=(1.98±0.03)≰1. Quantum correlations are observed in a nanoseconds timescale in the interference between the central exciting frequency and sidebands dislocated by the detuning to the atomic resonance. They prevail over the noise background coming from Rayleigh scattering from the same optical transition. These correlations are fragile with respect to processes that disturb the phase of the atomic excitation, but are robust to variations in number of atoms and to increasing light intensities.
We observe narrow band pairs of time-correlated photons of wavelengths 776 and 795 nm from nondegenerate four-wave mixing in a laser-cooled atomic ensemble of ^{87}Rb using a cascade decay scheme. Coupling the photon pairs into single mode fibers, we observe an instantaneous rate of 7700 pairs per second with silicon avalanche photodetectors, and an optical bandwidth below 30 MHz. Detection events exhibit a strong correlation in time [g((2))(τ = 0) ≈ 5800] and a high coupling efficiency indicated by a pair-to-single ratio of 23%. The violation of the Cauchy-Schwarz inequality by a factor of 8.4 × 10(6) indicates a strong nonclassical correlation between the generated fields, while a Hanbury Brown-Twiss experiment in the individual photons reveals their thermal nature. The comparison between the measured frequency bandwidth and 1/e decay time of g((2)) indicates a transform-limited spectrum of the photon pairs. The narrow bandwidth and brightness of our source makes it ideal for interacting with atomic ensembles in quantum communication protocols.
We report on a source of narrowband pairs of time-correlated photons generated via four-wave mixing in a laser-cooled atomic ensemble of87Rb atoms. We employ a cascade decay level scheme in which two copropagating pump beams of 780nm and 776nm excite the atoms to the 5D3/2, F = 3 level and the photon pairs of wavelength 762nm and 795nm from the cascade decay are coupled into single mode fibers and sent to silicon avalanche photodetectors. The photodetection events exhibit a strong correlation in time (g(2)(τ = 0) ≈ 5800), and a high coupling efficiency, as indicated by a pair-to-single ratio of 23%. An instantaneous pair generation rate of 7700 pairs per second (uncompensated for optical losses and detector efficiency) is observed with an increased 776nm pump power of 14mW. The violation of the Cauchy-Schwarz inequality by a factor of 8.4 × 106indicates a strong non-classical correlation between the generated fields, while a Hanbury-Brown-Twiss experiment in the individual photons reveals their thermal nature.