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Chinese Physics Letters

Generation of a Non-Classical Correlated Photon Pair via Spontaneous Four-Wave Mixing in a Cold Atomic Ensemble

Xiaosong Lu, Qunfeng Chen, Bao-Sen Shi, Guo Guang-Can

Published Jun 1, 2009• 33 citations
Publisher

Abstract

Counter propagated write and read lasers can be used to generate non-classical correlated photon pairs in an atomic ensemble. We experimentally investigate how the detuning of the write laser affects the non-classical correlation function between the Stokes photon and the anti-Stokes photon, which are generated via a spontaneous four-wave mixing process using an off-axis configuration in a cold 85 Rb atomic ensemble. The change of the time-resolved second-order correlated function between the Stokes and anti-Stokes photons is presented. The experimental result suggests that a suitable choice of detuning should be considered in such an experiment.

DOI
10.1088/0256-307x/26/6/064204

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Counter propagated write and read lasers can be used to generate non-classical correlated photon pairs in an atomic ensemble. We experimentally investigate how the detuning of the write laser affects the non-classical correlation function between the Stokes photon and the anti-Stokes photon, which are generated via a spontaneous four-wave mixing process using an off-axis configuration in a cold 85 Rb atomic ensemble. The change of the time-resolved second-order correlated function between the Stokes and anti-Stokes photons is presented. The experimental result suggests that a suitable choice of detuning should be considered in such an experiment.
We demonstrate a bright correlated photon-pair source using double-A four-wave mixing in a87Rb cold atomic ensemble prepared in a magneto-optical trap. A counter-propagating pump configuration, combined with the small-angle collection along the propagation direction, enables clear distinction of resulting Stokes (780nm) and anti-Stokes (795nm) photons from pump fields. From measurements, we infer a generation rate of correlated photon pairs up to 107count per second (cps) into a collection mode waist of 175 μm. This is competitive with previous results observed in cold atomic ensembles [1], [2]. Their narrow spectral bandwidth makes these photons highly suitable for direct interaction with atomic systems and long-distance fiber transmission.
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 present the quantum theory for creation of collective atomic memory and generation of nonclassically correlated photon pairs from an ensemble via the protocol of Duan et al. [Nature (London) 414, 413 (2001)]. The temporal evolution of photon numbers, photon statistics, and cross-correlation between the Stokes and anti-Stokes fields is found by solving the equation of motion for atomic spin-wave excitations. We consider a low-finesse cavity model with collectively enhanced signal-to-noise ratio, which remains still considerably large in the free-space limit. Our results describe analytically the dependence of quantum correlations on spin decoherence time and time delay between the write and read lasers and reproduce the observed data very well including the generated pulse shapes, strong violation of Cauchy-Schwarz inequality and conditional generation of anti-Stokes single-photon pulse. The approach we developed may be used also for quantum description of storage and retrieval of quantum information, especially when the statistical properties of nonclassical pulses are studied.
Abstract An experimental investigation of how ground‐state decoherence and phase mismatch influence biphoton generation in double‐Λ spontaneous four‐wave mixing (SFWM) within a cold atomic ensemble is presented. The results reveal significant asymmetry in the Stokes and anti‐Stokes photon generation rates, arising from the distinct effects of phase mismatch and ground‐state decoherence. While phase mismatch primarily drives this asymmetry under minimal decoherence, larger decoherence further amplifies it, underscoring the complex interplay between these factors. Using the coincidence count rate representation, insights are provided into pairing ratios, demonstrating that the stimulated four‐wave mixing process inherent in SFWM explains the observed phenomena. Interestingly, although ground‐state decoherence reduces the generation of temporally correlated photons, it paradoxically enhances biphoton purity, as confirmed through conditional autocorrelation measurements. This counterintuitive phenomenon is reported here for the first time. Furthermore, unconditional autocorrelation measurements show that the generated photons follow a thermal‐state distribution, consistent with theoretical predictions. This study advances the understanding of biphoton generation dynamics and temporal photon correlations in SFWM, offering valuable insights for optimizing SFWM‐based biphoton sources and their applications in quantum technologies.