Narrow Band Source of Transform-Limited Photon Pairs via Four-Wave Mixing in a Cold Atomic Ensemble

Abstract

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.

Time-resolved scattering of a single photon by a single atom

“…After molasses cooling, the trapped atom is optically pumped into the 5 S 1/2 , F ¼ 2, m F ¼ À 2 state. Probe photons are prepared by heralding on one photon of a time-correlated photon pair generated via four-wave-mixing (FWM) in a cloud of cold 87 Rb atoms 26,27 . The relevant energy levels are depicted in Fig.…”

Photon bandwidth dependence of light-matter interaction

“…Detecting photons at 776 nm then heralds single photons at 780 nm, resonant with the 87 Rb D2 transition. Due to collective effects in the atomic ensemble, the bandwidth of the probe photons is broader than the natural linewidth (Γ 0 /2π = 6.07 MHz) of the 5P 3/2 − 5S 1/2 transition [30,38]. We employ these effects to tune the bandwidth Γ p of the probe photons over a range of 6 to 2 Γ 0 by controlling the number and density of atoms in the cold ensemble via the gradient of the magnetic quadrupole field during the MOT phase.…”

“…To test the model presented in Section II, we prepare single photon states by heralding on a time-correlated photon-pair from a parametric process in a cold atomic ensemble [29,30]. These photons are focused onto a single atom trapped in a far-off-resonant optical dipole trap.…”

Observation of spatial quantum correlations in the macroscopic regime

“…Dv, 42.50.Ar, Under certain conditions, the quantum fluctuations in beams of light can be reduced below the shot noise limit (SNL) not only in the temporal domain, but also in the transverse spatial degree of freedom [1]. To date, most of the attention has focused on the study of quantum noise reduction, or squeezing, in the temporal domain [2][3][4][5][6][7][8][9][10][11][12]. Nevertheless, many areas in quantum optics, specifically quantum metrology and quantum imaging, could greatly benefit from the study of the quantum correlations directly in the spatial domain [13][14][15].…”

“…First, the photon pairs generated by FWM have narrow bandwidths (in the MHz regime, even when working with hot atoms) [11,28,29], therefore they are useful for atom-light interaction-based quantum protocols [33]. Second, the FWM process offers large gains even in a single pass configuration unlike SPDC [6,34].…”