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arXiv:1806.02079 · Physical Review A

Characterization of a photon-pair source based on a cold atomic ensemble using a cascade-level scheme

Alessandro Ceré, Bharath Srivathsan, Gurpreet Kaur Gulati, Brenda Chng, Christian Kurtsiefer

Published Aug 20, 2018• 7 citations
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Abstract

We characterize a source of photon pairs based on cascade decay in a cold $^{87}\mathrm{Rb}$ ensemble. This source is particularly suited to generate photons for interaction with $^{87}\mathrm{Rb}$ based atomic systems. We experimentally investigate the dependence of pair generation rate, single photon heralding efficiency, and bandwidth as a function of the number of atoms, detuning, and intensity of the pump beams. The observed power and detuning behaviors can be explained by the steady-state solution of an established three-level model of an atom. Measurements presented here provide a useful insight on the optimization of this kind of photon-pair source.

DOI
10.1103/physreva.98.023835

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We characterize a source of photon pairs based on cascade decay in a cold 87Rb^{87}\mathrm{Rb}87Rb ensemble. This source is particularly suited to generate photons for interaction with 87Rb^{87}\mathrm{Rb}87Rb based atomic systems. We experimentally investigate the dependence of pair generation rate, single photon heralding efficiency, and bandwidth as a function of the number of atoms, detuning, and intensity of the pump beams. The observed power and detuning behaviors can be explained by the steady-state solution of an established three-level model of an atom. Measurements presented here provide a useful insight on the optimization of this kind of photon-pair source.
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.
We prepare heralded single photons from a photon pair source based on nondegenerate four-wave mixing in a cold atomic ensemble via a cascade decay scheme. Their statistics shows strong antibunching with a value for the second order correlation at zero delay less than 0.03, indicating a near single photon character. In an optical homodyne experiment, we directly measure the temporal envelope of these photons and find, depending on the heralding scheme, an exponentially decaying or rising profile. Such a rising envelope is required for the efficient interaction between a single photon and a two level system. At the same time, their observation illustrates the breakdown of a realistic interpretation of the heralding process in terms of defining an initial condition of a physical system.
We experimentally realized a bright and robust polarization-entangled photon-pair source from Doppler-broadened atomic ensembles by exploiting the strong signal-idler polarization correlation. The photon pair is emitted from the cascade-type 5S1/2–5P3/2–5D5/2 transition of 87Rb. Without using an interferometric configuration, we robustly produce all four Bell states by applying the intrinsic polarization correlation achieved in a cascade-type atomic system. The coincidence count rate of the polarization-entangled photon pairs is measured as 10.0(1) kHz under a weak pump power of 20 μW. Our scheme enables the generation of a high rate of polarization-entangled photon pairs with a high fidelity of 0.933(2). We believe that our approach to realize bright and stable polarization-entangled photon-pair sources can contribute to the construction of practical scalable quantum networks based on atom–photon interactions.
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.
We experimentally study the generation of photon pairs via spontaneous four-wave mixing with two distinct laser pulses. We find that the dual-pump technique enables new capabilities: 1) a new characterization methodology to measure noise contributions, source brightness and photon-collection efficiencies directly from raw photon-count measurements; 2) an enhanced ability to generate heralded single photons in a pure quantum state; and 3) the ability to derive upper and lower bounds on heralded-photon quantum state purity from measurements of photon-number statistics even in the presence of noise. Such features are highly valuable in photon-pair sources for quantum applications.