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European Quantum Electronics Conference (EQEC)

Narrow-bandwidth paired photons with controllable waveform

Vlatko Balić, Danielle Braje, Pavel Kolchin, G. Y. Yin, Stephen E. Harris

Published Jan 18, 2006• 0 citations
Publisher

Abstract

This study reports the experimental demonstration of a high-brightness source of paired photons based on an double-/spl Lambda/ atomic level configuration in cold /sup 87/Rb atoms. The source is capable of producing pairs of correlated photons with bandwidths as small as 5 MHz at into a single spatial mode. Strong counter-propagating coupling and pump lasers are applied and the system spontaneously emits pairs of photons at Stokes and anti-Stokes frequencies. The backward-wave geometry ensures that a spontaneously emitted photons will be oppositely directed and that they can be detected without additional filtering.

DOI
10.1109/eqec.2005.1567243

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This study reports the experimental demonstration of a high-brightness source of paired photons based on an double-/spl Lambda/ atomic level configuration in cold /sup 87/Rb atoms. The source is capable of producing pairs of correlated photons with bandwidths as small as 5 MHz at into a single spatial mode. Strong counter-propagating coupling and pump lasers are applied and the system spontaneously emits pairs of photons at Stokes and anti-Stokes frequencies. The backward-wave geometry ensures that a spontaneously emitted photons will be oppositely directed and that they can be detected without additional filtering.
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 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 present an efficient and robust source of photons at the 87 Rb D1-line (795 nm) with a narrow bandwidth of δ = 226(1) MHz. The source is based on non-degenerate, cavity-enhanced spontaneous parametric down-conversion in a monolithic optical parametric oscillator far below threshold. The setup allows for efficient coupling to single mode fibers. A heralding efficiency of η heralded = 45(5) % is achieved, and the uncorrected number of detected photon pairs is 3.8 × 10 3 /(s mW). For pair generation rates up to 5 × 10 5 /s, the source emits heralded single photons with a normalized, heralded, second-order correlation function g c ( 2 ) < 0.01 . The source is intrinsically stable due to the monolithic configuration. Frequency drifts are on the order of δ /20 per hour without active feedback on the emission frequency. We achieved fine-tuning of the source frequency within a range of >2 GHz by applying mechanical strain.
Cavity-enhanced spontaneous parametric down-conversion is capable of efficient generation of single photons with suitable spectral properties for interfacing with the atoms. However, beside the remarkable progress of this technique, multimode longitudinal emission remains a major drawback. Here, we demonstrate a bright source of single photons that overcomes this limitation by a novel mode-selection technique based on the introduction of an additional birefringent element to the cavity. This enables us to tune the double resonance condition independent of the phase matching and thus to achieve single-mode operation without mode filters. Our source emits single-frequency-mode photons at 852 nm, which is compatible to the Cs D2 line, with a bandwidth of 10.9 MHz and a detected photon-pair rate of 2.5 kHz at 10 mW of pump power, while maintaining g(2)(0) = 0.13. The efficiency of our source is further underlined by detecting a four-photon rate of 0.28 Hz at 20 mW of pump power. These detected rates correspond to a photon-pair generation rate of 47.5 Hz, and a four-photon generation rate of 37 Hz. Such photon generation rates open up a variety of new applications reaching from hybrid light-matter interactions to optical quantum information tasks based on long temporal coherence.
The generation of nonclassical photon pairs with a long coherence time is key for applications that range from fundamental to quantum communication and metrology. Spontaneous four-wave mixing with electromagnetically induced transparency has been demonstrated as one of the most efficient methods; however, narrowing the bandwidth and producing photon pairs with a temporal length beyond 1 μs remains a technical challenge due to noise considerations and the need for cold atoms with a high optical depth (OD). In this work, we demonstrate the generation of narrowband photon pairs with a controllable coherence time up to 1.72 μs in a laser-cooled atomic ensemble with an OD as high as 130. At such a high OD, we find that the pump laser field spatial profile has a significant effect on the time–frequency entangled two-photon waveform. We also confirm the quantum particle nature of heralded narrowband single photons generated from this source.
We observe time-correlated four photons within a correlation window of 20ns from spontaneous four-wave mixing via a double-Lambda scheme in a cold cloud of Rb-87 atoms. In contrast to high-power pulsed pumping of chi^(2) nonlinear processes in crystals, our scheme generates correlated four-photon states by direct continuous-wave pumping at nominal powers. We verify the presence of genuinely correlated four-photon states over accidentals by higher-order intensity cross-correlation measurements and accidental subtraction. We infer a time-correlated four-photon generation rate of 2.5(4)x10^6 counts per second close to saturation. The photons produced are near-resonant with atomic transitions, and have a bandwidth in the order of MHz, making them readily compatible with quantum networking applications involving atoms.