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arXiv:1706.00605 · Optica

Quantum interference between autonomous single-photon sources from Doppler-broadened atomic ensembles

Taek Jeong, Yoon-Seok Lee, Jiho Park, Heonoh Kim, Han Seb Moon

Published Sep 25, 2017• 28 citations
arXivHTMLPDF

Abstract

To realize a quantum network based on quantum entanglement swapping, bright and completely autonomous sources are essentially required. Here, we experimentally demonstrate Hong-Ou-Mandel (HOM) quantum interference between two independent bright photon pairs generated via the spontaneous four-wave mixing in Doppler-broadened ladder-type 87Rb atoms. Bright autonomous heralded single photons are operated in a continuous-wave (CW) mode with no synchronization or supplemental filters. The four-fold photon coincidence counts per hour correspond to 5000 events for a temporal range of 10 ns, one order higher than those of previously reported for autonomous single photons. We observe an HOM dip with 83% visibility with the two autonomous heralded single photons for an effective measurement time of 900 s. The achievement of the HOM interference between the two bright autonomous photon-pair sources has made an important step towards a practical scalable quantum network.

DOI
10.1364/optica.4.001167

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To realize a quantum network based on quantum entanglement swapping, bright and completely autonomous sources are essentially required. Here, we experimentally demonstrate Hong-Ou-Mandel (HOM) quantum interference between two independent bright photon pairs generated via the spontaneous four-wave mixing in Doppler-broadened ladder-type 87Rb atoms. Bright autonomous heralded single photons are operated in a continuous-wave (CW) mode with no synchronization or supplemental filters. The four-fold photon coincidence counts per hour correspond to 5000 events for a temporal range of 10 ns, one order higher than those of previously reported for autonomous single photons. We observe an HOM dip with 83% visibility with the two autonomous heralded single photons for an effective measurement time of 900 s. The achievement of the HOM interference between the two bright autonomous photon-pair sources has made an important step towards a practical scalable quantum network.
Entanglement swapping forms a key concept in the realization of scalable quantum networks and large-scale quantum communication. For the practical implementation of entanglement swapping, completely autonomous entanglement sources and a joint Bell-state measurement (BSM) between two independent photons are essential. Here, we experimentally demonstrate entanglement swapping between two independent polarization-entangled photon-pair sources obtained via spontaneous four-wave mixing (SFWM) in a Doppler-broadened atomic ensemble of 87 R b atoms. From the joint BSM, we estimate the S parameter in the Clauser–Horne–Shimony–Holt (CHSH) form of Bell’s inequality and confirm the violation of the Bell–CHSH inequality by S = 2.32 ± 0.07 with 4.5 standard deviations. We believe that this work is an important step toward realizing practical scalable quantum networks.
Entanglement swapping enables the distribution of quantum states over long distances. In this protocol, a Bell state measurement projects photons, which were created in separate entangled photon sources, into an entangled state when their corresponding pairs interfere at a beamsplitter. This process is optimally realized when the photons are indistinguishable in all degrees of freedom, resulting in Hong-Ou-Mandel (HOM) interference. Since its initial proposal by [1], several experimental realizations have used pulsed, synchronized pumps, as in [2] to achieve temporal indistiguishability. In the case of continuous wave (CW) pumping, the sources are not synchronized, requiring post-selection of the coincidence events after detection. This requires detectors with a temporal resolution greater than the coherence time of the photons. Previous experiments have successfully observed HOM interference using periodically poled Lithium Niobate (ppLN) waveguides and superconducting nanowire single-photon detectors (SNSPDs) [3], also with hybrid sources to improve coincidence rates and HOM visibility [4], [5].
Quantum entanglement first attracted attention as the heart of the Einstein-Podolsky-Rosen (EPR) paradox and one of the most counterintuitive phenomena in the view of classical physics [1]. Hong-Ou-Mandel (HOM) [2] and Franson interferences [3] are regarded as important evidence of the non-classical nature of photon sources. Since a two-photon interference experiment with time-energy entangled photons was proposed by J. D. Franson [2], the fourth-order interferences beyond the one-photon coherence length have attracted considerable attention as important and interesting quantum phenomena in quantum optics. Surprisingly, Franson interference has been not experimentally demonstrated by use of two-photon pairs emitted from a cascade-type atomic system with continuous wave (CW) pumping, as originally proposed by Franson.
Quantum networks require scalable photon sources combining narrow linewidth, high efficiency, and high indistinguishability. Microresonator photon-pair sources are promising candidates, yet a source-only description based on the joint spectral amplitude (JSA) predicts near-zero Hong-Ou-Mandel (HOM) interference visibility between photons generated by independent, CW-driven microresonators. In this work, we show that the observed HOM interference is not determined by the JSA alone. By combining finite-time detection with cavity-enhanced spontaneous four-wave mixing, we characterize a detector-conditioned heralded state governed by the idler-photon detection window. We further demonstrate that independently optimizing the idler and signal detection windows allows both heralded-photon indistinguishability and intrinsic heralding efficiency to approach unity, without spectral filtering or complex source engineering. Utilizing integrated high-QQQ silicon nitride microresonators, we achieve HOM visibilities of 0.992(8) and 0.942(12), without background subtraction, at fourfold count rates of 4.5(3) and 12.2(6) Hz, respectively. Our work establishes CW-driven high-QQQ microresonators as a robust and scalable platform for quantum-network primitives.
This paper reports a novel single mode source of narrow-band entangled photon pairs at telecom wavelengths under continuous wave (CW) excitation, based on parametric down conversion. For only 7 mW of pump power it has a created spectral radiance of 0.08 pairs per coherence length and a bandwidth of 10 pm (1.2 GHz). The effectively emitted spectral brightness reaches 3.9 × 10 5 pairs s −1 pm −1 . Furthermore, when combined with low jitter single photon detectors, such sources allow for the implementation of quantum communication protocols without any active synchronization or path length stabilization. A Hong–Ou–Mandel (HOM)-dip with photons from two autonomous CW sources has been realized demonstrating the setup's stability and performance.
We present Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. Without any spectral filtering, we observe a visibility of $V=62\ifmmode\pm\else\textpm\fi{}4%$. After correcting for accidental coincidences, we obtain $V=93\ifmmode\pm\else\textpm\fi{}6%$. The observed interference demonstrates the compatibility of the two sources, forming the basis for an efficient quantum interface between different physical systems.