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Physical Review Letters

Time-Energy Entangled Photon Pairs from Doppler-Broadened Atomic Ensemble via Collective Two-Photon Coherence

Jiho Park, Taek Jeong, Heonoh Kim, Han Seb Moon

Published Dec 26, 2018• 46 citations
Publisher

Abstract

We experimentally demonstrate two-photon interference of a time-energy entangled photon pair generated via collective two-photon coherence in Doppler-broadened cascade-type ^{87}Rb atoms. The two photons originally proposed by J. D. Franson are realized as a photon pair due to collective effects, which are generated from the cascade atomic system with a relatively long lifetime of the initial state and a considerably shorter lifetime of the intermediate state. The achievement of two-photon interference with photon-pair sources generated from inhomogeneous atomic ensembles constitutes an important result for time-energy entanglement based on an atom-photon interaction.

DOI
10.1103/physrevlett.121.263601

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We experimentally demonstrate two-photon interference of a time-energy entangled photon pair generated via collective two-photon coherence in Doppler-broadened cascade-type ^{87}Rb atoms. The two photons originally proposed by J. D. Franson are realized as a photon pair due to collective effects, which are generated from the cascade atomic system with a relatively long lifetime of the initial state and a considerably shorter lifetime of the intermediate state. The achievement of two-photon interference with photon-pair sources generated from inhomogeneous atomic ensembles constitutes an important result for time-energy entanglement based on an atom-photon interaction.
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.
We experimentally demonstrate Franson interference of a time–energy entangled photon pair generated via collective two-photon coherence in the 5S1/2−5P3/2−5D5/2 transition of warm Rb87 atoms. The two unbalanced Michelson interferometers used in our setup are spatially separated in order to understand entanglement as a nonlocal property of the photon pairs from the warm atomic ensemble. We observe a Franson interference fringe with a high visibility of 99.1±1.3% with continuous-wave-mode photon pairs from the cascade-type atomic ensemble. To the best of our knowledge, this work is the first demonstration of the nonlocal two-photon interference experiment in separated photon channels by use of two-photon pairs emitted from a cascade-type atomic system as originally proposed by Franson [Phys. Rev. Lett.62, 2205 (1989)PRLTAO0031-900710.1103/PhysRevLett.62.2205].
We experimentally demonstrate nonlocal two-photon interference of energy-time entangled photon pairs via spontaneous four-wave mixing (SFWM) process in Doppler-broadened ladder-type 87Rb^{87}\mathrm{Rb}87Rb atoms. The Doppler-broadened ladder-type atomic system offers collective two-photon coherence which results in strong temporal correlation between the entangled photons. The spacelike separated photon pairs clearly show nonlocal two-photon interference with high visibility of $97.1\ifmmode\pm\else\textpm\fi{}2.5%$, which violates the Bell-CHSH (Clauser-Horne-Shimony-Holt) inequality by 10.5 standard deviations, in which the travel time of a single-photon towards the interferometer is much longer than the coincidence window. Our work will provide an atom-based entangled photon source which is applicable for long distance atom-memory-based quantum communication.
Abstract Photonic quantum states generated from atomic ensembles will play important roles in future quantum networks and long-distance quantum communication because their advantages, such as universal identity and narrow spectral bandwidth, are essential for quantum nodes and quantum repeaters based on atomic ensembles. In this study, we report the collectively coherent superposition of biphoton wavefunction emitted from different velocity classes in a Doppler-broadened cascade-type atomic ensemble. We experimentally demonstrate that the three times difference of temporal width of both biphoton temporal waveforms varies dependent on the wavelengths of the signal and idler photons from both 6S1/2–6P3/2–6D5/2 and –8S1/2 transitions of 133Cs, corresponding to the idler and signal wavelengths of 852 nm–917 nm and 852 nm–795 nm, respectively. Our results help understand the characteristics of biphoton sources from a warm atomic ensemble and can be applied to long-distance quantum networks and practical quantum repeaters based on atom–photon interactions.
We report a polarization-entangled photon-pair source obtained via spontaneous four-wave mixing (SFWM) in a Doppler-broadened atomic ensemble of ^{87}Rb atoms using a Sagnac interferometer. Collective two-photon coherence occurs in the Doppler-broadened ladder-type atomic system with bidirectional counterpropagating two-photon resonant pump and coupling fields; hence, polarization-entangled photon pairs are collectively radiated in the phase-matched direction. Without phase stabilization of the interferometry for polarization entanglement, we robustly produce all four Bell states via a polarization Sagnac configuration. The brightness, stability, and temporal purity advantages provided by our polarization-entangled SFWM photon-pair source have very important applications in the context of a practical scalable quantum network.
Abstract Bright, entangled multiphoton sources based on atom–photon interactions are an essential requirement in the realization of several quantum information and quantum computation schemes based on photonic quantum systems. Here, a four‐photon polarization‐entangled Greenberger–Horne–Zeilinger (GHZ) state obtained from Doppler‐broadened atomic ensembles of 87 Rb atoms is experimentally demonstrated. Owing to collective two‐photon coherence in the Doppler‐broadened cascade‐type atomic system, the setup enables the generation of robust four‐photon GHZ states with a fidelity of 82.1% and a measured four‐photon average coincidence rate of 0.58 s −1 . It is believed that the generation of such bright and stable multiphoton GHZ states from atomic media is an important step toward realizing photonic quantum computation and practical quantum networks based on atom–photon interactions.