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Applied Physics Letters

Direct generation of polarization-entangled photons from warm atomic ensemble

Jiho Park, Jin‐Hyuk Bae, Heonoh Kim, Han Seb Moon

Published Aug 16, 2021• 20 citations
Publisher

Abstract

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.

DOI
10.1063/5.0045567

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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 report the stable generation of a bright four-photon polarization-entangled Greenberger–Horne–Zeilinger (GHZ) state obtained by the spatial multiplexing of inherently polarization-entangled photons from a warm atomic ensemble of 87Rb. Owing to the total angular momentum conservation of the atomic state in the spontaneous four-wave mixing process of a cascade-type atomic system, the polarizations of the signal and idler photons in the two-photon decay are entangled. With a Rb vapor cell, we realize the generation of robust and bright four-photon GHZ states using two symmetrical spatial modes of inherently polarization-entangled photon pairs without applying an interferometric configuration. The four-photon average coincidence rate of the GHZ state with a fidelity of 0.88(2) is measured as 1.69(7) Hz under a weak pump power of 10 μW. The minimum Allan variance for the stability of the four-photon GHZ state is estimated as 0.92° for an average time of 10 h.
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.
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.
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.
Abstract The realization of high-quality and stable entangled states is crucial for the advancement of quantum information science and technology. The entangled photons generated from a warm atomic ensemble exhibit high spectral purity and indistinguishability, offering significant potential for atom–photon-based quantum networks. We report high-quality time–frequency entangled continuous-wave (CW) photon pairs via a spontaneous four-wave mixing process in the cascade-type atomic transition of 5S 1/2 –5P 3/2 –5D 5/2 from a warm 87 Rb atomic ensemble. We observed Franson interference with the visibility of 99.6(8) % using two independent Franson interferometers, obtained by post-selecting the temporally indistinguishable two-photon events. The interferometer phases were actively stabilized using the reference laser frequency-locked to the 133 Cs (6S 1/2 -6P 3/2 ) transition. Beyond stabilizing the interferometers, the active control of these phases allowed us to measure the CHSH S -value directly and achieve a near-maximal Bell inequality violation with S = 2.83(7). This result demonstrates that warm atomic ensembles can serve as bright sources of time–frequency entanglement, which combined with active interferometer stabilization enable stable high-visibility operation for practical applications in quantum network.