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Physical Review A

Self-stabilized narrow-bandwidth and high-fidelity entangled photons generated from cold atoms

Ying Yu, Dong-Sheng Ding, Ming‐Xin Dong, Shuai Shi, Weijie Zhang, Bao‐Sen Shi

Published Apr 9, 2018• 10 citations
Publisher

Abstract

Entangled photon pairs are critically important in fundamental quantum mechanics research as well as in many areas within the field of quantum information, such as quantum communication, quantum computation, and quantum cryptography. Previous demonstrations of entangled photons based on atomic ensembles were achieved by using a reference laser to stabilize the phase of two spontaneous four-wave mixing paths. Here, we demonstrate a convenient and efficient scheme to generate polarization-entangled photons with a narrow bandwidth of $57.2\ifmmode\pm\else\textpm\fi{}1.6$ MHz and a high-fidelity of $96.3\ifmmode\pm\else\textpm\fi{}0.8%$ by using a phase self-stabilized multiplexing system formed by two beam displacers and two half-wave plates where the relative phase between the different signal paths can be eliminated completely. It is possible to stabilize an entangled photon pair for a long time with this system and produce all four Bell states, making this a vital step forward in the field of quantum information.

DOI
10.1103/physreva.97.043809

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We demonstrate an efficient experimental scheme for producing polarization-entangled photon pairs from spontaneous four-wave mixing (SFWM) in a laser-cooled (85)Rb atomic ensemble, with a bandwidth (as low as 0.8 MHz) much narrower than the rubidium atomic natural linewidth. By stabilizing the relative phase between the two SFWM paths in a Mach-Zehnder interferometer configuration, we are able to produce all four Bell states. These subnatural-linewidth photon pairs with polarization entanglement are ideal quantum information carriers for connecting remote atomic quantum nodes via efficient light-matter interaction in a photon-atom quantum network.
Vector vortex beams with varying polarization on the transversal profile can be used to increase information capacity and found applications in optical communication, optical trapping, high-resolution imaging, and quantum information. Here, we experimentally generate polarization-vector vortex mode hybrid entangled photon pairs in warm 85 Rb atomic vapor via spontaneous four-wave mixing (SFWM). Specifically, we firstly generate non-classical biphotons with high g (2) and then achieve polarization entanglement via coherent superposition of two SFWM paths by using beam displacers in a phase self-stabilized interferometer configuration. Finally, we realize polarization-vector vortex mode hybrid entanglement by utilizing q-plates, and a fidelity of 94.92% is obtained. This work may benefit the use of vector vortex beams in a quantum network based on atoms.
Multi-photon entangled states not only play a crucial role in research on quantum physics but also have many applications in quantum information fields such as quantum computation, quantum communication, and quantum metrology. To fully exploit the multi-photon entangled states, it is important to establish the interaction between entangled photons and matter, which requires that photons have narrow bandwidth. Here, we report on the experimental generation of a narrowband four-photon Greenberger-Horne-Zeilinger state with a fidelity of 64.9% through multiplexing two spontaneous four-wave mixings in a cold Rb85 atomic ensemble. The full bandwidth of the generated GHZ state is about 19.5 MHz. Thus, the generated photons can effectively match the atoms, which are very suitable for building a quantum computation and quantum communication network based on atomic ensembles.
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.
In quantum communication and photonic quantum information processing, the requirement of quantum repeaters and quantum memory often imposes a strict bandwidth prerequisite for the entangled photons. At the same time, there is ever more increasing demand for entangling more degrees of freedom, i.e., hyperentanglement, for a photon pair. In this Letter, we report the direct generation of narrow-band orbital angular momentum (OAM) and polarization hyperentangled photons from cold atoms. The narrow-band photon pair is naturally entangled in polarization and OAM, in addition to time-frequency, degrees of freedom due to spin and orbital angular momentum conservation conditions in the spontaneous four-wave mixing process in a cold atom ensemble. The narrow-band hyperentangled photon pair source reported here is expected to play important roles in quantum memory-based long-distance quantum communication.