Cavity-Enhanced Atom-Photon Entanglement with Subsecond Lifetime

Xu-Jie Wang, Sheng-Jun Yang, Peng-Fei Sun, Bo Jing, Jun Li, Ming-Ti Zhou, Xiao-Hui Bao, and Jian-Wei Pan

Phys. Rev. Lett. 126, 090501 – Published 2 March 2021

Abstract

A cold atomic ensemble suits well for optical quantum memories, and its entanglement with a single photon forms the building block for quantum networks that give promise for many revolutionary applications. Efficiency and lifetime are among the most important figures of merit for a memory. In this Letter, we report the realization of entanglement between an atomic ensemble and a single photon with subsecond lifetime and high efficiency. We engineer dual control modes in a ring cavity to create entanglement and make use of three-dimensional optical lattice to prolong memory lifetime. The memory efficiency is 38% for 0.1 s storage. We verify the atom-photon entanglement after 1 s storage by testing the Bell inequality with a result of $S = 2.36 \pm 0.14$ .

Received 2 November 2020
Accepted 1 February 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.090501

Physics Subject Headings (PhySH)

Cavity quantum electrodynamics Entanglement manipulation Entanglement production Quantum information with atoms & light Quantum memories Quantum networks Quantum repeaters Quantum state engineering

Atomic, Molecular & OpticalQuantum Information, Science & Technology

Authors & Affiliations

Xu-Jie Wang ^1,2,*, Sheng-Jun Yang ^1,2,*, Peng-Fei Sun ^1,2, Bo Jing ^1,2, Jun Li ^1,2, Ming-Ti Zhou^1,2, Xiao-Hui Bao ^1,2, and Jian-Wei Pan^1,2

¹Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
²CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

^*These authors contributed equally to this work.

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Vol. 126, Iss. 9 — 5 March 2021

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Figure 1
Schematic experimental setup and atomic energy levels. The ${}^{87}{Rb}$ atoms are trapped in a 3D optical lattice formed by interfering four circularly polarized 1064 nm laser beams. The lattice beams are not shown in the picture for the sake of simplicity, and more details about the optical lattice can be found in our former publication [31]. The phase difference between two write-read paths is locked by a PID control loop. Angles between the write-out signal and the two write beams are 2.5° and $- 4.2 °$ , respectively. A three-mirror cavity is placed around the atomic ensemble to increase the retrieval efficiency with whose beam waist matching the write(read)-out signal. The cavity has a beam waist radius of $\sim 60 μ m$ , a finesse of $43.4 / 44.3$ for H/V polarized light, and a free spectral range of 484 MHz. The cavity is also locked by a PID control loop. PZT1 (piezoelectric ceramic transducer) is used to lock the cavity by displacing a HR mirror, and PZT2 is used to lock the W-R interferometer. Write-out and read-out photons are analyzed in polarization, and detected with single-photon detectors (SPD). We use a pumped Rb vapor cell to filter leakage from the read beam to the read-out channel. Some other parameters and symbols are listed as following. Cavity-locking beam: 796 nm, $0.5 μ W$ . Phase-locking beam: 796 nm, $0.5 μ W$ . Write beam: red detuned $- 30 MHz$ , $2 μ W$ . Read beam: blue detuned $+ 36 MHz$ , $320 μ W$ . Beam radii of the write and read lasers are both $300 μ m$ . Polarizing beam splitter (PBS), half-wave plate (HWP), quarter-wave plate (QWP). partially reflecting (PR) mirror with reflectivity of 90.4(2)%, highly reflecting (HR) mirror. Lens: focal length 250 mm. $δ$ : electronic delay around several $μ s$ .
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Figure 2
Comparison of one and two control mode retrieval efficiencies. (a) Intrinsic retrieval efficiencies at different lattice trap powers. Blue circles indicate memory efficiencies with a single control mode, and orange open circles indicate memory efficiencies with double control modes. (b) Double-mode efficiencies plotted as a function of single-mode efficiencies. Slopes of two dashed lines are 0.5 and 1, respectively. A solid curve in black refers to the theoretical model, which is discussed in detail in the Supplemental Material [49].
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Figure 3
Intrinsic retrieval efficiencies and entanglement visibilities versus storage time. (a) Blue circles represent efficiencies of a single-mode quantum memory at different storage time. We fit the data with the function $η (t) = A_{1} \exp (- t / τ_{1}) + A_{2} \exp (- t / τ_{2})$ , resulting in $A_{1} = 0.26 \pm 0.04$ , $A_{2} = 0.51 \pm 0.01$ , $τ_{1} = 0.047 \pm 0.018 ms$ , $τ_{2} = 697 \pm 32 ms$ . Orange circles represent efficiencies of the dual-mode storage. With the same fit function, we get the result of $A_{1} = 0.17 \pm 0.02$ , $A_{2} = 0.41 \pm 0.01$ , $τ_{1} = 0.060 \pm 0.021 ms$ , $τ_{2} = 703 \pm 31 ms$ . (b) Red solid circles show the $H / V$ (horizontal/vertical) visibilities of the entanglement and brown open circles show the $D / A$ (diagonal/anti-diagonal) visibilities. Red and brown dashed lines are the average values within 1 s. The gray dashed line is the threshold of 0.707 to violate the Bell inequality.
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