Spatial Multiplexing of Atom-Photon Entanglement Sources using Feedforward Control and Switching Networks

Long Tian, Zhongxiao Xu, Lirong Chen, Wei Ge, Haoxiang Yuan, Yafei Wen, Shengzhi Wang, Shujing Li, and Hai Wang

Phys. Rev. Lett. 119, 130505 – Published 29 September 2017

Abstract

The light-matter quantum interface that can create quantum correlations or entanglement between a photon and one atomic collective excitation is a fundamental building block for a quantum repeater. The intrinsic limit is that the probability of preparing such nonclassical atom-photon correlations has to be kept low in order to suppress multiexcitation. To enhance this probability without introducing multiexcitation errors, a promising scheme is to apply multimode memories to the interface. Significant progress has been made in temporal, spectral, and spatial multiplexing memories, but the enhanced probability for generating the entangled atom-photon pair has not been experimentally realized. Here, by using six spin-wave-photon entanglement sources, a switching network, and feedforward control, we build a multiplexed light-matter interface and then demonstrate a $\sim sixfold$ ( $\sim fourfold$ ) probability increase in generating entangled atom-photon (photon-photon) pairs. The measured compositive Bell parameter for the multiplexed interface is $2.49 \pm 0.03$ combined with a memory lifetime of up to $\sim 51 μ s$ .

Received 26 December 2016

DOI:https://doi.org/10.1103/PhysRevLett.119.130505

Physics Subject Headings (PhySH)

Entanglement production Quantum communication Quantum entanglement Quantum memories Quantum optics

Atomic ensemble

Atomic, Molecular & OpticalQuantum Information, Science & Technology

Authors & Affiliations

Long Tian, Zhongxiao Xu, Lirong Chen, Wei Ge, Haoxiang Yuan, Yafei Wen, Shengzhi Wang, Shujing Li, and Hai Wang^*

The State Key Laboratory of Quantum Optics and Quantum Optics Devices, Collaborative Innovation Center of Extreme Optics, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, People’s Republic of China

^*wanghai@sxu.edu.cn

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Vol. 119, Iss. 13 — 29 September 2017

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Images

Figure 1
Quantum repeater scheme based on the multiplexed interfaces. (a) Entanglement creation in an elementary link. $A$ and $B$ ensembles are located at the left and right end points of the link, respectively. Each one is synchronically excited by write-laser pulses, which generates $m$ SWPE sources, labeled by ${| Φ ⟩}_{A (B)}^{(1 st)}, \dots, {| Φ ⟩}_{A (B)}^{(i th)}, \dots, {| Φ ⟩}_{A (B)}^{(m th)}$ . The SWPE source ${| Φ ⟩}_{A}^{(i th)}$ [ ${| Φ ⟩}_{B}^{(i th)}$ ] emits a Stokes photon $S_{A_{i}} (S_{B_{i}})$ and creates one spin-wave excitation $M_{A_{i}} (M_{B_{i}})$ . The photons $S_{A_{i}}$ and $S_{B_{i}}$ are sent to the $i$ th middle station between $A$ and $B$ ensembles for the BSM. Conditioned on a successful BSM at the $k$ th station, for example, the ensembles $A$ and $B$ are projected into an entangled state ${| Φ ⟩}_{A B}^{(k th)}$ . (b) Entanglement swapping between two adjacent links. The ensemble $C$ ( $D$ ) also creates $m$ SWPE sources and a successful BSM at the $l th$ central station of the $C − D$ link, e.g., projects the $C$ and $D$ ensembles into an entangled state ${| Φ ⟩}_{C D}^{(l th)}$ . To performing an entanglement swapping between the $A − B$ and $C − D$ links, we convert the spin-wave excitation $M_{B_{k}} (M_{C_{l}})$ into an anti-Stokes photon $T_{B_{k}} (T_{C_{l}})$ and route it into a common channel $C_{B} (C_{C})$ by using a feedforward-controlled OSN.
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Figure 2
Overview of the experiment. (a) Experiment setup for the 6-SWPE sources in combination with feedforward control (Noted that we only plot 4-SWPE sources in the figure). PC. phase compensator (see Sec. II in Supplemental Material [56]); $A_{(1, 2, \dots, 6)}$ . acousto-optic modulators; ${WP}_{1, 2, \dots, 6, T}$ . half-wave or quarter-wave plates. In the measurements of the fidelities (Fig. 5) or polarization visibilities (Fig. 3), the ${WP}_{1, 2, \dots, 6, T}$ are half-wave (quarter-wave) plates when analyzing the photon polarization in the $D$ or $A$ ( $R$ or $L$ ) polarization setting and are removed when analyzing the photon polarization in $H$ or $V$ polarization setting (see Sec. V in Supplemental Material [56]). In the measurements of the Bell parameter, the ${WP}_{1, 2, \dots, 6, T}$ are half-wave plates and used for setting the polarization angles. (b) Relevant atomic levels. (c) Time sequence of an experimental cycle.
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Figure 3
Coincidences between the Stokes and anti-Stokes photons from the multiplexed LMEI at (a) $H − V$ , (b) $D − A$ , and (c) $R − L$ polarization settings for $p_{S}^{(m = 6)} \approx 1.26 %$ and $δ t = 1 μ s$ . Error bars represent $\pm 1$ standard deviation.
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Figure 4
The polarization visibility as functions of Stokes detection rate (a) and coincidence count rate (b) for the polarization setting of $H − V$ , respectively. The blue solid lines in (a) and (b) are the least-square fittings to the single-source data according to Eqs. (S48) and (S49) in Supplemental Material [56], respectively, while the red solid lines in (a) and (b) are the least-square fittings to the multiplexed data $V_{H − V}^{(M)}$ according to Eqs. (S57) and (S59) in Supplemental Material [56], respectively.
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Figure 5
Real and imaginary parts of the reconstructed density matrices $ρ_{r}^{(M, 1 st)}$ and $ρ_{r}^{(M, 3 rd)}$ of the two photons from the sources 1(a) and 3(b), respectively.
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Figure 6
Measurements of the Bell parameters $S^{(M)}$ and $\bar{S}$ as a function of $δ t$ for $p_{S}^{(6)} = 0.0297$ . Error bars represent $\pm 1$ standard deviation.
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