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arXiv:2111.00381 · Physical Review A

Noise suppression in a temporal-multimode quantum memory entangled with a photon via an asymmetrical photon-collection channel

Ya Li, Yafei Wen, Minjie Wang, Chao Liu, Hailong Liu, Shujing Li, Zhongxiao Xu, Hai Wang

Published Aug 15, 2022• 8 citations
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Abstract

Quantum interfaces (QIs) that generate entanglement between a multimode atomic memory and a photon form a multiplexed repeater node and could greatly improve quantum repeater rates. Recently, a temporal multimode spin-wave memory entangled with a photon was demonstrated with cold atoms [Wen et al., Phys. Rev. A 100, 012342 (2019)]. However, due to the additional noise generated in a multimode operation, the fidelity of the spin-wave--photon entanglement significantly decreases with the mode number. So far, improvements in the temporal-multimode entanglement fidelity associated with suppressing the additional noise have not been explored. In this work, we propose and experimentally demonstrate a scheme that can suppress the additional noise of a temporally multiplexed QI. The scheme uses an asymmetric channel to retrieve the photons coming from the temporally multiplexed QI. For comparison purposes, we also set up a QI that uses a symmetric channel for photon collection. When the QIs store 14 modes, the measured Bell parameters $S$ for the QIs using the asymmetric and the symmetric photon-collection channels are 2.36 \ifmmode\pm\else\textpm\fi{} 0.03 and 2.24 \ifmmode\pm\else\textpm\fi{} 0.04, respectively, which means that a QI using an asymmetric channel provides a 3% increase in the entanglement fidelity, i.e., a 1.7-fold decrease in the additional noise, compared with a QI using a symmetric channel. In addition, the 14-mode entanglement QIs that use the asymmetric and symmetric collections preserve the violation of a Bell inequality for storage times of up to \ensuremath{\sim}25 and \ensuremath{\sim}20 \ensuremath{\mu}s, respectively, showing that the asymmetric QI has better entanglement storage performance.

DOI
10.1103/physreva.106.022610

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Future quantum repeater architectures, capable of efficiently distributing information encoded in quantum states of light over large distances, rely on quantum memories for light [1]. Quantum repeaters can benefit from a modal multiplexing implementation of the memory, essentially scaling up the repeater's throughput [2]. In this work we demonstrate a temporally multiplexed quantum repeater node in a laser-cooled cloud of 87-Rb atoms (as proposed in [3]). We employ the DLCZ protocol where pairs of photons and single collective spin excitations (so called spin-waves) are created [4]. The latter can then be efficiently transferred into a second single photon. For selective readout, we need to control the dephasing and rephasing of the spin-waves created in different temporal modes. We achieve this by a magnetic field gradient, which induces an inhomogeneous broadening of the involved atomic hyperfine levels [5]. By employing this steering technique, combined with cavity-enhanced noise suppression and feed forward readout, we demonstrate distinguishable retrieval of up to 10 temporal modes. For each mode, we prove non-classical correlations between the first and second photon. Furthermore, an enhancement in rates of correlated photon-photon pairs is observed as we increase the number of temporal modes stored in the memory. The reported device is a crucial key element of a quantum repeater architecture implementing multiplexed quantum memories. [1] H.-J. Briegel, W. Dür, J. Cirac and P. Zoller; Phys. Rev. Lett. 81 5932 (1998) [2] C. Simon, H. de Riedmatten, M. Afzelius,N. Sangouard, H. Zbinden and N. Gisin; Phys. Rev. Lett. 98 190503 (2007) [3] C. Simon, H. de Riedmatten and M. Afzelius; Phys. Rev. A 82 010304(R) (2010) [4] L. Duan, M. Lukin, J. Cirac and P. Zoller, P; Nature 414 413 (2001) [5] B.Albrecht, P. Farrera, G. Heinze, M. Cristiani and H. de Riedmatten; Phys. Rev. Lett. 115 160501 (2015)

SPIE Quantum Technologies•Mar 30, 2020•1 citation
Quantum interfaces (QIs) that generate entanglement between a multimode atomic memory and a photon form a multiplexed repeater node and could greatly improve quantum repeater rates. Recently, a temporal multimode spin-wave memory entangled with a photon was demonstrated with cold atoms [Wen et al., Phys. Rev. A 100, 012342 (2019)]. However, due to the additional noise generated in a multimode operation, the fidelity of the spin-wave--photon entanglement significantly decreases with the mode number. So far, improvements in the temporal-multimode entanglement fidelity associated with suppressing the additional noise have not been explored. In this work, we propose and experimentally demonstrate a scheme that can suppress the additional noise of a temporally multiplexed QI. The scheme uses an asymmetric channel to retrieve the photons coming from the temporally multiplexed QI. For comparison purposes, we also set up a QI that uses a symmetric channel for photon collection. When the QIs store 14 modes, the measured Bell parameters SSS for the QIs using the asymmetric and the symmetric photon-collection channels are 2.36 \ifmmode\pm\else\textpm\fi{} 0.03 and 2.24 \ifmmode\pm\else\textpm\fi{} 0.04, respectively, which means that a QI using an asymmetric channel provides a 3% increase in the entanglement fidelity, i.e., a 1.7-fold decrease in the additional noise, compared with a QI using a symmetric channel. In addition, the 14-mode entanglement QIs that use the asymmetric and symmetric collections preserve the violation of a Bell inequality for storage times of up to \ensuremath{\sim}25 and \ensuremath{\sim}20 \ensuremath{\mu}s, respectively, showing that the asymmetric QI has better entanglement storage performance.
A source that can generate atom-photon quantum correlations or entanglement based on a quantum memory is a basic building block of quantum repeaters (QRs). To achieve high entanglement generation rates in ensemble-based QRs, spatial-, temporal-, and spectral-multimode memories are needed. Previous temporal-multimode memories are based on rephasing mechanisms in inhomogeneously broadened media. Here, by applying a train of write pulses in time, with each pulse coming from a different direction, to a homogeneously broadened atomic ensemble to induce Duan-Lukin-Cirac-Zoller-like Raman processes, we prepare up to 19 pairs of modes, namely, one spin-wave mode and one photonic time bin. Spin-wave-photon (i.e., atom-photon) entanglement is probabilistically produced in these mode pairs. Based on the stored spin-wave modes together with feedforward-controlled readout, we build a temporally multiplexed source and then demonstrate an 18.8-fold increase in the probability of the generation of spin-wave-photon entanglement compared to the sources that use individual modes. The measured Bell parameter for the multiplexed source is $2.30\ifmmode\pm\else\textpm\fi{}0.02$, and the memory lifetime is 30 \ensuremath{\mu}s.
Practical realization of quantum repeaters requires quantum memories with high retrieval efficiency, multi-mode storage capacities, and long lifetimes. Here, we report a high-retrieval-efficiency and temporally multiplexed atom-photon entanglement source. A train of 12 write pulses in time is applied to a cold atomic ensemble along different directions, which generates temporally multiplexed pairs of Stokes photons and spin waves via Duan-Lukin-Cirac-Zoller processes. The two arms of a polarization interferometer are used to encode photonic qubits of 12 Stokes temporal modes. The multiplexed spin-wave qubits, each of which is entangled with one Stokes qubit, are stored in a "clock" coherence. A ring cavity that resonates simultaneously with the two arms of the interferometer is used to enhance retrieval from the spin-wave qubits, with the intrinsic retrieval efficiency reaching 70.4%. The multiplexed source gives rise to a ∼12.1-fold increase in atom-photon entanglement-generation probability compared to the single-mode source. The measured Bell parameter for the multiplexed atom-photon entanglement is 2.21(2), along with a memory lifetime of up to ∼125 µs.
Abstract Quantum interfaces (QIs) that generate entanglement between photonic and spin-wave (atomic memory) qubits are basic building block for quantum repeaters. Realizing ensemble-based repeaters in practice requires quantum memory providing long lifetimes and multimode capacity. Significant progress has been achieved on these separate goals. The remaining challenge is to combine the two attributes into a single QI. Here, by establishing spatial multimode, magnetic-field-insensitive and long-wavelength spin-wave storage in laser-cooled atoms inside a phase-passively-stabilized polarization interferometer, we constructed a multiplexed QI that stores up to three long-lived spin-wave qubits. Using a feed-forward-controlled system, we demonstrated that a multiplexed QI gives rise to a 3-fold increase in the atom–photon (photon–photon) entanglement-generation probability compared with single-mode QIs. For our multiplexed QI, the measured Bell parameter is 2.51±0.01 combined with a memory lifetime of up to 1 ms. This work represents a key step forward in realizing fiber-based long-distance quantum communications.
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 ∼sixfold (∼fourfold) probability increase in generating entangled atom-photon (photon-photon) pairs. The measured compositive Bell parameter for the multiplexed interface is 2.49±0.03 combined with a memory lifetime of up to ∼51 μs.
Future quantum repeater architectures, capable of efficiently distributing information encoded in quantum states of light over large distances, will benefit from multiplexed photonic quantum memories. In this work we demonstrate a temporally multiplexed quantum repeater node in a laser-cooled cloud of ^{87}Rb atoms. We employ the Duan-Lukin-Cirac-Zoller protocol where pairs of photons and single collective spin excitations (so-called spin waves) are created in several temporal modes using a train of write pulses. To make the spin waves created in different temporal modes distinguishable and enable selective readout, we control the dephasing and rephasing of the spin waves by a magnetic field gradient, which induces a controlled reversible inhomogeneous broadening of the involved atomic hyperfine levels. We demonstrate that by embedding the atomic ensemble inside a low finesse optical cavity, the additional noise generated in multimode operation is strongly suppressed. By employing feed forward readout, we demonstrate distinguishable retrieval of up to 10 temporal modes. For each mode, we prove nonclassical correlations between the first and second photon. Furthermore, an enhancement in rates of correlated photon pairs is observed as we increase the number of temporal modes stored in the memory. The reported capability is a key element of a quantum repeater architecture based on multiplexed quantum memories.
Future quantum repeater architectures, capable of efficiently distributing information encoded in quantum states of light over large distances, rely on quantum memories for light [1]. Quantum repeaters can benefit from a modal multiplexing implementation of the memory, essentially scaling up the repeater's throughput [2]. In this work we demonstrate a temporally multiplexed quantum repeater node in a laser-cooled cloud of 87-Rb atoms (as proposed in [3]). We employ the DLCZ protocol where pairs of photons and single collective spin excitations (so called spin-waves) are created [4]. The latter can then be efficiently transferred into a second single photon. For selective readout, we need to control the dephasing and rephasing of the spin-waves created in different temporal modes. We achieve this by a magnetic field gradient, which induces an inhomogeneous broadening of the involved atomic hyperfine levels [5]. By employing this steering technique, combined with cavity-enhanced noise suppression and feed forward readout, we demonstrate distinguishable retrieval of up to 10 temporal modes. For each mode, we prove non-classical correlations between the first and second photon. Furthermore, an enhancement in rates of correlated photon-photon pairs is observed as we increase the number of temporal modes stored in the memory. The reported device is a crucial key element of a quantum repeater architecture implementing multiplexed quantum memories. [1] H.-J. Briegel, W. Dür, J. Cirac and P. Zoller; Phys. Rev. Lett. 81 5932 (1998) [2] C. Simon, H. de Riedmatten, M. Afzelius,N. Sangouard, H. Zbinden and N. Gisin; Phys. Rev. Lett. 98 190503 (2007) [3] C. Simon, H. de Riedmatten and M. Afzelius; Phys. Rev. A 82 010304(R) (2010) [4] L. Duan, M. Lukin, J. Cirac and P. Zoller, P; Nature 414 413 (2001) [5] B.Albrecht, P. Farrera, G. Heinze, M. Cristiani and H. de Riedmatten; Phys. Rev. Lett. 115 160501 (2015)