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arXiv:1705.03679 · Physical Review Letters

Multimode and Long-Lived Quantum Correlations Between Photons and Spins in a Crystal

Cyril Laplane, Pierre Jobez, Jean Etesse, Nicolas Gisin, Mikael Afzelius

Published May 24, 2017• 119 citations

Abstract

The realization of quantum networks and quantum repeaters remains an outstanding challenge in quantum communication. These rely on the entanglement of remote matter systems, which in turn requires the creation of quantum correlations between a single photon and a matter system. A practical way to establish such correlations is via spontaneous Raman scattering in atomic ensembles, known as the Duan-Lukin-Cirac-Zoller (DLCZ) scheme. However, time multiplexing is inherently difficult using this method, which leads to low communication rates even in theory. Moreover, it is desirable to find solid-state ensembles where such matter-photon correlations could be generated. Here we demonstrate quantum correlations between a single photon and a spin excitation in up to 12 temporal modes, in a ^{151}Eu^{3+}-doped Y_{2}SiO_{5} crystal, using a novel DLCZ approach that is inherently multimode. After a storage time of 1 ms, the spin excitation is converted into a second photon. The quantum correlation of the generated photon pair is verified by violating a Cauchy-Schwarz inequality. Our results show that solid-state rare-earth-ion-doped crystals could be used to generate remote multimode entanglement, an important resource for future quantum networks.

DOI: 10.1103/physrevlett.118.210501

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The generation and distribution of quantum correlations between photonic qubits is a key resource in quantum information science. For applications in quantum networks and quantum repeaters, it is required that these quantum correlations be stored in a quantum memory. In 2001, Duan, Lukin, Cirac, and Zoller (DLCZ) proposed a scheme combining a correlated photon-pair source and a quantum memory in atomic gases, which has enabled fast progress towards elementary quantum networks. In this Letter, we demonstrate a solid-state source of correlated photon pairs with embedded spin-wave quantum memory, using a rare-earth-ion-doped crystal. We show strong quantum correlations between the photons, high enough for performing quantum communication. Unlike the original DLCZ proposal, our scheme is inherently multimode thanks to a built-in rephasing mechanism, allowing us to demonstrate storage of 11 temporal modes. These results represent an important step towards the realization of complex quantum networks architectures using solid-state resources.

The process of storing information carried by single photons demands strong light-matter interaction. While interfacing single photon sources with quantum memories is still challenging, a possible short-cut by combining source and memory in the same ensemble was proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) [1]. It is already a quite common protocol in cold atomic ensemble QMs, however it is very demanding to realize in solid state systems. Here we demonstrate for the first time a DLCZ-type memory with a rare-earth ion-doped crystal (REIC) with photon counting, directly applicable to the well-known quantum repeater schemes which works with QMs. We find second order cross correlation values up to 21±4 and violate Cauchy-Schwarz equality by R = 44 ± 20. Although the original DLCZ protocol does not support temporal multi-modality, we store 11 temporal modes thanks to a modified scheme in which we combine the DLCZ and the Atomic Frequency Comb protocols. Generating and storing non-classical multiplexed photon pairs with a single scheme in a REIC paves the way of making scalable, solid-state quantum repeater architectures.

The process of storing information carried by single photons demands strong light-matter interaction. While interfacing single photon sources with quantum memories is still challenging, a possible short-cut by combining source and memory in the same ensemble was proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) [1]. It is already a quite common protocol in cold atomic ensemble QMs, however it is very demanding to realize in solid state systems. Here we demonstrate for the first time a DLCZ-type memory with a rare-earth ion-doped crystal (REIC) with photon counting, directly applicable to the well-known quantum repeater schemes which works with QMs. We find second order cross correlation values up to 21±4 and violate Cauchy-Schwarz equality by R = 44 ± 20. Although the original DLCZ protocol does not support temporal multi-modality, we store 11 temporal modes thanks to a modified scheme in which we combine the DLCZ and the Atomic Frequency Comb protocols. Generating and storing non-classical multiplexed photon pairs with a single scheme in a REIC paves the way of making scalable, solid-state quantum repeater architectures.

The Duan-Lukin-Cirac-Zoller (DLCZ) scheme is a potential method to establish remote entanglements and realize large-scale quantum networks. Here we propose a DLCZ-like scheme based on the noiseless photon echo in rare-earth ion-doped crystals. Correlated photon pairs with a controllable delay can be created by the direct optical rephasing. Theoretical analysis indicates that the protocol is efficient in the low-optical-depth regime. This protocol could be feasibly implemented to establish long-lived quantum correlations between a photon and a spin-wave excitation in rare-earth ion-doped crystals.

We perform the experimental generation of pairs of photons on a solid-state rare-earth ion doped crystal of Eu3+:Y2SiO5, by using a DLCZ-like protocol designed for inhomogeneously broadened media. The idea relies on the use of the atomic frequency comb technique, in order to rephase the atoms for the emission of the photons. A specificity of this protocol is its high temporal multimode capacity, as many pairs of photons can be emitted at different instants in time. A Cauchy-Schwarz inequality violation of 2.88>1 is witnessed, proving the non-classical correlations of the photon pairs that we produce. A detailed analysis of the source and detection imperfections is conducted, revealing ways of increasing the quality of the pairs that are produced.

The generation and distribution of entanglement are key resources in quantum repeater schemes. Temporally multiplexed systems offer time-bin encoding of quantum information which provides robustness against decoherence in fibers, crucial in long distance communication. Here, we demonstrate the direct generation of entanglement in time between a photon and a collective spin excitation in a rare earth ion doped ensemble. We analyze the entanglement by mapping the atomic excitation onto a photonic qubit and by using time-bin qubit analyzers implemented with another doped crystal using the atomic frequency comb technique. Our results provide a solid-state source of entangled photons with embedded quantum memory. Moreover, the quality of the entanglement is high enough to enable a violation of a Bell inequality by more than two standard deviations.