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arXiv:2306.06328

Decoherence of Single-Excitation Entanglement over DLCZ Quantum Networks Caused by Slow-Magnetic-Field Fluctuations and Protection Approach

Sun, Can, Ya Li, Yi-bo Hou, Minjie Wang, Shujing Li, H. Wang

Published Jun 10, 2023• 0 citations
arXivHTMLPDF

Abstract

Atomic spin-wave (SW) memory is a building block for quantum repeater. However, due to decoherence caused by atomic motions and magnetic-field gradients, SW retrieval efficiency decays with storage time. Slow magnetic-field fluctuations (SMFFs) lead to shot-to-shot phase noises of SWs, but have not been considered in previous repeater protocols. Here, we develop a SW model including a homogeneous phase noise caused by SMFFs, and then reveal that such phase noise induces decoherence of single-excitation entanglement between two atomic ensembles in a repeater link. For verify this conclusion, we experimentally prepare single-excitation entanglement between two SWs in a cold atomic ensemble and observe entanglement decoherence induced by SMFFs. Limited by SMFFs, the evaluated lifetime of the single-excitation entanglement over a repeater link is ~135 ms, even if the link uses optical-lattice atoms as nodes. We then present an approach that may overcome such limitation and extend this lifetime to ~1.7 s.

DOI
10.48550/arxiv.2306.06328

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Atomic spin-wave (SW) memory is a building block for quantum repeater. However, due to decoherence caused by atomic motions and magnetic-field gradients, SW retrieval efficiency decays with storage time. Slow magnetic-field fluctuations (SMFFs) lead to shot-to-shot phase noises of SWs, but have not been considered in previous repeater protocols. Here, we develop a SW model including a homogeneous phase noise caused by SMFFs, and then reveal that such phase noise induces decoherence of single-excitation entanglement between two atomic ensembles in a repeater link. For verify this conclusion, we experimentally prepare single-excitation entanglement between two SWs in a cold atomic ensemble and observe entanglement decoherence induced by SMFFs. Limited by SMFFs, the evaluated lifetime of the single-excitation entanglement over a repeater link is ~135 ms, even if the link uses optical-lattice atoms as nodes. We then present an approach that may overcome such limitation and extend this lifetime to ~1.7 s.
Abstract Atomic spin‐wave (SW) memory is a building block for quantum repeater. However, due to decoherence caused by atomic motions and magnetic‐field gradients, SW retrieval efficiency decays with storage time. Slow magnetic‐field fluctuations (SMFFs) lead to shot‐to‐shot phase noises of SWs, but have not been considered in previous repeater protocols. Here, a SW model including a homogeneous phase noise caused by SMFFs is developed, and then revealed that such phase noise induces decoherence of single‐excitation entanglement between two atomic ensembles in a repeater link. For verifying this conclusion, single‐excitation entanglement between two SWs in a cold atomic ensemble is experimentally prepared and observed entanglement decoherence induced by SMFFs. Limited by SMFFs, the evaluated lifetime of the single‐excitation entanglement over a repeater link is ≈135 ms, even if the link uses optical‐lattice atoms as nodes. Then an approach that may overcome such limitations and extend this lifetime to ≈1.7 s is presented.
Abstract Entanglement between a spin-wave qubit (memory qubit) and a photonic qubit is a basic building block for quantum repeaters. Duan-Lukin-Cirac-Zoller (DLCZ) scheme, which generates spin waves via spontaneous Raman scattering (SRS) of Stokes photons in atomic ensemble, provides a promising way to generate such entanglement. In a recent work [arXiv: 2006.05631, accepted by communications physics], DLCZ-like quantum memory that generates long-lived atom-photon entanglement has been experimentally demonstrated, where magnetic-field-insensitive (MFI) coherence is used to store spin waves. For realizing such MFI spin-wave storage, the atoms have to be initially prepared in a specific Zeeman sublevel, which is achieved by applying optical pumping lasers. Here, we demonstrate the memory lifetimes for the cases that the atoms are perfectly and imperfectly prepared in the specific Zeeman level, respectively. The experimental results show that the spin waves associated with magnetic-field-sensitive (MFS) and MFI coherences will be simultaneously created for the case that the atoms are imperfectly prepared in the Zeeman sublevel. Thus, the read outs will experience decay oscillations due to interferences between the two spin waves and the memory lifetime will be shorten due to dephasing of MFS coherence. A detailed theoretical analysis has been developed for explaining the experimental results. The present work will help one to understand decoherence of spin waves (SWs) and then enable one to obtain optimal lifetime of the entanglement storage in the cold atoms.
The quantum interface (QI) that generates non-classically-correlated pairs of spin-wave memories and photons is a building block (node) for DLCZ-like quantum repeater. The cross-correlation function, which characterizes quantum correlation of the pairs, and retrieval efficiency together with lifetime are important figures of merits of QIs. In this presented work, we generate highly retrievable spin-wave-photon pairs via cavity-enhanced DLCZ-like interaction between light and a cloud of cold atoms, where, the intrinsic retrieval efficiency reaches to ~77.1% together with a memory lifetime of 0.65 ms. The cross-correlation functions of the pairs as a function of the storage time and excitation probability are measured, respectively. More important, we develop a theoretical model to describe the measured results in quantity. The theoretical model includes the cavity-enhanced noise coming from the imperfect readouts and is in agreement with the measured dependence of the cross-correlation function on the storage time and excitation probability.
The quantum interface (QI) that generates non-classically-correlated pairs of spin-wave memories and photons is a building block (node) for DLCZ-like quantum repeater. The cross-correlation function, which characterizes quantum correlation of the pairs, and retrieval efficiency together with lifetime are important figures of merits of QIs. In this presented work, we generate highly retrievable spin-wave-photon pairs via cavity-enhanced DLCZ-like interaction between light and a cloud of cold atoms, where, the intrinsic retrieval efficiency reaches to ~77.1% together with a memory lifetime of 0.65 ms. The cross-correlation functions of the pairs as a function of the storage time and excitation probability are measured, respectively. More important, we develop a theoretical model to describe the measured results in quantity. The theoretical model includes the cavity-enhanced noise coming from the imperfect readouts and is in agreement with the measured dependence of the cross-correlation function on the storage time and excitation probability.
Entanglement swapping (ES) between memory repeater links is critical for establishing quantum networks via quantum repeaters. So far, ES with atomic-ensemble-based memories has not been achieved. Here, we experimentally demonstrated ES between two entangled pairs of spin-wave memories via Duan-Lukin-Cirac-Zoller scheme. With a cloud of cold atoms inserted in a cavity, we produce non-classically-correlated spin-wave-photon pairs in 12 spatial modes and then prepare two entangled pairs of spin-wave memories via a multiplexed scheme. Via single-photon Bell measurement on retrieved fields from two memories, we project the two remaining memories never entangled previously into an entangled state with the measured concurrence of C = 0.0124(0.003). The successful probability of ES in our scheme is increased by three times, compared with that in non-multiplexed scheme. Our presented work shows that the generation of entanglement (C>0) between the remaining memory ensembles requires the average cross-correlation function of the spin-wave-photon pairs to be >30 .
Entanglement swapping (ES) between memory repeater links is critical for establishing quantum networks via quantum repeaters. So far, ES with atomic-ensemble-based memories has not been achieved. Here, we experimentally demonstrate ES between different spatial modes for a cold-atom-ensemble quantum memory via the Duan-Lukin-Cirac-Zoller scheme. With a cloud of cold atoms inserted in a cavity, we produce non-classically correlated spin-wave-photon pairs in 12 spatial modes. We then prepare two entangled pairs of spin-wave modes through memory multiplexing. Via single-photon Bell-state measurement on retrieved fields from two spin-wave modes, we project the two remaining modes never entangled previously into an entangled state with a measured concurrence of C =0.0124±0.0030. The successful probability of ES in our scheme is increased by three times, compared with that in the scheme without memory multiplexing. Our presented work shows that the generation of entanglement ( C >0) between the remaining spin-wave modes requires the average cross-correlation function of the spin-wave-photon pairs to be ≥30. This presented work represents a significant step toward practical quantum repeaters.