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arXiv:2406.04272 · Optica Quantum

Entangling quantum memories at channel capacity

Prajit Dhara, Liang Jiang, Saikat Guha

Published Sep 22, 2025• 0 citations
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Abstract

Entangling quantum memories, mediated by optical-frequency or microwave channels, at high rates and fidelities is key for linking qubits across short and long ranges. All well-known protocols encode up to one qubit per optical mode, entangling one pair of memory qubits per transmitted mode over the channel, with probability η, the channel’s transmissivity. The rate is proportional to η ideal Bell states (ebits) per mode. The quantum capacity, C(η)=−log2⁡(1−η) ebits per mode, which ≈1.44η for high loss, thereby making these schemes near rate-optimal. However, C(η)→∞ as η→1, making the known schemes highly rate-suboptimal for shorter ranges. We propose a cavity-assisted memory–photon interface that can be used to entangle matter memories with Gottesman–Kitaev–Preskill (GKP) photonic qudits, which, along with dual-homodyne entanglement swaps that retain analog information, enables entangling memories at capacity-approaching rates at low loss. We benefit from the loss resilience of GKP-encoded states and the ability to encode multiple qubits in one mode. Our memory–photon interface further supports the preparation of needed ancilla GKP qudits. We expect our result to spur research in low-loss, high-cooperativity cavity-coupled qubits with high-efficiency optical coupling and demonstrations of high-rate short-range quantum links.

DOI
10.1364/opticaq.570931

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Entangling quantum memories, mediated by optical-frequency or microwave channels, at high rates and fidelities is key for linking qubits across short and long ranges. All well-known protocols encode up to one qubit per optical mode, entangling one pair of memory qubits per transmitted mode over the channel, with probability η, the channel’s transmissivity. The rate is proportional to η ideal Bell states (ebits) per mode. The quantum capacity, C(η)=−log2⁡(1−η) ebits per mode, which ≈1.44η for high loss, thereby making these schemes near rate-optimal. However, C(η)→∞ as η→1, making the known schemes highly rate-suboptimal for shorter ranges. We propose a cavity-assisted memory–photon interface that can be used to entangle matter memories with Gottesman–Kitaev–Preskill (GKP) photonic qudits, which, along with dual-homodyne entanglement swaps that retain analog information, enables entangling memories at capacity-approaching rates at low loss. We benefit from the loss resilience of GKP-encoded states and the ability to encode multiple qubits in one mode. Our memory–photon interface further supports the preparation of needed ancilla GKP qudits. We expect our result to spur research in low-loss, high-cooperativity cavity-coupled qubits with high-efficiency optical coupling and demonstrations of high-rate short-range quantum links.
Single photons are the flying qubits of choice for distributing entanglement in a quantum internet. Quantum memories embedded in quantum repeaters are crucial to overcome transmission loss and enhance the rate of quantum communication. A multimode memory can further boost the channel capacity. However, benchmarking and building a practical quantum memory that simultaneously optimizes multiple performance metrics poses two key challenges. Here, we introduce quantum interconnect rate to comprehensively quantify quantum memories, and further demonstrate a high-performance quantum memory that simultaneously integrates three essential criteria at once: large multimode capacity, high efficiency, and high fidelity. Operating on 11-dimensional spatial modes, our memory achieves a uniform efficiency exceeding 80% and qubit storage fidelities above 99%, enabling the efficient storage of high-dimensional qudits. Based on these capabilities, we estimate a distribution of 3.56 bits of quantum information over a 1000-km repeater link in one minute, highlighting a practical pathway toward scalable quantum interconnects and quantum networks.
Gottesman-Kitaev-Preskill (GKP) states have been demonstrated to pose significant advantages when utilized for fault-tolerant all optical continuous-variable quantum computing as well as for quantum communications links for entanglement distribution. However interfacing these systems to long-lived solid-state quantum memories has remained an open problem. Here we propose an interface between quantum memories and GKP qubit states based on a cavity-mediated controlled displacement gate. We characterize the quality of memory-GKP entanglement as a function of cavity parameters suggesting optimal regimes of operation for high-quality state transfer between either qubit states. We further extend this protocol to demonstrate the creation of GKP cluster states by avoiding the requirement of ancillary optical quadrature-squeezed light. Utilizing post-selected entanglement swapping operations for GKP qubits, we demonstrate the utility of our protocol for high-rate entanglement generation between quantum memories. Extensions and derivatives of our proposal could enable a wide variety of applications by utilizing the operational trade-offs for qubits encoded in memory and in the GKP basis.
Scalable quantum networks require the efficient generation and storage of entanglement between photonic qubits and quantum memories. Quantum repeaters based on absorptive rare-earth-ion photonic memories offer a promising route toward highly multiplexed quantum networking, but unifying spectrally matched photon sources and quantum memories within a common architecture remains a major challenge. Here we demonstrate an integrated photonic architecture for telecom photon-memory entanglement generation based on dual self-similar silicon carbide microring resonators. Connected by a fiber link, one resonator operates as an entangled photon-pair source, while the other functions as a cavity-enhanced atomic-frequency-comb quantum memory. The memory resonator reaches an ensemble cooperativity of 1.9 after hyperfine initialization and is spectrally matched to the source, enabling storage of entangled telecom photons without spectral modification. We generate and characterize photon-memory entanglement from a quantum interference visibility preserved before and after storage. Harnessing the strong source correlations and the high multimode capacity of the memory, we access high-dimensional entanglement spanning 63 temporal modes, reaching a maximum photon information efficiency of 5.1 Ebits per detected photon and a peak on-chip photon-memory entanglement rate of 5.6 kEbits per second. These results establish a route toward chip-scale quantum networking hardware operating over telecommunications infrastructure.
Quantum repeaters are a promising platform for realizing long-distance quantum communication and thus could form the backbone of a secure quantum internet, a scalable quantum network, or a distributed quantum computer. Repeater protocols that encode information in single- or multiphoton states are limited by transmission losses and the cost of implementing entangling gates or Bell measurements. In this work, we consider implementing a quantum repeater protocol using Gottesman-Kitaev-Preskill (GKP) qubits. These qubits are natural elements for quantum repeater protocols, because they allow for deterministic Gaussian entangling operations and Bell measurements, which can be implemented at room temperature. The GKP encoding is also capable of correcting small displacement errors. At the cost of additional Gaussian noise, photon loss can be converted into a random displacement error channel by applying a phase-insensitive amplifier. Here we show that a similar conversion can be achieved in two-way repeater protocols by using phase-sensitive amplification applied in the postprocessing of the measurement data, resulting in less overall Gaussian noise per (sufficiently short) repeater segment. We also investigate concatenating the GKP code with higher-level qubit codes while leveraging analog syndrome data, postselection, and path-selection techniques to boost the rate of communication. We compute the secure key rates and find that GKP repeaters can achieve a comparative performance relative to methods based on photonic qubits while using orders-of-magnitude fewer qubits.
High-order coherent-state modulation and collective midpoint measurements enable multi-ebit entanglement distribution between cavity-coupled atomic memories. An SRM-inspired 16-QAM receiver achieves 1.751.751.75 ebits per network mode use at 0.50.50.5~dB end-to-end loss, exceeding single-qubit-per-mode benchmarks by 5.85.85.8~dB and lying 3.83.83.8~dB below the half-link capacity bound. The advantage persists for per-interface loss below 0.20.20.2~dB and calibrated phase errors below 0.2π0.2π0.2π. For 4-QAM, an optimized POVM with twice as many outcomes raises the achievable rate by 4.0%4.0\%4.0%, revealing measurement-design headroom.