Hybrid Quantum Repeater Chains with Atom-based Quantum Processing Units and Quantum Memory Multiplexers

TL;DR

This work proposes a hybrid quantum repeater that integrates SPDC photon sources, AFC quantum memories, and cavity-coupled atom-based QPUs to achieve high-rate, long-distance entanglement distribution. By leveraging heavy spectro-temporal multiplexing and a staged loading of photonic entanglement into matter qubits, the architecture enables deterministic processing after heralded remote entanglement generation. The authors develop error-suppression strategies (PNR detectors, EPL distillation, and the RE trick) and demonstrate, through numerical simulations, that the hybrid design can surpass atom-based repeaters in key regimes, with best performance tied to optimized multiplexing and improved local efficiencies. The study provides a concrete, near-term blueprint for scalable quantum networks, while identifying practical challenges and avenues for nested distillation and deterministic loading to extend performance across many hops.

Abstract

Quantum repeaters enable the generation of reliable entanglement across long distances despite the underlying channel noise. Nevertheless, realizing quantum repeaters poses a difficult engineering challenge due to various device constraints and design tradeoffs. Herein, we propose and analyze an efficient hybrid quantum repeater design that integrates atom-based quantum processing units, spontaneous parametric down-conversion photon sources, and atomic frequency comb quantum memories. Our design leverages the strong spectro-temporal multiplexing capability of the quantum memory to enable high-rate elementary-link entanglement generation between repeater nodes. Transferring the photonic entanglement into matter-qubit entanglement, together with deterministic quantum operations, further enables reliable long-distance entanglement distribution. We analyze photon-loss channels in the hybrid architecture and propose suitable error-suppression strategies that are natively incorporated into our repeater protocol. Using numerical simulations, we demonstrate the advantages of our hybrid design for end-to-end secret key rates in a linear repeater-chain model. With continued advances in relevant hardware technologies, we envision that the proposed hybrid design is well-suited for large-scale quantum networks.

Figures (12)

• Figure 1: An elementary link for atom-based quantum repeaters. An elementary link of the atom-based repeater comprises two atom-based QPUs and an entanglement swapper. Each QPU prepares atom-photon entanglement locally and sends the photon to a remote entanglement swapper. The inset shows the energy diagram of the atom in the QPU. The cycling transition $| B \rangle\leftrightarrow| e \rangle$ couples to the cavity. A single-click event in the entanglement swapper heralds the generation of entanglement between remote QPUs.
• Figure 2: An elementary link for SPDC-based quantum repeaters. The SPDC sources are first pumped to generate entangled photon states in the Fock basis. For each entangled state, one mode is sent to a remote entanglement swapper, and another mode is stored in a QM (we assumed an AFC-type QM). The center entanglement swapper heralds the generation of entanglement between remote QMs.
• Figure 3: Frequency multiplexed remote entanglement generation. (a) The frequency spectrum of pump, signal, and idler modes when frequency multiplexing is used. The shaded area corresponds to particular frequency bins for a signal-idler pair (indexed by $m$, centered around the pump frequency). The frequency of the signal mode is denoted by $f_{s,m}$ and the frequency of the idler mode is denoted by $f_{i,m}$. (b) Illustration for the SPDC emission modes and their respective destinations.
• Figure 4: Architecture for the hybrid quantum repeater (a one-hop chain). A one-hop linear hybrid quantum repeater chain is illustrated. Each entanglement generation unit comprises an SPDC source, an AFCQM, a loading swapper, and an atom-based QPU. SPDC sources and remote entanglement swappers generate the memory-memory entanglement heralded by a single-photon detection at the remote entanglement swapper. The entanglement is then loaded into QPUs with additional local entanglement swapping and thus can be processed with deterministic quantum operations. The intermediate QPU has an additional optical switch (OS) such that the intermediate QPU can generate entanglement with both sides. The blue dashed lines indicate the elementary links of the hybrid repeater chain.
• Figure 5: The extreme photon loss (EPL) protocol. The EPL protocol takes two noisy entangled pairs, performs bilateral CNOTs, and measures the target qubits. If the measurement outcomes are both $1$ , the unmeasured pair is kept. Otherwise, it is discarded.