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arXiv:2505.02702 · Physical Review Letters

Efficient and High-Fidelity Entanglement in Cavity QED without High Cooperativity

S. Goswami, C.-H. Chien, N. Sinclair, B. Grinkemeyer, Shayne Bennetts, Y.-C. Chen, H. H. Jen

Published Dec 9, 2025• 1 citation
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Abstract

The so-called state-carving protocol generates high-fidelity entangled states at an atom-cavity interface without requiring high cavity cooperativity. However, this protocol is limited to 50% efficiency, which restricts its applicability. We propose a simple modification to the state-carving protocol to achieve efficient entanglement generation, with unit probability in principle. Unlike previous two-photon schemes, ours employs only one photon which interacts with the atoms twice-avoiding separate photon detections which causes irrecoverable probability loss. We present a detailed description and performance evaluation of our protocol under nonideal conditions. High fidelity of 0.999 can be achieved with cavity cooperativity of only 34. Efficient state carving paves the way for large-scale entanglement generation at cavity interfaces for modular quantum computing, quantum repeaters, and creating arbitrary shaped atomic graph states, essential for one-way quantum computing.

DOI
10.1103/n9wg-k6q9

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The so-called state-carving protocol generates high-fidelity entangled states at an atom-cavity interface without requiring high cavity cooperativity. However, this protocol is limited to 50% efficiency, which restricts its applicability. We propose a simple modification to the state-carving protocol to achieve efficient entanglement generation, with unit probability in principle. Unlike previous two-photon schemes, ours employs only one photon which interacts with the atoms twice-avoiding separate photon detections which causes irrecoverable probability loss. We present a detailed description and performance evaluation of our protocol under nonideal conditions. High fidelity of 0.999 can be achieved with cavity cooperativity of only 34. Efficient state carving paves the way for large-scale entanglement generation at cavity interfaces for modular quantum computing, quantum repeaters, and creating arbitrary shaped atomic graph states, essential for one-way quantum computing.
We demonstrate entanglement generation of two neutral atoms trapped inside an optical cavity. Entanglement is created from initially separable two-atom states through carving with weak photon pulses reflected from the cavity. A polarization rotation of the photons heralds the entanglement. We show the successful implementation of two different protocols and the generation of all four Bell states with a maximum fidelity of (90±2)%. The protocol works for any distance between cavity-coupled atoms, and no individual addressing is required. Our result constitutes an important step towards applications in quantum networks, e.g., for entanglement swapping in a quantum repeater.
Abstract Scalable graph states are essential for measurement-based quantum computation and many entanglement-assisted applications in quantum technologies. Generation of these multipartite entangled states requires a controllable and efficient quantum device with delicate design of generation protocol. Here we propose to prepare high-fidelity and scalable graph states in one and two dimensions, which can be tailored in an atom-nanophotonic cavity via state carving technique. We propose a systematic protocol to carve out unwanted state components, which facilitates scalable graph states generations via adiabatic transport of a definite number of atoms in optical tweezers. An analysis of state fidelity is also presented, and the state preparation probability can be optimized via multiqubit state carvings and sequential single-photon probes. Our results showcase the capability of an atom-nanophotonic interface for creating graph states and pave the way toward novel problem-specific applications using scalable high-dimensional graph states with stationary qubits.
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Summary form only given. Neutral atoms trapped inside an optical cavity provide an ideal platform for the implementation of quantum networks [1]. In such a network, nodes containing multiple atomic qubits are essential for the construction of a quantum repeater as they allow for entanglement swapping and thus the generation of entanglement between qubits over long distances. Here we will show the realisation of such a multi-qubit network node containing two87Rb atoms (see Figure. 1) in an optical cavity. Local entanglement between the two atoms is created with an experimental technique called quantum state carving. It relies on the reflection of weak coherent pulses from the cavity and subsequent measurement of the photons polarisation [2]. Postselection on the cases where the photon changed its linear polarisation by 90° after the reflection projects the atomic state onto an entangled state. This measurement technique is intrinsically probabilistic, but heralded. We will show the generation of all four maximally entangled atomic Bell states, a key resource for a plethora of quantum communication protocols. We will give an overview over the necessary experimental toolbox including the collective manipulation and readout of the two atomic qubits.
We propose a heralded high-probability scheme to generate remote entanglement between moderate-cooperativity spin-cavity registers with high fidelity. In conventional single-shot interfaces, limited cooperativity restricts the spin-conditional optical response and thus strongly suppresses the success probability. Our proposal instead recycles a single incident photon for repeated interactions with the spin-cavity register, such that a small spin-conditional phase shift acquired on each round trip accumulates coherently to enable remote entanglement. Moreover, the repeated scheme enables higher spin-photon encoding efficiency by using a spectral-width-scaling photon pulse with a shorter duration. We show that, for realistic imperfections and losses, this repeated phase-encoding approach produces high-fidelity entangled states with an appreciable success probability even at cooperativity C∼1C\sim1C∼1. Our protocol is particularly well suited to weakly coupled, cavity-based solid-state spin platforms and provides a route toward hybrid, photon-loss-tolerant distributed quantum computing.
We propose a versatile and efficient method to generate a broad class of complex entangled states of many atoms via the detection of a single photon. For an atomic ensemble contained in a strongly coupled optical cavity illuminated by weak single- or multifrequency light, the atom-light interaction entangles the frequency spectrum of a transmitted photon with the collective spin of the atomic ensemble. Simple time-resolved detection of the transmitted photon then projects the atomic ensemble into a desired pure entangled state. This method can be implemented with existing technology, yields high success probability per trial, and can generate complex entangled states such as mesoscopic superposition states of coherent spin states with high fidelity.