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Physical Review A

Carving entangled multiparticle states with exponentially improved fidelity

Joshua Ramette, Josiah Sinclair, Zeyang Li, Vladan Vuletić

Published May 14, 2025• 3 citations
Publisher

Abstract

We propose a method that uses the ``no-jump'' evolution of a probe to generate entangled multiparticle states of high fidelity. The probe is coupled to a target ensemble of qubits and engineered to exponentially decay at a rate depending on the target collective spin, such that postselecting on observing no probe decay precisely removes select faster-decaying spin components. When a probe and $N$-qubit target interact via a cavity mode of cooperativity $C$, our procedure generates entangled states with infidelities of ${e}^{\ensuremath{-}C/N}$, an exponential improvement over previous carving schemes. This scheme, which we call ``counterfactual'' carving, can generate complex entangled states for applications in quantum metrology and quantum computing.

DOI
10.1103/physreva.111.052426

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We propose a method that uses the no-jump'' evolution of a probe to generate entangled multiparticle states of high fidelity. The probe is coupled to a target ensemble of qubits and engineered to exponentially decay at a rate depending on the target collective spin, such that postselecting on observing no probe decay precisely removes select faster-decaying spin components. When a probe and $N$-qubit target interact via a cavity mode of cooperativity $C$, our procedure generates entangled states with infidelities of ${e}^{\ensuremath{-}C/N}$, an exponential improvement over previous carving schemes. This scheme, which we call counterfactual'' carving, can generate complex entangled states for applications in quantum metrology and quantum computing.
We propose a new method, "counter-factual" carving, that uses the "no-jump" evolution of a probe to generate entangled many-body states of high fidelity. The probe is coupled to a target ensemble of qubits and engineered to exponentially decay at a rate depending on the target collective spin, such that post-selecting on observing no probe decay precisely removes select faster-decaying spin components. When probe and NNN-qubit target interact via a cavity mode of cooperativity CCC, counter-factual carving generates entangled states with infidelities of e−C/Ne^{-C/N}e−C/N, an exponential improvement over previous carving schemes. Counter-factual carving can generate complex entangled states for applications in quantum metrology and quantum computing.
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 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.
We explore an intriguing alternative for a fast and high-fidelity generation of steady-state entanglement. By exponentially enhancing the atom-cavity interaction, we obtain an exponentially enhanced effective cooperativity of the system, which results in a high fidelity of the state generation. Meanwhile, we modulate the amplitudes of the driving fields to accelerate the population transfer to a target state, e.g., a Bell state. An exponentially shortened stabilization time is thus predicted. Specifically, when the cooperativity of the system is C=30C=30C=30, the fidelity of the acceleration scheme reaches 98.598.5%98.5, and the stabilization time is about ten times shorter than that without acceleration. Moreover, we find from the numerical simulation that the acceleration scheme is robust against systematic and stochastic (amplitude-noise) errors.
A scheme is presented for generating entangled states of multiple atoms in a cavity. In the scheme the atoms simultaneously interact with a cavity mode, with the first atom driven by two classical fields and the other atoms driven by a classical field. Our scheme is valid even if the cavity decay rate is larger than the effective coupling strength, which is important for experiment. The generation of entangled states is conditional on the detection of a photon decaying from the cavity and thus the fidelity of the entangled state is insensitive to the detection inefficiency. Furthermore, the scheme can be applied to the case with any number of atoms in principle.
The efficient generation of high-fidelity entangled states is the key element for long-distance quantum communication, quantum computation, and other quantum technologies, and at the same time the most resource-consuming part in many schemes. We present a class of entanglement-assisted entanglement purification protocols that can generate high-fidelity entanglement from noisy, finite-size ensembles with improved yield and fidelity as compared to previous approaches. The scheme utilizes high-dimensional auxiliary entanglement to perform entangling nonlocal measurements and determine the number and positions of errors in an ensemble in a controlled and efficient way, without disturbing the entanglement of good pairs. Our protocols can deal with arbitrary errors, but are best suited for few errors, and work particularly well for decay noise. Our methods are applicable to moderately sized ensembles, as will be important for near term quantum devices.