Qubit--which-path entanglement and Heisenberg-limited interferometry

A crucial requirement for large-scale quantum information processing and quantum communication will be the development of quantum networks, with quantum information distributed at long range between nodes of stationary qubits coupled to quantum cavities. Most strategies for creating such a quantum network require difficult-to-realize single-photon sources and single-photon detectors. I will discuss new strategies that instead use classical (coherent-state) pulses and simpler measurements (homodyne detection). In particular, I will describe a novel quantum-optical effect that can be used to generate “which-path” entanglement between a stationary qubit and a coherent-state light pulse [1]. The which-path entangled state can then be used for long-range entanglement distribution between nodes in a quantum network. Moreover, in contrast to previous work suggesting that photon-counting is required to reach the Heisenberg limit of sensitivity with analogous path-entangled states, we show that qubit--which-path states and entangled coherent states can be used to reach the maximum sensitivity allowed by quantum mechanics (saturate the quantum Cramér-Rao bound) via simple homodyne detection [2].

[1] Z. M. McIntyre and W. A. Coish Phys. Rev. Lett. 132, 093603 (2024)

[2] Z. M. McIntyre and W. A. Coish Phys. Rev. A 110, L010602 (2024)