Building a Quantum Repeater Using Optomechanical Oscillators as On-Demand Entanglement Sources

The core building block of a long-range quantum network is the repeater, which uses the principle of entanglement swapping to connect two spatially separated nodes. The key requirement is a reliable Einstein-Podolsky-Rosen (EPR) source that can create entanglement between a pair of spatially separated network nodes. A canonical entanglement swapping requires a Bell-basis joint detection of two photons (each is half of the entangled qubit pair in each node). The lack of reliable and deterministic EPR sources has been the major roadblock toward practical quantum repeaters. Here, we propose using an optomechanical oscillator, which produces a photon-phonon entangled pair, as a breakthrough on-demand EPR source. This is enabled by two key characteristics of optomechanical oscillators. First, phonon modes feature an extremely long lifetime. As such, if one oscillator’s phonon mode is populated first, the phonon qubit can be held until the other oscillator’s phonon mode is populated as well. Second, the photon-phonon coupling in an optomechanical oscillator is dynamically tunable. Consequently, once both oscillators’ phonon modes are populated, the coupling in both oscillators can be switched on for long enough such that a maximally entangled qubit pair is generated in each. We derive the entanglement swapping fidelity as a function of optical fiber loss, demonstrating a fidelity of 98% in the limit of negligible fiber loss.



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