Phonon-mediated quantum state transfer and remote qubit entanglement. (arXiv:1903.05672v1 [quant-ph])

Phonons, and in particular surface acoustic wave phonons, have been proposed
as a means to coherently couple distant solid-state quantum systems. Recent
experiments have shown that superconducting qubits can control and detect
individual phonons in a resonant structure, enabling the coherent generation
and measurement of complex stationary phonon states. Here, we report the
deterministic emission and capture of itinerant surface acoustic wave phonons,
enabling the quantum entanglement of two superconducting qubits. Using a 2
mm-long acoustic quantum communication channel, equivalent to a 500 ns delay
line, we demonstrate the emission and re-capture of a phonon by one qubit;
quantum state transfer between two qubits with a 67\% efficiency; and, by
partial transfer of a phonon between two qubits, generation of an entangled
Bell pair with a fidelity of $\mathcal{F}_B = 84 \pm 1$ %

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