QUANTUM ACOUSTICS: Hong-Ou-Mandel Interference with Itinerant Surface Acoustic Phonons

Two-particle interference is a fundamental property of indistinguishable quantum-mechanical particles that has no classical counterpart. When two identical photons enter a beam splitter with a perfect temporal overlap, interference causes both photons to exit the beam splitter in the same output channel, with zero probability of single photons in either output channel, as first established by Hong, Ou and Mandel [C. K. Hong, Z. Y. Ou, L. Mandel, Phys. Rev. Lett. 59, 2044–2046 (1987)]. Here we demonstrate the Hong-Ou-Mandel (HOM) experiment with individual itinerant surface acoustic wave phonons. Single phonons released on demand by two superconducting qubits interfere in a phononic beam splitter, with the beam splitter output detected by the same two qubits. We observe the classic HOM “dip” in coincident output phonons with a visibility of 0.910±0.013. We also systematically study the temporal waveform and frequency indistinguishability of the two phonons. We further demonstrate a Mach-Zehnder-like interferometer, where a single phonon is “split” by the beam splitter, enabling subsequent phase-dependent interference. This experiment completes the toolbox for linear phononic circuits, analogous to linear photonic computing, with possible future extensions to other fundamental quantum acoustics experiments and applications in quantum information processing with phonons.