A single-phonon directional coupler

Integrated photonics has enabled countless technologies in telecommunications, spectroscopy, metrology, quantum optics, and quantum information processing. Using highly confined guided optical modes is the key that has made integrated circuits possible and has lead to scaling of complex designs, benefiting from their small footprint. At the same time, the field of quantum acoustics has recently gained significant attention due to its various potential advantages over its photonic counterparts, including smaller mode volume, lower energy, and orders of magnitude slower propagation speeds, as well as the potential for interconnecting distinct quantum systems. Developing analogous integrated phononic technology is critical for realizing the full potential of phonons and could lead to groundbreaking new applications, such as scalable quantum computing and hybrid quantum devices. In this work, we demonstrate for the first time a 4-port directional coupler for quantum mechanical excitations - a crucial component for integrated phononic circuits. Adjusting the length of the coupling region allows to realize phononic beam splitters with varying splitting ratios. By sending a single-phonon Fock state onto one of these phononic splitters, we demonstrate the capability of using the directional coupler directly in the quantum regime. Our work provides an essential step towards an integrated phononic platform for both classical and quantum technologies applications.