A single-phonon directional coupler

TL;DR

This paper introduces the first 4-port directional coupler for quantum phonons, enabling controllable phononic beam splitting and demonstrating quantum-level operation, which is crucial for scalable quantum phononic circuits.

Contribution

It presents the first integrated 4-port directional coupler for phonons, demonstrating controllable splitting ratios and quantum-level performance for the first time.

Findings

Achieved controllable phononic beam splitting.

Validated quantum operation with single-phonon Fock states.

Established a foundational component for scalable quantum phononic circuits.

Abstract

Integrated photonics has revolutionized fields such as telecommunications, quantum optics, and metrology by enabling compact, scalable circuits through highly confined optical modes. Within the field of quantum acoustics, phonons have emerged as a compelling alternative, offering advantages such as lower energy, smaller mode volume, and low propagation speeds, which make them ideal for interfacing diverse quantum systems. Developing integrated phononic circuits is thus essential for unlocking the full potential of quantum acoustics and advancing technologies such as quantum computing and hybrid systems. In this work, we demonstrate the first 4-port directional coupler for quantum mechanical excitations - a key building block for phononic circuits. By tuning the coupling region length, we achieve phononic beam splitters with controllable splitting ratios. We validate quantum-level performance by sending a single-phonon Fock state through the device. This work represents a foundational advance toward scalable, integrated phononic platforms for both classical and quantum applications.