Ultra-compact transverse mode quantum interference device on silicon

Quantum photonic integrated circuits (QPICs) offer enhanced stability and scalability for quantum communications, sensing, and computing. Transverse modes in multimode waveguides enable high-dimensional scalability and versatile photon manipulation, but practical adoption requires compact and fabrication-tolerant quantum interference devices. Here, we present an ultra-compact taper-stepped beamsplitter that enables quantum interference between photon pairs in different transverse modes, and cascade it to realize NOON state interferometry. We experimentally achieve high visibilities of 93.9% for HOM interference and 86.5% for NOON state interference, demonstrating that efficient mode interference with active tuning can be realized on this platform.