Abstract 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.
Li et al. (2025) studied this question.