Quantum beat of two single photons in the transverse momentum space

Two-photon Hong-Ou-Mandel (HOM) interference is a pivotal quantum optical phenomenon, with broad applications in quantum information and metrology. Although conventional HOM interferometry loses sensitivity for transverse spatial separations beyond the photons' coherence length, we reveal experimentally that momentum-space measurements preserve this information through phase-sensitive two-photon interference fringes. Notably, we achieve micrometer-scale precision in determining transverse spatial separations an order of magnitude larger than the coherence length with only hundreds of photons. Our work establishes momentum-space interferometry as an important platform for quantum-enhanced metrology and information processing, overcoming the constraints of standard HOM techniques while providing new insights into quantum interference.