Phase-sensitive wave function collapse in two-photon spin Hall effect

Photonic spin Hall effect (SHE) is a fundamental and important optical phenomenon, which originates from the spin–orbit interactions (SOI) of light, forming the cornerstone for a wide range of optical precision measurements. However, the wave function collapse in the single-photon SHE is unpredictable as spin collapse has inherent randomness governed by the Born rule. Here, nonlocal spin bunching and anti-bunching of polarization-entangled photon pairs called two-photon SHE is realized to predict and manipulate the wave function collapse, which is fundamentally different from the single-photon SHE. The two-photon SHE is sensitive to the relative phase among entangled photon pairs, enabling continuous control from nonlocal spin bunching to anti-bunching. Owing to the phase sensitivity of the two-photon SHE, an accurate and simple quantitative phase imaging scheme is realized. The two-photon SHE may pave the way for unveiling the wave function evolution in the SOI of light and manipulating spin-dependent photonic quantum effects.