Testing the Bell inequality on frequency-bin entangled photon pairs using time-resolved detection

Entanglement, describing the inseparability of a quantum multiparty system, is one of the most intriguing features of quantum mechanics. Violation of Bell inequality, for ruling out the possibility of local hidden-variable theories, is commonly used as a strong witness of quantum entanglement. In previous Bell test experiments with photonic entanglement based on two-photon coincidence measurement, the photon temporal wave packets were absorbed completely by the detectors. That is, the photon coherence time is much shorter than the detection time window. Here we demonstrate the generation of frequency-bin entangled narrowband biphotons, and for the first time, to the best of our knowledge, test the Clauser–Horne–Shimony–Holt (CHSH) Bell inequality |𝑆|≤2 for their nonlocal temporal correlations with time-resolved detection. We obtain a maximum |𝑆| value of 2.52±0.48, which violates the CHSH inequality. Our result will have applications in quantum information processing involving time-frequency entanglement.