Observation and Manipulation of Optical Parametric Downconversion in the Langevin Regime

Quantum fluctuation plays a key role in parametric down-conversion in the Langevin regime. In this Letter, we report the experimental realization of optical parametric down-conversion in the Langevin regime on a chip. By precisely controlling the loss inherently tied to fluctuation, we observe the asymmetric Hong-Ou-Mandel dip-a hallmark of quantum fluctuation in the fluctuation-driven parametric down-conversion (PDC)-and the fluctuation-induced compression of single photons by nearly one order of magnitude. These findings pave the way for the manipulation of quantum fluctuation, quantum states, and system-reservoir interaction.