Efficient and stable single-photon counting at 1.55 μm by intracavity frequency upconversion

A single-photon signal at 1.55 μm was converted to the visible region by sum-frequency mixing with a strong pumping beam at 1064 nm in a periodically poled lithium niobate crystal placed in a diode-pumped Nd:YVO⁴ laser cavity. As the intracavity pump laser could be automatically stabilized without cavity lock, robust long-term stability was demonstrated for single-photon frequency upconversion, with a conversion efficiency of 74.3%. Such a stable single-photon upconversion was demonstrated to be efficient and robust for single-photon counting at 1550 nm, and the corresponding background noise was measured at less than 420✗10³s^-1.