Superresolving Phase Measurement with Short-Wavelength NOON States by Quantum Frequency Up-Conversion

Precise measurements are the key to advances in all fields of science. Quantum entanglement shows higher sensitivity than that which is achievable by classical methods. Most physical quantities including position, displacement, distance, angle, and optical path length can be obtained by optical phase measurements. Reducing the photon wavelength of the interferometry can further enhance the optical-path-length sensitivity and imaging resolution. By quantum frequency up-conversion, we realize a short-wavelength two-photon number entangled state. Nearly perfect Hong-Ou-Mandel interference is achieved after both 1547-nm photons are up-converted to 525 nm. Optical phase measurement of the two-photon entanglement state yields a visibility greater than the threshold to surpass the standard quantum limit. A spectra change of the photon pair after being up-converted is observed and well explained. These results offer alternative ways for high-precision quantum metrology using a short-wavelength quantum entanglement number state and offer a potential all-optical spectra engineering technique for the photon pair source.