  Precise measurements are the key to advances in all fields of science.
Quantum entanglement shows higher sensitivity than 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 realized 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 two photon
entanglement state yields a visibility greater than the threshold to surpass
the standard quantum limit. These results offer new ways for high precision
quantum metrology using short wavelength quantum entanglement number state.
