Quantum nondemolition measurement of the photon number via the optical Kerr effect

This paper proposes a quantum nondemolition measurement scheme for the photon number. The signal and probe optical waves interact via the optical Kerr effect. The optical phase of the probe wave is selected as the readout observable for the measurement of the photon number of the signal wave. The measurement accuracy Δn and the imposed phase noise Δφ of the signal wave satisfy Heisenberg’s uncertainty principle with an equality sign, 〈(Δn${)}^2$〉〈(Δφ${)}^2$〉=(1/4). .AE