Fig. 6.

Normalized probability density function of the phase measurement error at different average signal level ($I_{sig}/\left(hc/\lambda \right)=A_{sig}^2/\left(hc/\lambda \right)$) – 100, 10, 1, 0.1, 0.01 photon(s). The PDF is built with the Monte-Carlo simulation of 4 step phase shifting method for ${10}^5$ number of modes. The error is increased as the signal is decreased. Though it is wide, peak around 0 is observed even at signal photon number smaller than 1.