  An ellipsometer is a vital precision tool used for measuring optical
parameters with wide applications in many fields, including accurate
measurements in film thickness, optical constants, structural profiles, etc.
However, the precise measurement of photosensitive materials meets huge
obstacles because of the excessive input photons, therefore the requirement of
enhancing detection accuracy under low incident light intensity is an essential
topic in the precision measurement. In this work, by combining a
polarization-entangled photon source with a classical transmission-type
ellipsometer, the quantum ellipsometer with the PSA (Polarizer-Sample-Analyzer)
and the Senarmount method is constructed firstly to measure the phase
retardation of the birefringent materials. The experimental results show that
the accuracy can reach to nanometer scale at extremely low input intensity, and
the stability are within 1% for all specimens tested with a compensator
involved. Our work paves the way for precision measurement at low incident
light intensity, with potential applications in measuring photosensitive
materials, active-biological samples and other remote monitoring scenarios.
