  Performing reliable measurements in optical metrology, such as those needed
in ellipsometry, requires a calibrated source and detector, or a
well-characterized reference sample. We present a novel interferometric
technique to perform reliable ellipsometric measurements. This technique relies
on the use of a non-classical optical source, namely polarization-entangled
twin photons generated by spontaneous parametric downconversion from a
nonlinear crystal, in conjunction with a coincidence-detection scheme.
Ellipsometric measurements acquired with this scheme are absolute, i.e., they
require neither source nor detector calibration, nor do they require a
reference.
