Entangled-photon ellipsometry
  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. 
Ayman F. Abouraddy, Kimani C. Toussaint, Jr., Alexander V. Sergienko,   Bahaa E. A. Saleh, and Malvin C. Teich