Self-calibrating optical low-coherence reflectometry with energy-time entangled photons for absolute distance measurements

Optical low-coherence reflectometry is capable of unambiguously measuring positions of stacked, partially reflective layers in a sample object. It relies on the low coherence of the light source and the absolute distances are obtained from the position reading of a mechanical motor stage. We show how to exploit the simultaneous high and low coherence properties of energy-time entangled photon pairs to directly calibrate the position scale of an optical low coherence reflectometry scan with a reference laser wavelength. In the experiment, a precision of 1.6 nm and good linearity is demonstrated.