Subattosecond x-ray Hong-Ou-Mandel metrology.

We show that subattosecond delays and subangstrom optical path differences can be measured by using Hong-Ou-Mandel interference measurements with x-rays. Our scheme relies on the subattosecond correlation time of photon pairs that are generated by x-ray spontaneous parametric down-conversion, which leads to a dip in correlation measurements with a comparable width. Therefore, the precision of the measurements is expected to be better than 0.1 attosecond. We anticipate that the scheme we describe in this work will lead to the development of various techniques of quantum measurements with ultra-high precision at x-ray wavelengths.