We demonstrate a point-to-point clock synchronization protocol based on
bidirectionally exchanging photons produced in spontaneous parametric down
conversion (SPDC). The technique exploits tight timing correlations between
photon pairs to achieve a precision of 51ps in 100s with count rates of order
200s−1. The protocol is distance independent, secure against symmetric
delay attacks and provides a natural complement to techniques based on Global
Navigation Satellite Systems (GNSS). The protocol works with mobile parties and
can be augmented to provide authentication of the timing signal via a Bell
inequality check.Comment: 5 pages, 5 figure