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

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