A clock directly linking time to a particle's mass.

Historically, time measurements have been based on oscillation frequencies in systems of particles, from the motion of celestial bodies to atomic transitions. Relativity and quantum mechanics show that even a single particle of mass m determines a Compton frequency ω(0) = mc(2)/[formula: see text] where c is the speed of light and [formula: see text] is Planck's constant h divided by 2π. A clock referenced to ω(0) would enable high-precision mass measurements and a fundamental definition of the second. We demonstrate such a clock using an optical frequency comb to self-reference a Ramsey-Bordé atom interferometer and synchronize an oscillator at a subharmonic of ω(0.) This directly demonstrates the connection between time and mass. It allows measurement of microscopic masses with 4 × 10(-9) accuracy in the proposed revision to SI units. Together with the Avogadro project, it yields calibrated kilograms.