Bee Hossenfelder blogged about this. Too beautiful not to share. http://backreaction.blogspot.com/2013/01/how-particle-tells-time.html The experiment was done at UC Berkeley Precise measurement of Compton frequency of particle This was published online 10 January 2013 by Science journal. http://www.sciencemag.org/content/early/2013/01/09/science.1230767 A Clock Directly Linking Time to a Particle's Mass Shau-Yu Lan1, Pei-Chen Kuan1, Brian Estey1, Damon English1, Justin M. Brown1, Michael A. Hohensee1, Holger Müller1,2,* 1Department of Physics, 366 Le Conte Hall MS7300, University of California, Berkeley, CA 94720, USA. 2Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, USA. *To whom correspondence should be addressed. E-mail: firstname.lastname@example.org ABSTRACT 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 ω0 = mc2/ ħ, where c is the speed of light and ħ is the reduced Planck constant. A clock referenced to ω0 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 ω0. 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.