CUA - Center for Ultracold Atoms

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MIT-Harvard Center for Ultracold Atoms, A National Science Foundation Physics Frontier Center

Atomic Clock Beats the Quantum Limit

Got the Time? Inherent quantum mechanical fuzziness limits how well you can know the position of a clock’s hand (red), but by “squeezing” this uncertainty into the unimportant “length” dimension, its angle can be determined more precisely (blue). Researchers played an analogous trick with an atomic clock.
Published 6.25.2010

Ian D. Leroux, Monika H. Schleier-Smith, and Vladan Vuletić

The best atomic clocks are limited by the uncertainty principle–quantum-scale fluctuations prevent measurements from being perfectly precise. But reports in the 19 February and 25 June Physical Review Letters show that the fluctuations can be moved into other measurable quantities that don’t affect the time measurement. Although the new clock isn’t as good as the best ones now available, it shows that the quantum limit can be evaded for future precision experiments and satellite-aided navigation.

Atomic clocks are based on atoms that have a choice of two states, a ground state and an excited state. The difference in energy between the states determines the frequency of an intrinsic oscillation that is set up when the atom starts out in a combination state. Experimentalists can’t measure this oscillation directly, but it’s often represented by a point that continually rotates around the circumference of a circle or the surface of a sphere. The angle through which this point has moved since the beginning of an experiment is called the phase. A precise measurement of the phase is the goal of a clock because the rotation speed is a known quantity, fixed by the difference in energy of the two atomic states.

But quantum fluctuations embodied in the uncertainty principle limit the precision with which this phase can be measured, and for a decade the best atomic clocks have approached this limit. In principle, though, researchers have known how to do better. The inherent uncertainty applies to the product of two variables, such as the momentum and position of a particle, so the uncertainty in one variable can be reduced at the cost of increasing the uncertainty in the other, a trick called squeezing.

Continue to the American Physical Society’s report.