Argonne Physics Division Colloquium - 1 Feb 2013

11:00 AM, Building 203 auditorium

It is a key principle of quantum mechanics that plane matter waves are
proportional to exp(-ip_{μ}x^{μ})=exp(-iω_{0}), where p_{&mu}; and x^{μ} are
respectively 4-momentum and position, and τ is the proper time measured
along the particle's trajectory. Thus, the quantum state of a free
particle of mass m accumulates the same phase as a clock ticking at the
particle's Compton frequency of ω_{0}=mc^{2}/h-bar travelling along the particle's
trajectory. This implies that a single particle can be a reference for
a clock. In principle, such a clock could be built by annihilating
particle-antiparticle pairs and counting the frequencies of the
generated photons. This would provide a frequency reference with
virtually infinite quality factor Q and unsurpassed stability against
systematic influences. The frequency (3x10^{25} Hz for a Cesium
atom), however, is far beyond modern counting techniques. A method to
divide it into a technically accessible range is thus required.

We demonstrate a "Compton clock," a clock referenced to ω_{0}, using an
optical frequency comb to self-reference a Ramsey-Borde atom
interferometer and synchronize an oscillator at a subharmonic of ω_{0}.
The interferometer is based on n-photon Bragg diffraction. It is
self-referenced by locking the laser to the Nth multiple of the measured
recoil frequency. The clock's frequency is then fully
determined by ω_{0} and the known numerical factors. The clock
has an accuracy and stability of 4x10^{-9}. It highlights the intimate
connection between frequency and mass: The Compton frequency can serve
as a frequency reference directly, without requiring the particle to be
annihilated. It allows measurement of microscopic masses with 4x10^{-9}
accuracy in the proposed revision to SI units. Together with the
Avogadro project, it yields calibrated kilograms. We will survey other
applications of matter waves as clocks, such as testing relativity and
verifying the gravitational Aharonov-Bohm effect.

Argonne Physics Division Colloquium Schedule