New
Measurement of the Electron Magnetic Moment and the Fine Structure Constant
Remarkably, the famous
UW measurement of the electron magnetic moment has stood since 1987. With QED theory, this measurement has
determined the accepted value of the fine structure constant. This colloquium is about a new Harvard
measurement of these fundamental constants.
The new measurement has an uncertainty that is about six times smaller,
and it shifts the values by 1.7 standard deviations. One electron suspended in a Penning trap is
used for the new measurement, like in the old measurement. What is different is that the lowest quantum
levels of the spin and cyclotron motion are resolved, and the cyclotron as well
as spin frequencies are determined using quantum jump spectroscopy. In addition, a 0.1 mK Penning trap that is
also a cylindrical microwave cavity is used to control the radiation field, to
suppress spontaneous emission by more than a factor of 100, to control cavity
shifts, and to eliminate the blackbody photons that otherwise stimulate
excitations from the cyclotron ground state.
Finally, great signal-to-noise for one-quantum transitions is obtained
using electronic feedback to realize the first one-particle self-excited
oscillator. The new methods may also
allow a million times improved measurement of the 500 times small antiproton
magnetic moment.