Electron cycling in quantum confines

From Long Beach, Calif., at a meeting of the American Physical Society

Like a race car forced to stay within certain lanes of a speedway, a lone electron in a circular path should whirl along exclusively in specific allowed orbits, the rules of quantum mechanics say.

In a Harvard University laboratory, physicists have devised an extraordinarily small, low-energy cyclotron in which a single orbiting electron vividly demonstrates that predicted quantum behavior.

Moreover, this artificial atom without a nucleus, as Gerald Gabrielse and his colleagues refer to their diminutive apparatus, promises to make possible the most precise measurements yet of certain fundamental constants. “We think we can measure the strength of the bar magnet in the electron, called its magnetic moment, 10 times better than ever before,” Gabrielse predicts.

He and his colleagues use carefully controlled magnetic and electric fields to make the electron do laps in a vacuum within a small metal can at the heart of the apparatus. The can, about a centimeter across, is cooled to as low as 70 millikelvins—less than a 50th of the previous record chill for a trap containing a lone fundamental particle. At that temperature, the electron zips around for hours at a time in its lowest energy state, Gabrielse says. This so-called ground state is the orbital equivalent of the inner lane on the racetrack.

If the device warms a little, the cavity walls emit a few microwave photons. When the frequency, or energy, of the photons is just right, they can bump the electron temporarily to a higher energy state. The excited electron traces a circular path with a larger radius.

To nail down the magnetic moment, the Harvard group aims to measure accurately the photon frequency that most effectively boosts the electron from the ground state to the next-wider orbit, Gabrielse says. The team can identify that frequency by transmitting microwaves into the cavity and noting which frequency induces the most leaps to the higher level.

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