Spin Gyroscopes

In an atomic clock, the frequency of a periodic signal—the number of “ticks” per second of the clock—is referenced to the energy difference between two quantum states-that frequency is a constant of nature. Clocks can also thus detect external fields with high sensitivity.

Atomic magnetometry exploits this field-sensing gyroscopic aspect of an atom, in detecting a shift in the relative energy between various atomic states in presence of an external magnetic field. As a spinning top on Earth “wobbles,” or precesses, due to torque from gravity the magnetic dipole associated with an atomic spin, precess at a well-defined "Larmor" frequency if it experiences a torque from a magnetic field.

Exotic (non magnetic) fields could also exert a torque on the atomic spins, but because the effects are expected to be so small, researchers must eliminate the “prosaic” magnetic external perturbations to see them.

In this paper an atomic spin gyroscope acheives a precession-frequency stability of better than 7 nHz which means that the Princeton team’s atomic spin gyroscope can sense torques so small that, in the absence of other effects, they would cause atomic spins to precess with periods of longer than four years.