Application Description

Optically pumped magnetometers (OPMs), also referred to as atomic magnetometers, are one of the most sensitive tools to probe very weak magnetic fields, with demonstrated sensitivity reaching the fT/√Hz regime. OPMs rely on the precession of a macroscopic spin state in an ensemble of (alkali) atoms in a magnetic field. The frequency of the precession, i.e., the Larmor frequency, is proportional to the magnetic field strength. Hence, the field strength can be determined by measuring the Larmor frequency or the corresponding change in the spin direction.

There are many types of atomic magnetometers suitable for different ranges of field strength. For instance, Larmor magnetometers can measure magnetic fields with magnitudes comparable to the earth’s magnetic field (~50 μT) and are therefore typically used for exploration tasks. For ultralow magnetic fields below 10 nT, the OPMs operating in the spin-exchange relaxation-free (SERF) regime have demonstrated record sensitivities, making them the ideal choice for a variety of applications – from the exploration of human-brain interfaces to the study of axion-like dark matter.

Measurement Strategies

Regardless of the specific configuration, all OPMs follow the same underlying principle. Figures 1 and 2 show the central component, a glass cell that typically contains alkali atoms, which serves as the sensing element. After polarization, the spin of these atoms precesses within the magnetic field, which is unknown.
To polarize the alkali-atom vapor, circularly polarized pump light at a specific frequency brings the atoms into the same state with the same spin direction. To measure the precession of this spin, the Faraday rotation of a linearly polarized probe laser beam is used. The Faraday rotation causes the polarization axis of the probe laser to rotate proportionally to the spin’s projection onto the probe laser axis. The Zurich Instruments MFLI Lock-in Amplifier can be used to actuate both pump and probe lasers, as well as to acquire the photodiode signals efficiently.