Alkali vapor optically pumped magnetometers use alkali metals including Cesium, Potassium or Rubidium. The cell containing the metal must be continuously heated to approximately 45 to 55 degrees Celsius to render the metal in gaseous form.

These magnetometers operate on virtually the same principle as illustrated,
in part, below.

A glass vapour cell containing gaseous metal is exposed (or pumped) by light of very specific wavelength – an effect called light polarization. The frequency of light is specifically selected and circularly polarized for each element (i.e. the D₁ spectral line) to shift electrons from the ground level 2 to the excited metastable state 3 (Figure below).

Electrons at level 3 are not stable, and they spontaneously decay to both energy levels 1 and 2. Eventually, the level 1 is fully populated (i.e. level 2 is depleted). When this happens, the absorption of polarizing light stops and the vapour cell becomes more transparent.

This is when RF depolarization comes into play. RF power corresponding to the energy difference between levels 1 and 2 is applied to the cell to move electrons from level 1 back to level 2 (and the cell becomes opaque again). The frequency of the RF field required to repopulate level 2 varies with the ambient magnetic field and is called Larmor frequency.

Quantum mechanics of alkali vapor system

Depolarization by a circular magnetic field at the Larmor frequency will rebalance populations of the two ground levels and the vapour cell will start absorbing more of the polarizing light. The effect of polarization and depolarization is that light intensity becomes modulated by the RF frequency. By detecting light modulation and measuring the frequency, we can obtain a value of the magnetic field.

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GEM Advanced Magnetometers.
Our World is Magnetic.