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How Atomic Magnetometers Work

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Atomic magnetometers work by measuring the precession frequency of certain atoms in a magnetic field. Atoms with a magnetic moment may be visualized as spinning tops. A spinning top’s angular momentum precesses around the gravitational field vector. Similarly, an atom’s magnetic moment precesses around the magnetic field vector, with a frequency proportional to the magnetic field being measured. Since frequency is a quantity that is easy to measure to very high precision, the atomic precession frequency, and therefore the magnetic field, may be determined to very high precision.

 

Individual alkali atoms have intrinsic magnetic moments due to their unpaired electron. In a gas, however, these moments are all randomly aligned and therefore cancel each other out at the macroscopic level. In a magnetometer, therefore, the first step required is to create a macroscopic magnetic moment through the mechanism of optical pumping. By shining polarized light through the gas, the magnetic moments of individual atoms tend to become aligned in the same direction (along the light path) and hence a measurable magnetic moment is obtained.

The precession of this magnetic moment around the magnetic field in turn affects the absorption of the light shining through the gas. As the moments precess away from the light direction, they tend to absorb more of the light. In this manner, the frequency of the atomic precession may be measured by the effect it has on the intensity of the light passing through the gas.

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Precessing atoms modulate the intensity of a light beam. This is the basic principle of the operation of an atomic magnetometer. The blue arrow represents the magnetic field, while the yellow arrow represents the magnetic moment. At top left, the moment is parallel to the light beam, and does not absorb the light. As the moment precesses, or rotates, around the magnetic field arrow, it absorbs varying amounts of light, thereby modulating the amplitude of the beam.

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