SERF

This is done by using a high (1014 cm−3) density of potassium atoms and a very low magnetic field.

Under these conditions, the atoms exchange spin quickly compared to their magnetic precession frequency so that the average spin interacts with the field and is not destroyed by decoherence.

[1] A SERF magnetometer achieves very high magnetic field sensitivity by monitoring a high density vapor of alkali metal atoms precessing in a near-zero magnetic field.

SERF magnetometers are among the most sensitive magnetic field sensors and in some cases exceed the performance of SQUID detectors of equivalent size.

[3] They are vector magnetometers capable of measuring all three components of the magnetic field simultaneously.

In this regime of fast spin-exchange, all atoms in an ensemble rapidly change hyperfine states, spending the same amounts of time in each hyperfine state and causing the spin ensemble to precess more slowly but remain coherent.

for atoms with low polarization experiencing slow spin-exchange can be expressed as follows:[4] where

is the "slowing-down" constant to account for sharing of angular momentum between the electron and nuclear spins:[5] where

The atoms suffering fast spin-exchange precess more slowly when they are not fully polarized because they spend a fraction of the time in different hyperfine states precessing at different frequencies (or in the opposite direction).

In an optimal configuration, a density of 1014 cm−3 potassium atoms in a 1 cm3 vapor cell with ~3 atm helium buffer gas can achieve 10 aT Hz−1/2 (10−17 T Hz−1/2) sensitivity with relaxation rate

A typical SERF atomic magnetometer can take advantage of low noise diode lasers to polarize and monitor spin precession.

An orthogonal probe beam detects the precession using optical rotation of linearly polarized light.

In a typical SERF magnetometer, the spins merely tip by a very small angle because the precession frequency is slow compared to the relaxation rates.

[2] The underlying physics governing the suppression spin-exchange relaxation was developed decades earlier by William Happer[4] but the application to magnetic field measurement was not explored at that time.

The name "SERF" was partially motivated by its relationship to SQUID detectors in a marine metaphor.

Alkali metal atoms with hyperfine state indicated by color precessing in the presence of a magnetic field experience a spin-exchange collision which preserves total angular momentum but changes the hyperfine state, causing the atoms to precess in opposite directions and decohere.
Alkali metal atoms in the spin-exchange relaxation-free (SERF) regime with hyperfine state indicated by color precessing in the presence of a magnetic field experience two spin-exchange collisions in rapid succession which preserves total angular momentum but changes the hyperfine state, causing the atoms to precess in opposite directions only slightly before a second spin-exchange collision returns the atoms to the original hyperfine state.
Relaxation rate as indicated by magnetic resonance linewidth for atoms as a function of magnetic field. These lines represent operation with potassium vapor at 160, 180 and 200 °C (higher temperature provides higher relaxation rates) using a 2 cm diameter cell with 3 atm He buffer gas, 60 Torr N 2 quenching gas. The SERF regime is clearly apparent for sufficiently low magnetic fields where the spin-exchange collisions occur much faster than the spin precession.
Atomic magnetometer principle of operation, depicting alkali atoms polarized by a circularly polarized pump beam, precessing in the presence of a magnetic field and being detected by optical rotation of a linearly polarized probe beam.
SERF components mockup