A spin-exchange relaxation-free (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.cite journal
author=Allred, J. C., Lyman, R. N., Kornack, T. W., Romalis, M. V.
title=High-Sensitivity Atomic Magnetometer Unaffected by Spin-Exchange Relaxation
journal=Phys Rev Lett | volume=89 | pages=130801 | year=2002
doi = 10.1103/PhysRevLett.89.130801
] The sensitivity of SERF magnetometers improves upon traditional atomic magnetometers by eliminating the dominant cause of atomic spin decoherence caused by spin-exchange collisions among the alkali metal atoms. SERF magnetometers are among the most sensitive magnetic field sensors and in some cases exceed the performance of SQUID detectors of equivalent size.cite journal
author=Kominis, I. K., Kornack, T. W., Allred, J. C., Romalis, M. V.
title=A subfemtotesla multichannel atomic magnetometer
journal=Nature|year=2003|month=April 10|volume=422|pages=596–599
] They are vector magnetometers capable of measuring all three components of the magnetic field simultaneously.Fact|date=June 2008

pin-exchange relaxation

Spin-exchange collisions preserve total angular momentum of a colliding pair of atoms but can scramble the hyperfine state of the atoms. Atoms in different hyperfine states do not precess coherently and thereby limit the coherence lifetime of the atoms. However, decoherence due to spin-exchange collisions can be completely eliminated if the spin-exchange collisions occur much faster than the precession frequency of the atoms. 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 causng the spin ensemble to precess more slowly but remain coherent. This so-called SERF regime can be reached by operating with sufficiently high alkali metal density (at higher temperature) and in sufficiently low magnetic field.cite journal
author=Happer, W. and Tam, A. C.
title=Effect of rapid spin exchange on the magnetic-resonance spectrum of alkali vapors
journal=Physical Review A | year=1977 | volume=16 | number=5 | pages=1877–1891

The spin-exchange relaxation rate R_{se} for atoms with low polarization experiencing slow spin-exchange can be expressed as follows::R_{se} = frac{1}{2 pi T_{se left( frac{2 I(2 I -1)}{3(2I+1)^2} ight)where T_{se} is the time between spin-exchange collisions, I is the nuclear spin, u is the magnetic resonance frequency, gamma_e is the gyromagnetic ratio for an electron.

In the limit of fast spin-exchange and small magnetic field, the spin-exchange relaxation rate vanishes for sufficiently small magnetic field::R_{se} = frac{gamma_e^2 B^2 T_{se} }{2 pi} frac{1}{2}left( 1-frac{(2I+1)^2}{Q^2} ight)where Q is the "slowing-down" constant to account for sharing of angular momentum between the electron and nuclear spins:cite journal
author=Savukov, I. M., and Romalis, M. V.
title=Effects of spin-exchange collisions in a high-density alkali-metal vapor in low magnetic fields.
journal=Physical Review A | year=2005 | volume=71 | pages=023405
] :Q(I=3/2)=4left( 2 - frac{4}{3+P^2} ight)^{-1}:Q(I=5/2)=6left( 3 - frac{48(1+P^2)}{19+26 P^2+3 P^4} ight)^{-1}:Q(I=7/2)=8left( frac{4(1+7P^2+7P^4+P^6)}{11+35P^2+17P^4+P^6} ight)^{-1}where P is the average polarization of the atoms. 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).


The sensitivity delta B of atomic magnetometers are limited by the number of atoms N and their spin coherence lifetime T_2 according to :delta B = frac{1}{gamma} sqrt{frac{2 R_{tot} Q}{F_z N} }where gamma is the gyromagnetic ratio of the atom and F_z is the average polarization of total atomic spin F = I+S.cite journal
author=I. M. Savukov, S. J. Seltzer, M. V. Romalis, and K. L. Sauer
title=Tunable Atomic Magnetometer for Detection of Radio-Frequency Magnetic Fields
journal=Physical Review Letters | volume=95 | pages=063004 | year=2005
doi = 10.1103/PhysRevLett.95.063004

In the absence of spin-exchange relaxation, a variety of other relaxation mechanisms contribute to the decoherence of atomic spin::R_{tot} = R_D + R_{sd,self} + R_{sd,mathrm{He + R_{sd,mathrm{N_2 where R_D is the relaxation rate due to collisions with the cell walls and R_{sd,X} are the spin destruction rates for collisions among the alkali metal atoms and collisions between alkali atoms and any other gasses that may be present.

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 R_{tot} ≈ 1 Hz.

Typical operation

Alkali metal vapor of sufficient density is obtained by simply heating solid alkali metal inside the vapor cell. A typical SERF atomic magnetometer can take advantage of low noise diode lasers to polarize and monitor spin precession. Circularly polarized pumping light tuned to the D_1 spectral resonance line polarizes the atoms. 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.

Advantages and disadvantages

SERF magnetometers compete with SQUID magnetometers for use in a variety of applications. The SERF magnetometer has the following advantages:
* Equal or better sensitivity per unit volume
* Cryogen-free operation
* All-optical measurement limits enables imaging and eliminates interference.Potential disadvantages:
* Can only operate near zero field.
* Sensor vapor cell must be heated.


Applications utilizing high sensitivity of SERF magnetometers potentially include:
* High-performance magnetoencephalographic imaging.cite journal
author=H. Xia, A. Ben-Amar Baranga, D. Hoffman, and M. V. Romalis
title=Magnetoencephalography with an atomic magnetometer
journal=Applied Physics Letters | year=2006 | volume=89 | pages=211104
* Sample magnetization measurement, especially rock samples.


The SERF magnetometer was developed by Michael V. Romalis at Princeton University in the early 2000s. The underlying physics governing the suppression spin-exchange relaxation was developed decades earlier by William Happer 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.


External links

* [ Photographs of a SERF magnetometer] from the Romalis Group at Princeton University.

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