Editing Magnetometer (section)

====Zero-field optically-pumped magnetometers====
Magnetometers based on atomic gasses can perform vector measurements of the magnetic field in the low field regime, where the decay of the atomic coherence becomes faster than the [[Larmor frequency]]. The physics of such magnetometers is based on the [[Hanle effect]]. Such zero-field optically pumped magnetometers have been tested in various configurations and with different atomic species, notably [[Alkali metal|alkali]] (potassium, rubidium and cesium), [[helium]] and [[Mercury (element)|mercury]]. For the case of alkali, the coherence times were greatly limited due to spin-exchange relaxation. A major breakthrough happened at the beginning of the 2000 decade, Romalis group in Princeton demonstrated that in such a low field regime, alkali coherence times can be greatly enhanced if a high enough density can be reached by high temperature heating, this is the so-called [[SERF|SERF effect]].

The main interest of optically-pumped magnetometers is to replace SQUID magnetometers in applications where cryogenic cooling is a drawback. This is notably the case of medical imaging where such cooling imposes a thick thermal insulation, strongly affecting the amplitude of the recorded biomagnetic signals. Several startup companies are currently developing optically pumped magnetometers for biomedical applications: those of TwinLeaf,<ref>{{Cite web |title=MicroSERF Twinleaf magnetometers |url=https://twinleaf.com/vector/microSERF/}}</ref> quSpin<ref>{{Cite web |title=quSpin QZFM magnetometers |url=https://quspin.com/products-qzfm/}}</ref> and FieldLine<ref>{{Cite web |title=FieldLine website |url=https://fieldlineinc.com/}}</ref> being based on alkali vapors, and those of Mag4Health on metastable helium-4.<ref>{{Cite web |title=Mag4Health website |url=https://www.mag4health.com/technology/mag4health-quantum-sensors/}}</ref>

=====Spin-exchange relaxation-free (SERF) atomic magnetometers=====
{{Main|SERF}}
At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free ([[SERF]]) atomic magnetometers containing [[potassium]], [[caesium]], or [[rubidium]] vapor operate similarly to the caesium magnetometers described above, yet can reach sensitivities lower than 1 fT Hz<sup>−{{1/2}}</sup>. The SERF magnetometers only operate in small magnetic fields. The Earth's field is about 50 [[tesla (unit)|μT]]; SERF magnetometers operate in fields less than 0.5 μT.

Large volume detectors have achieved a sensitivity of 200 aT Hz<sup>−{{1/2}}</sup>.<ref>{{cite journal|last1=Kominis |first1=I.K. |last2=Kornack |first2=T.W. |last3=Allred |first3=J.C. |last4=Romalis |first4=M.V.|doi=10.1038/nature01484 |date=4 February 2003|title=A subfemtotesla multichannel atomic magnetometer|bibcode= 2003Natur.422..596K |volume=422 |issue=6932 |pmid=12686995| journal=Nature| pages=596–9|s2cid=4204465 }}</ref> This technology has greater sensitivity per unit volume than SQUID detectors.<ref>{{cite journal |last1=Budker |first1=D. |last2=Romalis |first2=M.V. |date=2006 |title=Optical Magnetometry |arxiv=physics/0611246 |doi=10.1038/nphys566 |volume=3 |issue=4 |journal=Nature Physics |pages=227–234| bibcode=2007NatPh...3..227B |s2cid=96446612 }}</ref> The technology can also produce very small magnetometers that may in the future replace coils for detecting radio-frequency magnetic fields.{{Citation needed|date=January 2012}} This technology may produce a magnetic sensor that has all of its input and output signals in the form of light on fiber-optic cables.<ref>{{Cite book |last1= Kitching |first1= J. |last2= Knappe |first2= S. |last3= Shah |first3= V. |last4= Schwindt |first4= P. |last5= Griffith |first5= C. |last6= Jimenez |first6= R. |last7= Preusser |first7= J. |last8= Liew |first8= L. -A. |last9= Moreland |first9= J. |chapter= Microfabricated atomic magnetometers and applications |doi= 10.1109/FREQ.2008.4623107 |title= 2008 IEEE International Frequency Control Symposium |pages= 789 |year= 2008 |isbn= 978-1-4244-1794-0 |s2cid= 46471890 |chapter-url= https://zenodo.org/record/1232197 }}</ref> This lets the magnetic measurement be made near high electrical voltages.