However, a magnetic field is a vector, and in order to know all of its properties one has to measure all three of its components. Therefore, it is not surprising that many attempts have been undertaken to give new vector features to existing scalar devices. The most common principle used for this is the law of vector summation, according to which the unknown vector may be calculated by adding to it a set of known vectors and measuring the modulus of the resultant field vectors. Unfortunately, devices that employ systems of additional artificial magnetic fields have a common disadvantage: the impossibility of absolute measurements, since the contribution of these additional fields is comparable to the measured field.
An alternative proposed in 1972 by Fairweather and Usher is based on the fact that the amplitude and phase of the Larmor signal detected by a pumping beam depend on the direction of the pumping beam wavevector relative to the magnetic field vector. In particular, the signal amplitude is zero if these vectors are strictly parallel, so a feedback maintaining parallelism of the magnetic field to the pumping beam is easily implemented. However, the feedback scheme of Fairweather and Usher also requires additional weak magnetic fields. In a scheme proposed forty years later by the author of this summary, no additional magnetic fields were used, and the direction of the pumping beam was adjusted instead. The authors of this Optics Express article take the concept further by using four pumping beams instead of one, thus replacing the real-time rotation of the wavevector of light by off-line analysis of the Larmor signal data carried by these four beams.
Another important step taken by the authors involves using free atomic spin precession instead of a forced one. This allows the disturbances caused by the resonant radiofrequency field to be excluded, at the cost of using short and relatively strong spin-manipulation radiofrequency pulses.
The vector device described in the paper is primarily intended for use in neutron electric dipole moment search experiments. Yet it has extensive capabilities: for example, in geomagnetic observatories it can replace both vector and precise scalar magnetometers. In all, the results reported by the authors suggest that this device may be closer to being a “perfect magnetometer” than any instrument created up to this point.
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