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Advanced Quantum Magnetometer Technologies

GEM leads in developing advanced quantum magnetometer technologies, including the Overhauser, optically pumped Potassium (K-Mag) and Proton Precession magnetometers. The company is recognized as the successful commercial developer of both the Overhauser and Potassium magnetometer / gradiometer. Moreover, its Proton Precession magnetometer / gradiometer is the latest technology system in its class.

Quantum magnetometers take advantage of the spin of subatomic particles (nuclei and unpaired valence electrons). Through a process of polarization, particles are caused to precess in the earth’s ambient magnetic field. The resulting frequency of precession can be translated directly into magnetic field units. Quantum results are scalar (total field intensity) as opposed to vector (i.e. from fluxgate geophysical instruments or GEM’s Suspended dIdD technology).

The spin of nuclei and unpaired valence electrons is associated with the magnetic moment and is characteristic for each particular particle. Coupling of each particle’s magnetic moment with the applied field is quantized or limited to a discrete set of values as determined by quantum mechanical rules.

In the ambient magnetic field, there are 2I + 1 orientations for electrons and for nuclei (i.e. protons and Helium 3). For each of these, I = ½. There are therefore, only 2 orientations allowed (parallel and anti-parallel to magnetic field). Since the populations of each of the orientations are different, an assembly of magnetic moments will produce a tiny net macroscopic magnetization that is aligned with the magnetic field.

Macroscopic nuclear or electron spin magnetization is static. If elementary magnetic moments are forced out of alignment with the direction of the ambient magnetic field, the corresponding particles precess (i.e. rotate) around the field in a plane of precession perpendicular to the field direction. They precess with an associated angular frequency, called the Larmor frequency which is in general proportional to the magnetic field value.

However, in weak magnetic fields, such as the Earth’s, the signals of all scalar magnetometers are just too weak for simple measurement of Larmor frequency. They must be boosted in intensity or “polarized” to ensure sufficient sensitivity of measurement. Due to the distribution of local magnetic fields, all particles in the sensor precess with naturally different frequencies and lose synchronism over time. The signal associated with the precession decays exponentially and the characteristic time of decay is called “transversal” relaxation time T2.

Similarly, if we apply a magnetic field to an assembly of spins, it takes time to establish macroscopic magnetization. The increase is again exponential with the time constant, T1, called “longitudinal” relaxation time. The intensity of magnetization is proportional to the strength of the applied magnetic field.

The strength of the magnetization and therefore, of the detectable precession signal, depends on the difference in populations of the two orientations of magnetic moments.

Increasing that difference is called polarization and can be achieved in three ways in quantum magnetometers:

  • Application of strong auxiliary magnetic field (actually flux density) to polarize nuclear, usually protons.
  • Transfer of natural polarization of auxiliary electrons to protons (Overhauser effect).
  • Optical manipulation or “pumping” of electrons by elevating them to a higher state selectively.

Note: In practice, T2 is very short in solid samples. All quantum magnetometers therefore use liquid or gaseous sensors. In liquids and gases T1 and T2 assume values between a fraction of a second to several seconds. An exception is Helium 3, which has a T2 value of several hours or even days.

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