Optically Pumped Magnetometry

cell in rf coil
A glass cell filled with rubidium vapour, positioned in a transverse field coil.

In this project, we use a glass cell that is filled with rubidium vapour to measure magnetic fields. Using laser light, the quantum mechanical spins of  all atoms in the vapour can be “optically pumped”, such that they are all aligned. Since atomic spin is linked to a magnetic moment, the spins precess when they are exposed to a magnetic field.

We can learn something about the momentary alignment of the spin by measuring the change in light polarisation that a subsequent probing pulse experiences. By evaluating the temporal behaviour of this signal, we can determine about which axis and how fast the spins precess. The observation of this motion enables us to tell the strength and orientation of the present magnetic field.

The figure below shows a schematic of the experimental setup. At the centre of the setup is a rubidium filled vapour cell, which sits inside a magnetic shield. The shield is needed to evaluate the high-sensitivity of our magnetometer, because the environmental field noise is much stronger than the small fields we are interested in, e.g. magnetic fields produced by human organs like the heart, intestines, or even the brain. A pair of laser polarised beams (pump + repump, travelling to the left) first aligns the atomic spins, before a polarised laser beam (travelling to the right) interacts with the atomic vapour.

Vector Voigt setup

Spin precession signal
The raw signal magnetometer signal. After optically pumping the vapour for 5 ms, the spin precession causes oscillations in  light polarisation.

When probing the spin precession, we make use of the Voigt effect, or the linear birefringence of the vapour. Laser light will experience a varying refractive index that depends on the beam’s polarisation. And in this case, the two orthogonal components of linear polarisation parallel and orthogonal to the alignment of spin will travel at different speeds of light. This leads to a measurable phase shift, which allows us to observe the spin precession.

The spectral analysis of the precession signal is used to extract three components that can be mapped onto the three directions of the magnetic field. The figure below shows experimental data for this mapping, where the grid lines in the egg-shaped signal maps correspond to scanning two orthogonal components of the magnetic field. A change in the third component of the magnetic field result in differently sized “coordinate eggs”.3D map

Knowledge of the signal map allows us to fully invert the signal and reconstruct the underlying magnetic field. Our current goal is to build a magnetic field camera with three “colour channels” to measure the complete field distribution. In a scanning setup, we demonstrated this idea using the magnetic field that arises from three electric wires. The figure below shows the comparison between expected field lines, the colour coded signals (grey means zero field), and the field vectors that were reconstructed from the image.

Vector image

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