Low field laser magnetometry: From biomedical diagnostics to fundamental science
Antoine Weis (Physics Department, University of Fribourg, Switzerland)

I will describe the basic principle of optical pumping magnetometers (OPM), and in particular of modern variants thereof, in which a laser (LsOPM) rather than a spectral discharge lamp (LpOPM) is used for their operation. Our research team uses LsOPMs in applied research (detection of cardiomagnetic fields), and in fundamental research (search for a neutron electric dipole moment).

<span style="font-weight: bold;">Biomagnetism: </span>The tiny electrical currents associated with the electrophysiological activity of the human heart produce very weak magnetic fields, which can be measured outside the human chest as (time dependent) magnetocardiograms (MCG). We have developed a gradiometric LsOPM system using cesium vapor [1], which allows us to record MCGs maps, which can then be represented as movies [2]. Compared to SQUID magnetometers used so far for MCG detection, LsOPMs do not need cryogenic cooling liquids. They offer therefore a very promising alternative for the wider spreading of MCG diagnostics, which has proven advantages over electrocardiography (ECG). The magnetometers for MCG detection have to combine a highest achievable sensitivity with spatial resolution and a high bandwidth, which are mutually exclusive properties. The device used so far has an intrinsic sensitivity of 63 fT/Hz^½ with a bandwidth of 140 Hz and a spatial resolution of 28 mm.

<span style="font-weight: bold;">Fundamental Physics:</span> Our team participates in an international research effort for measuring the electric dipole moment (EDM) of the neutron with a projected 10-100-fold improved sensitivity. That experiment requires the precise measurement and active control of a 2 µT magnetic field and its gradients in a large volume. It is planned that a large number of LsOPMs shall be used to achieve that task. We have developed a modular system of LsOPM heads, which can be driven by a single frequency stabilized laser. With a sensor cell of 70 mm diameter we have demonstrated an intrinsic sensitivity of 10 fT for a 1 second integration time and a bandwidth of 1 kHz. These specifications fulfill the sensitivity requirements of the n-EDM experiment.

<span style="font-weight: bold;">References:</span>

[1] Bison G, Wynands R, Weis A., Appl. Phys.
2003; B76, 325-329.

[2] Bison G, Wynands R, Weis A., Opt. Express 2003a; 11, 904-909.