The Faraday effect is a magneto-optical phenomenon; the rotation of the polarization of light as it passes through a medium. In addition to being a measure of the density of the medium, the polarization rotation is additionally sensitive to magnetic fields, and so this technique can be used to probe the local magnetic field on a sample.

In our experiment we cool dilute clouds of Rubidium down to ultra-cold temperatures (<100 nk) using laser cooling and magnetic trapping techniques, resulting in a quantum state of matter called a Bose-Einstein condensate (BEC). For many experiments where we want exacting control over the magnetic properties of our BEC, it is critical to reduce and control the background magnetic field to extremely low levels (< milliGauss). One method to do this is to use the atoms themselves as a probe for the magnetic field, through the Faraday effect.

The Rb atoms have strong optical transitions at the D1 (795 nm) and D2 (780 nm) lines. Typically, the Faraday effect is strongest in close proximity to these lines, but light at these wavelengths has the disadvantage of causing unwanted scattering of photons, which re-heats the atoms. Also, the light itself creates a strong electric field, which creates an additional unwanted potential on the atoms. At so-called magic wavelengths (790 nm for Rb) between the two lines, the unwanted scattering is greatly reduced, and the electric field contributions of the light cancel. This means that the light at these wavelengths can be used to continuously probe the cloud of atoms without disturbing the trapping or the temperature of our atoms.

BEC Lab, Centre of Excellence for Engineered Quantum Systems (EQUS).

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