Magneto-optics with cold lithium atoms

The first measurements of magneto- and paramagneto-rotational effects on an ensemble of cold atoms are presented. 7 Li was trapped and cooled to less than a Microkelvin in a magneto-optical trap before being exposed to an external magnetic field having variable direction and intensity. A probing laser beam with an intensity below saturation was directed through the cold atomic ensemble and its change of polarisation was detected by means of a balanced measurement scheme. The rotation of the plane of polarisation was explained by Faraday-, Voigt- and paramagnetic effects. The change of polarisation was measured as a function of the frequency, intensity and initial polarisation of the probe beam, as well as of the direction and power of the magnetic field. Additionally, temporal changes of the detected signal due to optical hyperfine and Zeeman pumping during the measurement were investigated. The measurements are found to be in good agreement with simulations. Due to the small excitation line-width of cold atoms the Faraday effect shows a high sensitivity towards the magnetic field. This enables the realisation of a 'pointlike' magnetometer with the dimension of the atomic cloud. (author)