A laser-cooled atomic probe for low variations of static magnetic fields.

Neutral atom trapping and laser cooling has opened the way to large prospects for the present generation of atomic frequency standards (1). A cold cesium fountain device has been built for the first time at the Observatoire de Paris and is now operating since 1994. Its frequency stability, 4 10 -14  -1/2 where  is the integration time, has been the best one ever measured for a while. This letter shows how accurate can be the measurement of low static magnetic fields within the fountain device and the variations along the longitudinal and transverse axis. Thus the device is no more operating as a clock but constitutes a very precise magnetometer. I. Introduction An atomic frequency standard based on a fountain of cold cesium atoms has been in operation at the Observatoire de Paris since about a decade. It has been included in the calculation of the international atomic time because of its great performances. The flight time between both Ramsey interactions is lengthened to about 500ms so that Ramsey fringes as narrow as 700mHz have been measured (1) for cesium atoms of few K using high power laser diodes at the infrared wavelength 852nm. This frequency standard is three to four orders of magnitude better than classical thermal beam devices in frequency stability and accuracy. II. The prototype