Continuous nondemolition measurement of the Cs clock transition pseudospin.

We demonstrate a weak continuous measurement of the pseudospin associated with the clock transition in a sample of Cs atoms. Our scheme uses an optical probe tuned near the D1 transition to measure the sample birefringence, which depends on the component of the collective pseudospin. At certain probe frequencies the differential light shift of the clock states vanishes, and the measurement is nonperturbing. In dense samples the measurement can be used to squeeze the collective clock pseudospin and has the potential to improve the performance of atomic clocks and interferometers.