  We demonstrate a weak continuous measurement of the pseudo-spin 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 z-component of the collective pseudospin. At certain probe
frequencies the differential light shift of the clock states vanishes and the
measurement is non-perturbing. In dense samples the measurement can be used to
squeeze the collective clock pseudo-spin, and has potential to improve the
performance of atomic clocks and interferometers.
