A Continuous Non-demolition Measurement of the Cs Clock Transition   Pseudo-spin
  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. 
Souma Chaudhury, Greg A. Smith, Kevin Schulz, and Poul S. Jessen