Backaction noise produced via cavity-aided nondemolition measurement of   an atomic clock state
  We use a quantum nondemolition measurement to probe the collective pseudospin of an atomic ensemble in a high-finesse optical cavity. We analyze the backaction antisqueezing produced by the measurement process to show that our protocol could create conditional spin squeezing in the atomic ensemble. 
Igor Teper, Geert Vrijsen, Jongmin Lee, and Mark A. Kasevich