Measurement-induced spin squeezing in a cavity

Spin squeezing of an ensemble of N atoms in a high-Q cavity is enhanced by continuous measurement of photons in the cavity. A degree of spin squeezing proportional to $N^{-1/3}$ is achieved as a quasi-steady-state over a broad set of initial states of atoms and photons. Our method of spin squeezing is robust against the imperfect photodetection efficiency in current experiments.