The quadratic spin squeezing operators

We discuss generic spin squeezing operators (quadratic in angular momentum operators) capable of squeezing out quantum-mechanical noise from a system of two-level atoms (spins) in a coherent state. Such systems have been considered by Kitagawa and Ueda (1991 Phys. Rev. Lett. 67 1852, 1993 Phys. Rev. A 47 5138) in this context and a Hamiltonian of this nature governs the Lipkin model (Lipkin et al 1965 Nucl. Phys. 62 188) which is relevant to nuclear physics.