Abstract. We show that an excellent approximation to the exact quantum solution of the ground state of the Tavis–Cummings model is obtained by means of a semi-classical projected state. This state has an analytical form in terms of model parameters and, in contrast to the exact quantum state, allows for an analytical calculation of the expectation values of field and matter observables, entanglement entropy between field and matter, squeezing parameter and population probability distributions. The fidelity between this projected state and the exact quantum ground state is very close to 1, except for the region of classical phase transitions. We compare the analytical results with those of the exact solution obtained through direct Hamiltonian diagonalization as a function of atomic separation energy and matter–field coupling.

PACS numbers: 42.50.Ct, 03.65.Fd, 64.70.Tg

Print publication: Issue 5 (November 2009)
Received 3 July 2009, accepted for publication 16 September 2009
Published 27 October 2009