On the Dimension of the Space of Dark States in the Tavis–Cummings Model

The space of minimal energy of a qubit system is the dark subspace of quantum states of a system of two-level atoms in the finite-dimensional Tavis–Cummings (TC) model of quantum electrodynamics. The two-level atoms in the dark state do not interact with the electromagnetic field, which makes this subspace free from decoherence. An exact expression is obtained for the dimension of the dark subspace in the exact TC model and for the rotating wave approximation (RWA).