  We study the emergence of a parity breaking coherent photonic state of a
photon mode coupled to finite array of two-level systems, represented by
pseudospins 1/2. The pseudospin-photon interaction is realised via a shift of
the photonic oscillator equilibrium position by an amount linear in Cartesian
component of the total pseudospin. We demonstrate analytically, that the
instability is manifested in an upturn from concave to convex of the ground
state energy dependence on the total pseudospin component coupled to the
photons. The perturbation, sufficient for parity breaking, tends to zero in the
ultrastrong limit of light-matter coupling. We present phase diagram of finite
pseudospin-photon system, that demonstrates this feature. Evolution of Husimi
Q-functions of the pseudospin and photon subsystems, and of the pseudospin
entropy, along different trajectories across the phase diagram is presented.
