**Abstract**
We consider a system consisting of many two-level atoms in an optical cavity. Due to the strong interaction between atoms and cavity photons, this system has a quantum phase transition in ground states.

We now investigate the same system subject to dissipation and laser driving. It is found that stationary states, which the system reaches eventually, exhibit novel out-of-equilibrium phase transitions due to the interplay of microscopic Hamilton dynamics, external driving, and the dissipative processes [1]. It is stressed that microscopic quantum interference effect is greatly amplified into a macroscopic level due to long-range interactions among atoms mediated by cavity photons.

Analysis given here is based on the mean-field theory, which makes it tractable to study stationary states of the many body system. However, there is no approximation at this point; it is rigorously shown that the mean-field theory is exact in the thermodynamic limit [2].

[1] T. Shirai, T. Mori, and S. Miyashita, arXiv:1204.5516

[2] T. Mori, arXiv:1212.6726; to appear in J. Stat. Mech.