An analogy between the laser threshold region and a second-order phase transition is presented. The electric field acts as the order parameter for the laser while the atomic-population inversion plays the role of a temperature. The quantum-statistical theory of a laser including a symmetry-breaking external signal is discussed in order to establish a correspondence with the problem of a ferromagnet (treated in the molecular-field approximation) situated in an externally applied magnetic field. It is demonstrated that the results of the laser analysis may be discussed in terms of an energy function which is similar to the free energy of a thermodynamic system.

• Received 29 December 1969

DOI:https://doi.org/10.1103/PhysRevA.2.1170