Entropy evolution law in a laser process

•We apply the thermo-field dynamics method to the master equation of a laser.•We find the operator-sum (Kraus) representation for the density operator.•We find both the normally ordered and compact forms of ρ(t)ρ(t) for ρ0=|z〉〈z|ρ0=|z〉〈z|.•We find the exact expression of the laser’s entropy.•Our results reveal quantitatively how a laser beam can be generated in a laser.

For the first time, we obtain the entropy variation law in a laser process after finding the Kraus operator of the master equation describing the laser process with the use of the entangled state representation. The behavior of entropy is determined by the competition of the gain and damping in the laser process. The evolution formula for the number of photons is also obtained.