A two-level system of N = + particles is distributed among two eigenstates 1 and 2 with eigenenergies E and E, respectively. The system is in contact with a heat reservoir at temperature T. If a single quantum emission into the reservoir occurs, population changes n-1 and m m +1 take place in the system. For m1 and 1, the entropy change of the two level system is O O O KB In 2 kg ln kg ln 1 n n n2 M n 2 kg In (2)