  Lasers essentially consist of single-mode optical cavities containing
two-level atoms with a supply of energy called the pump and a sink of energy,
perhaps an optical detector. The latter converts the light energy into a
sequence of electrical pulses corresponding to photo-detection events. It was
predicted in 1984 on the basis of Quantum Optics and verified experimentally
shortly thereafter that when the pump is non-fluctuating the emitted light does
not fluctuate much. Precisely, this means that the variance of the number of
photo-detection events observed over a sufficiently long period of time is much
smaller than the average number of events. Light having that property is said
to be ``sub-Poissonian''. The theory presented rests on the concept introduced
by Einstein around 1905, asserting that matter may exchange energy with a wave
at angular frequency $\omega$ only by multiples of $\hbar\omega$. The optical
field energy may only vary by integral multiples of $\hbar\omega$ as a result
of matter quantization and conservation of energy. A number of important
results relating to isolated optical cavities containing two-level atoms are
first established on the basis of the laws of Statistical Mechanics. Next, the
laser system with a pump and an absorber of radiation is treated. The
expression of the photo-current spectral density found in that manner coincides
with the Quantum Optics result. The concepts employed in this paper are
intuitive and the algebra is elementary. The paper supplements a previous OQE
tutorial paper in establishing a connection between the theory of laser noise
and Statistical Mechanics.
