The propagating or resonant modes in many commonplace physical systems, including common laser oscillators, are not governed by hermitian operators, and the modes of such systems are not "normal modes," i.e., are not power-orthogonal in the usual sense. Many of the standard results of classical or quantum noise theory must, as a result, be modified or extended when applied to these systems. Laser oscillators employing nonnormal cavity modes, for example, experience increased spontaneous emission or quantum noise effects, as has been confirmed in recent years by observations of substantially increased Schawlow-Townes quantum linewidths in such lasers.
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