A phonon laser operating at an exceptional point
The basic quanta of light (photon) and sound (phonon) are bosonic particles that largely obey similar rules and are in general very good analogs of one another. Physicists have explored this analogy in recent experimental investigations of a phonon laser to provide insights into a long-debated issue of how a laser—or more specifically, its line width—is affected when operated at an exceptional point (EP). Exceptional points are singularities in the energy functions of a physical system at which two light modes coalesce (combine into one mode) to produce unusual effects. Until recently, the concept mainly existed only in theory, but received renewed attention with experimental demonstrations in optical systems such as lasers and photonic structures. The experimental studies involved systems with parity-time symmetry for balanced gain and loss of material, to ensure robust light intensity, immune to backscatter. While closed and lossless physical systems are described by Hermitian operators in...