Fifty years after the invention of the optical laser, two separate research groups have independently made important steps toward making phonon lasers - a type of laser that emits very high-frequency, coordinated sound rather than light waves - a reality. The studies, published in the current issue of the journal Physical Review Letters, could lead to a completely new kind of laser that could find interesting applications in medical imaging.

The quantum nature of light means its' possible to emit coherent photons of the same frequency and phase, in a process called "stimulated emission". This was predicted in 1917 by Albert Einstein and first put into practice in 1960, when the first optical laser was built.

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Despite some fundamental differences, light and sound waves are both formed by quanta, meaning that sound lasers (or "sasers") are also possible. Researchers have been looking at sound lasers for some time, but haven't been able to build one working at very high (terahertz) frequencies just yet.

The interest around sound lasers is not just purely academic: sound propagates at a speed that is about 100,000 smaller than the speed of light, and therefore has a proportionately smaller wavelength, along with lower energy levels. The combination of these two factors means sound lasers would allow for extremely precise imaging of living tissue without damaging it in the process (as is often the case with optical imaging).

The main obstacle to the implementation of a high-frequency saser is also what makes it so attractive: the shorter wavelengths make it harder to coordinate the quantum particles to travel coherently and realize the "stimulated emission" in phonons.

Two research teams from the US and the UK tackled the problem using different approaches, and both made important progress towards making sasers a reality. A group from Caltech assembled a pair of microscopic cavities that only permit specific frequencies of phonons to be emitted, effectively producing a resonator that ensures the waves are always in phase with each other.

A second group from the University of Nottingham in the UK took a different approach: they built their device out of electrons moving through a series of structures known as "quantum wells": whenever an electron hops from one quantum well to the next, it produces a phonon. While this system doesn't have the properties of a true phonon lasing, the system showed it amplifies high-frequency sound and could be used in the future as a fundamental building block of the first sound laser.

Both these studies are important breakthroughs that will one day bring to practical, high-frequency phonon lasers. While it's hard to predict right away what repercussion this could have in the long run — the optical laser was deemed next to useless shortly after being invented — medical imaging would surely benefit greatly from its development even in the short term.

Papers: Phonon Laser Action in a Tunable Two-Level System, Coherent Terahertz Sound Amplification and Spectral Line Narrowing in a Stark Ladder Superlattice

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