Content area
Full Text
QUANTUM PHYSICS
A sensitive cold-ion experiment probes sound at the level of phonons, the fundamental quantum units of vibration. It shows that phonons mix in such a way that they can be classified as 'bosonic' particles, like photons.
The phenomenon of wave interference is observed in various settings, including optics, electronics and acoustics. In constructive interference, the crests and troughs of interfering waves reinforce each other, whereas in destructive interference they cancel each other out. Although we think of sound as consisting of macroscopic waves, it has a quantum nature. The energy of a sound wave is an integer multiple of a fundamental quantum of vibrational energy called a phonon. On page 74 of this issue, Toyoda et al.1 report the effect of two-phonon interference, and show that the interfering phonons 'stick together' - they are never observed to go different ways.
The interference of sound waves is not just of academic interest. For instance, it is the operating principle of noise-cancelling headphones. These create their own sound vibrations, which are tuned to destructively interfere with external vibrations. The two vibrations cancel at the ear, and so no sound is heard. By contrast, they constructively interfere at other locations, away from the ear.
On a smaller scale, a quantum-mechanics principle dictates that when the number of phonons (or particles such as photons or electrons) is accurately known for a system, the locations of the crests and troughs of the waves associated with these particles cannot be known with certainty. However, one...