Quantum sound waves stick together

Abstract

To observe the interference of two phonons, Toyoda et al. start by 'feeding' one phonon to each ion. Because the two ions are positively charged, they repel each other, so that when one vibrates, the other one gently wiggles. The authors' results are a crucial test of the quantum theory of sound, and definitively prove that phonons are bosons rather than fermions (bosons, such as photons, are particles that have integer spin angular momentum, whereas fermions, such as electrons, have halfinteger spin).