Atoms in a crystal form a regular lattice, in which they can move over small distances from their equilibrium positions. Such phonon excitations are represented by quantum states. A superposition of phonon states defines a so-called phonon wavepacket, which is connected with collective coherent oscillations of the atoms in the crystal.
Click here for original story, Symmetry breaking by ultrashort light pulses opens new quantum pathways for coherent phonons
Source: Phys.org