In a recent study, researchers at the University of Colorado have resolved phonon Fock states in the spectrum of a superconducting qubit coupled to a multimode acoustic cavity. Fock states (or number states) are quantum states with a clearly defined number of particles. These states play a crucial part in the second quantization formulation of quantum mechanics.
Source:
https://phys.org/news/2019-07-coupling-qubits-multimode-cavity.html