singlephonon

Single phonon refers to the quantum of vibrational energy associated with a normal mode of a crystal lattice. In the harmonic approximation of a solid, lattice vibrations can be decomposed into normal modes, each described as a quantum harmonic oscillator. A single phonon excitation in a mode with wavevector k has energy E = ħω_k and crystal momentum ħk, and it obeys Bose-Einstein statistics. The occupancy of a mode is described by the phonon number n_k, with n_k = 1 corresponding to a single-phonon Fock state.

Phonons can be created and annihilated by processes such as scattering of light or electrons with the lattice, carrier recombination, or coupling to external fields. In experiments, single-phonon states are prepared in nanomechanical resonators, optomechanical systems, or via emission from quantum emitters coupled to a lattice, and detected through changes in optical, electrical, or spin degrees of freedom, or by direct spectroscopy such as inelastic scattering.

Techniques for observing and manipulating single phonons include Raman and inelastic neutron scattering, Brillouin scattering for phonons, and, in modern platforms, the use of superconducting qubits or optomechanical readout to resolve single-phonon excitations. In some systems, a single phonon can be stored as a long-lived vibrational excitation and acts as a quantum of information or a transduction resource linking different physical domains.

Key challenges include short lifetimes due to damping, thermal population, and coupling to the environment; achieving reliable single-phonon generation and readout at cryogenic temperatures remains an active area of research. Potential applications span quantum information processing, phonon-mediated quantum networks, and transduction between optical and microwave signals.