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Photoinduced phase transition is the cooperative change of the electronic states and/or the crystal structures of a material induced by ultrashort laser pulses. Its mechanism is closely related to the dynamics of elementary excitations which modulate the quantum-mechanical states of strongly correlated electron systems and/or strongly coupled electron-phonon systems in a coherent regime. As slight fluctuations trigger the macroscopic change of the system in phase transitions, the transient dynamics of light-induced fluctuations play an important role in controlling the photoinduced phase transitions. It has been theoretically revealed that the generation dynamics of photoinduced quantum entanglement causes the aforementioned fluctuations.

A model of remote electron-phonon systems which is not directly interacting with each other was selected. The manner in which the entanglement between the phonons of these systems appears as photoirradiation proceeds was calculated. When two systems are sufficiently separated in space, there is no direct quantum-mechanical correlation between them, and the quantum entanglement is generated between them when “quantized light” is irradiated. The problem that requires the quantum nature of light has not been raised frequently although the relationship between the nonequilibrium state and the entanglement is considered a fundamental problem of physics. The results of this study elucidate the role of light quantization on the transient dynamics of condensed matter.

The results of this study will be extended to the study of photoinduced fluctuations leading to macroscopic change of the physical properties or phase transitions. Simultaneously, it is expected to be closely related to the research in the field of quantum information as the proposed theoretical framework is similar to the one for the fundamental problem of quantum information. Specifically, the photoinduced entanglement generation between the remote qubits can be used as a quantum memory. The results of this study show that a large degree of freedom of phonon states will enable a larger quantum entanglement between phonons than qubits, and that an effective quantum entanglement storage/memory is expected. The quantum many-body problems regarding the electrons and bosons (phonons and photons) have a long history, and it is expected that further detailed studies will continue in the future.

(Written by K. Ishida on behalf of all the authors)