Generation and dynamics of phononic cat states after optical excitation of a quantum dot

Abstract

We study theoretically the fluctuation properties of optical phonons generated after optical excitation of a quantum dot. If the quantum dot exciton is optically manipulated by ultrafast laser pulses, the electronic system and the phonon system can become entangled, which strongly influences the fluctuation properties of the phonons. When reduced to the phonon system, such an entanglement corresponds to a mixed phonon state. We discuss excitations with one or two ultrafast laser pulses. For a single pulse excitation, in general, a statistical mixture of two coherent states is found. For more pulses, a statistical mixture of superpositions of coherent states builds up in the phonon system. With the help of the Wigner function, which provides an intuitive picture of the generated phonon states, we explain how these states are formed depending on the excitation conditions and illustrate their time evolution. From this the fluctuation properties of the corresponding states can be well interpreted and the conditions for obtaining phonon squeezing are identified.