  Single-photon emitters in solid-state systems are important building blocks
for scalable quantum technologies. Recently, quantum light emitters have been
discovered in the wide-gap van der Waals insulator hBN. These color centers
have attracted considerable attention due to their quantum performance at
elevated temperatures and wide range of transition energies. Here, we
demonstrate coherent state manipulation of a single hBN color center with
ultrafast laser pulses and investigate in our joint experiment-theory study the
coupling between the electronic system and phonons. We demonstrate that
coherent control can not only be performed resonantly on the optical transition
giving access to the decoherence but also phonon-assisted, which reveals the
internal phonon quantum dynamics. In the case of optical phonons we measure
their decoherence, stemming in part from their anharmonic decay. Dephasing
induced by the creation of acoustic phonons manifests as a rapid decrease of
the coherent control signal when traveling phonon wave packets are emitted.
Furthermore, we demonstrate that the quantum superposition between a
phonon-assisted process and the resonant excitation causes ultrafast
oscillations of the coherent control signal. Our results pave the way for
ultrafast phonon quantum state control on the nanoscale and open up a new
promising perspective for hybrid quantum technologies.
