  Molecules immersed in liquid helium are excellent probes of superfluidity.
Their electronic, vibrational and rotational dynamics provide valuable clues
about the superfluid at the nanoscale. Here we report on the experimental study
of the laser-induced rotation of helium dimers inside the superfluid
$^4\mathrm{He}$ bath at variable temperature. The coherent rotational dynamics
of $\mathrm{He}_2^*$ is initiated in a controlled way by ultrashort laser
pulses, and tracked by means of time-resolved laser-induced fluorescence. We
detect the decay of rotational coherence on the nanosecond timescale and
investigate the effects of temperature on the decoherence rate. The observed
temperature dependence suggests a non-equilibrium evolution of the quantum
bath, accompanied by the emission of the wave of second sound. The method
offers new ways of studying superfluidity with molecular nano-probes under
variable thermodynamic conditions.
