  Superfluid 4He, the first superfluid ever discovered, is in some ways the
least well understood. Unlike 3He superfluid, or the variety of Bose-Einstein
condensates of ultracold gases, superfluid 4He is a very dense liquid of
strongly interacting quasiparticles. The theory is then necessarily
phenomenological: the quasiparticle properties are found from experiment, and
controversies over their description still remain, notably regarding vortex
dynamics and the nature of rotons and roton pair creation. It is therefore
important to develop new experimental tools for probing the system far from
equilibrium. Here we describe a method for locally perturbing the density of
superfluid helium through the excitation of roton pairs with ultrashort laser
pulses. By measuring the time dependence of this perturbation, we track the
non-equilibrium evolution of the two-roton states on a picosecond timescale.
Our results reveal an ultrafast cooling of hot roton pairs as they thermalize
with the colder gas of other quasiparticles. We anticipate that these findings,
as well as future applications of the introduced ultrafast laser technique to
different temperature and pressure regimes in bulk liquid 4He, will stimulate
further experimental and theoretical investigations towards better
understanding of superfluidity.
