  We experimentally demonstrate a controlled transfer of angular momentum to
roton pairs in superfluid helium. The control is executed with an optical
centrifuge and detected with coherent time- and frequency-resolved Raman
scattering. We show that the sign of the Raman shift, and hence the orientation
of the angular momentum transferred from the laser field to the rotons, is
dictated by the centrifuge. The magnitude of the shift reflects the two-roton
energy and indicates that the centrifuge-induced hot roton pairs are far from
the equilibrium with the colder quantum bath. The observed decay of the
coherent Raman signal suggests that the decoherence is governed by the
scattering on thermal rotons and phonons. The demonstrated method offers ways
of examining microscopic origins of superfluidity by controlling collective
excitations in superfluids.
