  We observe quantum beats in the nanosecond-scale photoluminescence decay of
rubrene single crystals after photoexcitation with short laser pulses in a
magnetic field of 0.1 to 0.3 T. The relative amplitude of the quantum beats is
of the order of 5\%. Their frequency is $1.3$ GHz when the magnetic field is
oriented parallel to the two-fold rotation axis of the rubrene molecules and
decreases to $0.6$ GHz when the magnetic field is rotated to the crystal's
molecular stacking direction. The amplitude of the quantum beats decays
alongside the non-oscillatory photoluminescence background, which at low
excitation densities has an exponential decay time of $ 4.0 \pm 0.2$~ns. We
interpret this as the effective lifetime of a multiexciton state that
originates from singlet-fission and can undergo geminate recombination back to
the singlet state.
