  It has been recently suggested that probabilities of different events in the
multiverse are given by the frequencies at which these events are encountered
along the worldline of a geodesic observer (the "watcher"). Here I discuss an
extension of this probability measure to quantum theory. The proposed extension
is gauge-invariant, as is the classical version of this measure. Observations
of the watcher are described by a reduced density matrix, and the frequencies
of events can be found using the decoherent histories formalism of Quantum
Mechanics (adapted to open systems). The quantum watcher measure makes
predictions in agreement with the standard Born rule of QM.
