  We study synchronization of two dissipatively coupled Van der Pol oscillators
in the quantum regime. Due to quantum noise strict frequency locking is absent
and is replaced by a crossover from weak to strong frequency entrainment. We
discuss the differences to the behavior of one quantum Van der Pol oscillator
subject to an external drive. Moreover, we describe a possible experimental
realization of two coupled quantum van der Pol oscillators in an optomechanical
setting.
