Synchronization is a universal phenomenon that is important both in fundamental studies and in technical applications. Here we investigate synchronization in the simplest quantum-mechanical scenario possible, i.e., a quantum-mechanical self-sustained oscillator coupled to an external harmonic drive. Using the power spectrum we analyze synchronization in terms of frequency entrainment and frequency locking in close analogy to the classical case. We show that there is a step-like crossover to a synchronized state as a function of the driving strength. In contrast to the classical case, there is a finite threshold value in driving. Quantum noise reduces the synchronized region and leads to a deviation from strict frequency locking. PACS numbers: 05.45.Xt, Synchronization is an intriguing phenomenon exhibited by a wide range of physical, chemical, and biological systems [1]. The basic setting consists of coupled self-oscillating systems synchronizing their motion; examples include such different phenomena as orbital resonances in planetary motion or the rhythm of muscle cells in mammal hearts. A paradigmatic and widely studied model of synchronization is the Kuramoto model of coupled limit-cycle oscillators [2,3].The most fundamental scenario of classical synchronization is the frequency locking of a self-sustained oscillator which is externally driven by a harmonic force [1,4]. A self-sustained oscillator takes energy from a source, e.g. by negative damping, and can therefore maintain stable oscillatory motion and an undetermined phase in the presence of dissipation. If the oscillator is additionally driven by a harmonic force, there is a finite range of detuning for which the oscillator is frequencylocked to the drive, and noise can reduce or destroy this range of synchronization [1]. There are also regimes of frequency entrainment in which the oscillator frequency is pulled towards the drive frequency, but does not reach it. In this case, the frequency of the driven oscillator, the observed frequency ω obs , differs from both the natural frequency of the oscillator and the drive frequency. The simplest model exhibiting these effects is the van der Pol oscillator [1] which allows the analysis of the complex phenomenology of synchronization.Recently, the question if synchronization exists in quantum systems has attracted a lot of interest. There have been important first attempts to address this problem theoretically, from communities as diverse as trapped atomic ensembles, Josephson junctions, and nanomechanical systems [6][7][8][9][10][11][12][13][14][15][16]. Optomechanical systems [17] appear to offer a particularly promising approach. Recent experiments have reported classical synchronization of nanomechanical oscillators [18,19], the quantum many-body dynamics of an array of identical optomechanical cells has been predicted to show synchronized behavior [9], and quantitative measures for quantum synchronization based on the Heisenberg uncertainty principle have been applied to two and many coupled optomechani...