“…Recent progress in experimental studies has revealed that synchronization can take place in coupled nonlinear oscillators with intrinsically quantum-mechanical origins, such as micro and nanomechanical oscillators [16][17][18][19][20], spin torque oscillators [21], and cooled atomic ensembles [22,23]. Moreover, theoretical studies have been performed on the synchronization of nonlinear oscillators which explicitly show quantum signatures , such as optomechanical oscillators [24][25][26], cooled atomic ensembles [27,28], trapped ions [29][30][31], spins [32], and superconducting circuits [33]. In particular, a number of studies have analyzed the quantum van der Pol (vdP) oscillator [29], which is a typical model of quantum self-sustained oscillators, for example, synchronization of a quantum vdP oscillator by harmonic driving [26,34] or squeezing [35], mutual synchronization of coupled quantum vdP oscillators [30,36], and quantum fluctuations around oscillating and locked states of a quantum vdP oscillator [40,41].…”