Phase synchronization was proved to be unbounded in quantum level, but the witness of phase synchronization is always expensive in terms of the quantum resource and non-local measurements involved. Based on the quantum uncertainty relation, we construct two local criterions for the phase synchronization in this paper. The local criterions indicate that the phase synchronization in the quantum level can be witnessed only by the local measurements, and the deduction has been verified in the optomechanics system in numerical way. Besides, by analyzing the physical essence of the phase synchronization in quantum level, we show that one can prepare a state, which describes two synchronized oscillators with no entanglement between them. Thus, the entanglement resource is not necessary in the occurrence of the ideal phase synchronization, and also the reason for this phenomenon is discussed.PACS number(s): 03.65.Yz; 03.65.Ud
I. IntroductionSpontaneous synchronization refers to the phenomenon that the two or more weakly coupled systems with different natural frequencies synchronize their motions only due to their mutual interaction [1][2][3][4]. The occurrence of synchronization is first discovered by Huygens in two coupled pendulum clocks, and then has been observed in so many different settings, such as the collective lightning of fireflies, the beating of heart cells and chemical reaction [2]. In classical mechanics, the spontaneous synchronization has been widely studied [5][6][7][8], and there exist standard methods to verify whether the motion of two systems is synchronized [2].In quantum level, the spontaneous synchronization has been considered from different aspects: clock synchronization [9][10][11][12], synchronization in oscillator networks [13][14][15][16][17][18][19][20][21], and synchronization between two atomic ensembles [22]. Notably, due to the absence of the phase space trajectories, the extension of the notion of phase synchronization from classical mechanics to its quantum counterpart is not straightforward [23]. Lots of work has been done and great progress has been