Synchronization of two cavity-coupled qubits measured by entanglement
  Some nonlinear radiations such as superfluorescence can be understood as cooperative effects between atoms. We regard the cooperative radiation as a manifested effect secondary to the intrinsic synchronization among the atoms and propose a time-resolved measure of synchronization on a cavity-coupled dual-qubit system using multipartite concurrence. Comparing the variation of the concurrence over time with that of an asynchronicity measure, we find that the synchronization between the qubits features a time delay characteristic to the initiation of superfluorescent pulses. The delay coincides with the duration for the qubits to establish cooperation and emit the collective radiation, after which the concurrence monotonically increases to a stationary value while the asynchronicity dives to a steady minimum. Furthermore, the establishment of synchronization is determined by the qubit-cavity coupling strength. Asynchronicity shows that synchronization is only possible when the coupling enters strong regime and sustains to a high level when the coupling becomes ultra-strong. 
Tiantian Huan, Ri-gui Zhou, Hou Ian
quant-ph