Pulse control of sudden transition for two qubits in XY spin baths and   quantum phase transition
  We study the dynamics of two initially correlated qubits coupled to their own separate spin baths modeled by a XY spin chain and find the explicit expression of the quantum discord for the system. A sudden transition is found to exist between classical and quantum decoherence by choosing certain initial states. We show that the sudden transition happens near the critical point, which provides a new way to characterize the quantum phase transition. Furthermore, we propose a scheme to prolong the transition time of the quantum discord by applying the bang-bang pulses. 
Da-Wei Luo, Hai-Qing Lin, Jing-Bo Xu and Dao-Xin Yao
quant-ph