Quantum-feedback-control induced bifurcation and its application to qubit readout

Sensitive measurement of the state of a qubit is crucial to the implementation of quantum information processing. This paper proposes a strategy to utilize a static bifurcation induced by nonlinear quantum feedback control to read out the state of a charge qubit that is dispersively coupled to a transmission line resonator. Compared with the traditional dispersive qubit readout strategy, the main advantage of our proposal is that even a weak qubit-resonator coupling is hopeful to lead to a strong measurement by varying the nonlinear feedback control parameter. This work opens up new perspectives for the applications of nonlinear quantum feedback control.