We investigate the influence of environmental decoherence on the dynamics of a coupled qubit system and quantum correlation. We analyse the relationship between concurrence and the degree of initial entanglement or the purity of initial quantum state, and also their relationship with quantum discord. The results show that the decrease of the purity of an initial quantum state can induce the attenuation of concurrence or quantum discord, but the attenuation of quantum discord is obviously slower than the concurrence's, correspondingly the survival time of quantum discord is longer. Further investigation reveals that the robustness of quantum discord and concurrence relies on the entanglement degree of the initial quantum state. The higher the degree of entanglement, the more robust the quantum discord is than concurrence. And the reverse is equally true. Birth and death happen to quantum discord periodically and a newborn quantum discord comes into being under a certain condition, so does the concurrence.
We demonstrate the controllable generation of multi-photon Fock states in circuit quantum electrodynamics (circuit QED). The external bias flux regulated by a counter can effectively adjust the bias time on each superconducting flux qubit so that each flux qubit can pass in turn through the circuit cavity and thereby avoid the effect of decoherence. We further investigate the quantum correlation dynamics of coupling superconducting qubits in a Fock state. The results reveal that the lower the photon number of the light field in the number state, the stronger the interaction between qubits is, then the more beneficial to maintaining entanglement between qubits it will be.
We investigate the relation between the speed of quantum NOT gate and the asymmetry or detuning of the potential in system of the interaction of a two-level rf-SQUID qubit with a classical microwave pulse. The rf-SQUID is characterized by an asymmetric double well potential that gives rise to diagonal matrix elements. Then in resonance, we compare the gate speeds for three-level and two-level quantum NOT gates. We show that in general, a three-level gate is much faster than the conventional two-level gate.
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