Quantum Phase Transition and Quantum Dynamics of the Qubits in Dissipative Environment

摘要

An analytic approach is proposed to study the quantum phase transition and quantum dynamics of one- and two-qubit system coupled to a dissipative environment. First, we analyze the ground state phase diagram for Ohmic and sub-Ohmic bath using an analytic ground state wavefunction which goes beyond the Silbey-Harris ground state with lower ground state energy. The quantum critical point α_c is determined as the transition point from non-degenerate to degenerate ground state and our calculated α_c agrees with previous numerical results. Second, a modified Bloch-Redfield approach is proposed which goes beyond the ordinary Bloch-Redfield approach but still keeps its simplicity. Then, we investigate the system's dynamical behavior arising from the complex qubit-bath and qubit-qubit interactions.