Interaction Of Superconducting Quantum Bit With The Environment

Quantum computation has been shown to perform certain tasks much faster than classical computers. Solid-state quantum bits (qubits) are promising candidates for the realization of a scalable quantum computer. However, they are usually strongly limited by decoherence due to the many extra degrees of freedom of a solid-state system. Considering the initial states of the environment (heat bath), we investigate solid-state qubits interacting with the heat bath.Firstly, the time evolution properties of a superconducting Josephson charge qubit coupled with the heat bath is studied. We find that the initial states of the heat bath play an important role on it. If initially the charge qubit is in the excited state and under the conditions of exact resonance (Δ=0), its evolution is similar to spontaneous emission of the two-levels atom with the heat bath being in the vacuum state. When the initial state of the heat bath is the number state, the probability amplitudes of the ground and excited state are in the forms of cosine summation and sine summation respectively.Secondly, the dissipative properties of the Josephson charge qubit is investigated. Comparing two cases for the heat bath being different temperatures and initial states, we know that: when the charge qubit is in the higher temperature (500mK) and a heat bath is in the number state, then the decay time of its flip operator is longer than that in the equilibrium state, in other words, it has a longer coherent time and the asymptotic value of population operators for the｜0｝state is also smaller. However, the dynamics properties of the charge qubit in the lower temperature ( 50 mK) are unrelated to the initial states for the heat bath.Finally, the decoherence of the dissipative qubits with a driving field is studied under the two-level approximation. The results show that, the decoherence of the qubits is decreased and the decoherent time is increased by applying the external field. On this basis, we analyse the decoherence for three kinds of different coupling superconducting qubits.