Relaxation of a Coherent Quantum System During Premeasurement.

Recent experiments on superconducting loops have demonstrated coherent superpositions between macroscopic quantum states. In the experiment of the superconducting persistent-current qubit, a dc SQUID measures the magnetic flux generated by the persistent currents of the macroscopic quantum states. Due to the inductive interaction between the qubit and the SQUID, the SQUID has various back effects on the qubit. For instance, the relaxation and dephasing of the qubit are limited by the noise from the environment of the dc SQUID. Here, we calculated the noise transmitted to the qubit from the environment of SQUID and analyzed its effect on qubit dynamics within the spin-boson formalism. The results can be applied to optimizing the measurement circuit for the best signal-to-noise ratio. We also studied the intrinsic limitation of the SQUID measurement scheme and developed new method to improve the measurement efficiency.