Coherent Quantum Control In Superconducting Josephson Qubits

In this thesis, we report on two of our theoretical works based on recent experimentation. One is about how to use the triangle pulse to control the state of a multi-anticrossing system. The other is about how to use Rabi oscillation in the rf SQUID to improve the resolution of a magnetometer.In chapter 1, we propose a universal analytical method to study the dynamics of a multi-anticrossing system subjected to driving by one single large-amplitude triangle pulse, within its time scales smaller than the dephasing time. Our approach can explain the main features of the Landau-Zener-Stuckelberg interference patterns recently observed in a tripartite system [Nature Communications 1:51 (2010)]. In particular, we focus on the effects of the size of anticrossings on interference and compare the calculated interference patterns with numerical simulations. In addition, Fourier transform of the patterns can extract information on the energy level spectrum.In chapter 2, we propose a macroscopic quantum coherence magnetometer, namely, MAQCOM, which is based on the macroscopic quantum coherence in a superconducting quantum circuit. It is an artificial two-level quantum system and the photon induced transition between quantum levels is used to detect the magnetic field. At current experimental setup, the flux resolution of this scheme can be as small as 2.5μΦ0, about 100 times better than that of the under-damped dc SQUID. The small size and high sensitivity make it a very promising device to read out the quantum nano-devices.