Josephson junction quantum bits and logic gates

Abstract

Low-capacitance Josephson junctions can serve as quantum bits, combining the coherence of the superconducting state with control mechanisms of single-charge systems. With logic states differing by one Copper-pair charge, single- and two-bit gates can be performed by applying voltage and flux pulses. The phase coherence time is long enough to allow a series of these steps. In addition to the manipulations the resulting quantum state has to be read out, which can be accomplished by coupling the qubit to a single-electron transistor. To describe this quantum measurement process we study the time evolution of the density matrix of the coupled system. The system can be realized by present-day technology.