Stochastic Amplifier Based on Superconducting Interferometer with 4-Terminal Josephson Junction for Flux Qubit State Readout

This paper proposes one of possible solutions to arrange the weak continuous measurements of the state of the flux qubit based on a superconducting interferometer ring, with a stochastic amplifier which provides the minor back action.The amplifier exploits the stochastic resonance (SR) effect and is embodied as a superconducting interferometer ring incorporating the four-terminal (4-t) Josephson junction . The two leads of the 4-t Josephson junction are closed by the amplifier-interferometer ring while the rest two are connected to the superconducting magnetic antenna (flux transformer) that senses the flux qubit state. In this arrangement, a variation of the qubit flux which corresponds to a certain qubit state superposition results in a change in the antenna loop current. Further, the stochastic amplifier gain, as it will be shown below, depends on the antenna loop current thus monitoring the cubit state.