JOSEPHSON FLUX QUBIT

Yasunobu Nakamura, Fumiki Yoshihara, K. Harrabi +1 more

Anthony J. Przybysz, E. Crowe, H. Dharma Kwon +6 more

C. Granata, B. Ruggiero, M. Russo +3 more

We present niobium-based Josephson devices as prototypes for applications to quantum computation with flux qubits. The most interesting feature of this device is the use of a Josephson vertical interferometer for a fine tuning of flux qubit and interquibit coupling. These devices allow a control of the off-diagonal Hamiltonian terms of flux qubit and of the flux transfer function of a superconducting transformer. In the vertical interferometer, the Josephson current is precisely modulated from a maximum to zero with fine control and precision by a small transversal magnetic field parallel to the radio-frequency superconducting loop plane and to the flux transformer.

David K. Ferry

Wenchao Huang, Qifeng Liang, Dao‐Xin Yao +1 more

We propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero-energy Majorana bound states transport 4 π period Josephson currents in the nanowire junction. The interplay between this 4 π period Josephson effect and the convectional 2 π period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest-energy levels of the SQUID are isolated from the other levels. These two levels show counterpropagating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.