Employment of Submicron YBA₂CU₃O_7-x Grain Boundary Junctions for the Fabrication of “ Quiet” Superconducting Flux-Qubits

Abstract

The implementation of superconducting flux-qu-bits represents a challenge for the future. Superconductors offer both the scalability, typical of solid state technology, and the properties of a macroscopic quantum system. However, the real employment of superconducting devices passes through the reduction of decoherence time of a single qu-bit as well as a number of entangled qu-bits. Decoherence can be reduced by using appropriate technological choices, like the ones minimizing the coupling of the quantum system to the environment. The discovery of unconventional superconductors offered the possibility to study new Josephson junction based circuitries that, in principle, can reduce the coupling to external room temperature electronics. We re-examined the five junction flux qu-bit proposed in 2001 by Blatter and coworkers, composed of a superconducting loop with four conventional junctions and one π-junction, characterized by an energy minimum shifted by π with respect to conventional Josephson junctions. In this scheme, the π -junction acts as a simple phase shifter, frustrating the superconducting loop, and allowing the formation of a double degenerate ground state. In our proposal, the π-junction is a particular device based on the properties of Andreev reflections to form bound states at the Fermi energy.