Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits withπ-shifters

Abstract

We describe fabrication and testing of composite flux qubits combining Nb-and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing π-phase shifters fabricated using well-established Nbbased technology of superconductor-ferromagnet-superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in-situ Ar etching using e-beam gun. Shadow-evaporated Al/AlO x /Al Josephson junctions with Nb bias pads show the expected current-voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb π-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without πjunction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.

Thermal conductance of Nb thin films at sub-kelvin temperatures

“…In this case, a possible candidate for a good thermal insulator is superconducting niobium (Nb), whose critical temperature is higher, 9.2 K in bulk. Nb has been used as a component in a variety of mesoscopic devices, for instance, thermometers and coolers 13 , quantum processors 14 15 16 and in space applications 17 . In addition, superconducting resonators coupled to qubits are employed to detect and manipulate weak electromagnetic fields 18 19 20 21 22 23 .…”

Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

“…The estimated Q 0 values at 4.2 K are up to two orders of magnitude higher compared to those of standard SFS heterostructures that typically operate in the overdamped regime like SNS JJs, with β ranging from 10 −3 to 10 −1 [33,34,57]. Q 0 values are of the same order of magnitude of conventional SIS junctions commonly used to drive and for the read-out of components in quantum and classical circuits [58,59]. Moreover, the Q 0 values increase up to one order of magnitude for the 4.0 nm thick barrier, when lowering T to 300 mK.…”

Ferromagnetic Josephson Junctions for High Performance Computation

“…The tunability of the critical current in different types of ferromagnetic junctions leading to their memory capabilities make them possibly useful in the long range not only for the integration in SFQ circuits, but also for the development of specific types of qubits. Apart from the phase qubit [18][19][20], a transmon could benefit of the notions developed for ferromagnetic JJs. The ratio E J /E C would be no longer tuned by a dc-SQUID, but using a single magnetic JJ (MJJ), via magnetic pulses or RF trains.…”

“…The use of RF fields to control the critical current of magnetic Josephson junctions adds to the well-established capabilities to use ferromagnetic junctions for phase qubits [18][19][20].…”