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Italian Quantum Information Science Conference (IQIS)

Ferromagnetic Josephson Junctions for High Performance Computation

Roberta Caruso, D. Massarotti, Alessandro Miano, V. V. Bol’ginov, Aymen Ben Hamida, Liubov N Karelina, I. V. Vernik, V. V. Ryazanov, Oleg A. Mukhanov, Giovanni Piero Pepe, Francesco Tafuri

Published Jun 25, 2019• 0 citations
Publisher

Abstract

Josephson junctions drive the operation of superconducting qubits and they are the key for the coupling and the interfacing of superconducting qubit components with other quantum platforms. They are the only means to introduce non linearity in a superconducting circuit and offer direct solutions to tune the properties of a superconducting qubit, thus enlarging the possible qubit layouts. Junctions performances and tunability can take advantage of using a large variety of barriers and their special functionalities. We mention pertinent results on the advances in understanding the properties of ferromagnetic junctions, which makepossible the use of these devices either as memory elements and as core circuit elements.

DOI
10.3390/proceedings2019012016

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Josephson junctions drive the operation of superconducting qubits and they are the key for the coupling and the interfacing of superconducting qubit components with other quantum platforms. They are the only means to introduce non linearity in a superconducting circuit and offer direct solutions to tune the properties of a superconducting qubit, thus enlarging the possible qubit layouts. Junctions performances and tunability can take advantage of using a large variety of barriers and their special functionalities. We mention pertinent results on the advances in understanding the properties of ferromagnetic junctions, which makepossible the use of these devices either as memory elements and as core circuit elements.
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We propose to exploit currently available tunnel ferromagnetic Josephson junctions to realize a hybrid superconducting qubit. We show that the characteristic hysteretic behavior of the ferromagnetic barrier provides an alternative and intrinsically digital tuning of the qubit frequency by means of magnetic field pulses. To illustrate functionalities and limitation of the device, we discuss the coupling to a readout resonator and the effect of magnetic fluctuations. The possibility to use the qubit as a noise detector and its relevance to investigate the subtle interplay of magnetism and superconductivity is envisaged.
The nontrivial integration of qubits with classical computing systems and the lack of quantum-memory devices remain a key obstacle in the development of quantum technologies. Here, we present a theoretical platform for hybrid quantum devices with memory characteristics. In these current-driven hybrid memristors, a ferroelectric layer serves as the tunneling barrier in a superconducting Josephson junction; thus, the potential barrier is history-dependent. A memory-integrated phase qubit and a Lagrangian description of the superconducting circuit are also presented, paving the way to highly functional, self-modulated Josephson-junction technologies.
In recent years, various novel phenomena such as π-state (π-junction) and long-ranged supercurrents emerged in ferromagnetic Josephson junctions (superconductor/ferromagnet/superconductor junctions) have been studied so actively. An attractive device application of the ferromagnetic π-junction is a flux-bias-free superconducting flux quantum bit (qubit). By inserting the π-junction in the superconducting loop of the flux qubit, an external flux bias corresponding to half flux quantum, which is required for the operation of the conventional flux qubits, becomes unnecessary. This flux-bias-free feature is a great advantage in the realization of large-scale quantum circuits with many qubits. In the talk, we will present recent progress on the development of NbN-based ferromagnetic Josephson junctions which is suitable for the quantum circuits. We also show the experimental results on the several types of the quantum circuits with the NbN-based ferromagnetic π-junction.
Josephson junctions with ferromagnetic barriers (FJJs) enable self-biased operation and inductorless logic, improving energy efficiency and scalability of superconducting digital circuits. This review summarizes the physics of FJJs, the materials and existing devices, junction dynamics, and provides an overview of circuit design schemes that incorporate FJJs, offering practical insights for future applications. The authors discuss challenges in the implementation of these approaches, and highlight directions in device and circuit design.
We review the physical basis of and recent developments in ferromagnetic Josephson junctions with a focus on the π state, which is useful for applications to superconducting devices. Among various novel phenomena that emerge from the interaction between the superconductor and ferromagnet, the π state (junction) works as a phase shifter in superconducting devices, and several proposals and demonstrations of applications to quantum computing elements (qubits), classical superconducting logic circuits, and cryogenic memory have been presented. As a new research trend in superconducting spintronics, we also introduce recent interesting works on the long-range Josephson effect caused by spin-triplet Cooper pairs.