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.
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.
In superconducting spintronics, ferromagnetic \pi Josephson junctions are attractive for qubits, cryogenic memory, and phase shifters in logic circuits, offering reduced cell size and superior coherence and scalability. This study demonstrates such junctions based on niobium nitride, which can be grown epitaxially on MgO and is compatible with existing hardware. Its results provide physical insight into superconducting spintronics, as well as an interesting component for applications in quantum information processing.
The physics and applications of superconducting phase shifts and their control in superconducting systems are reviewed herein. The operation principle of almost all superconducting devices is related to the superconducting phase, and an efficient control of the phase is crucial for improving the performance and scalability. Furthermore, employing new methods to shift or control the phase may lead to the development of novel superconducting device applications, such as cryogenic memory and quantum computing devices. Recently, as a result of the progress in nanofabrication techniques, superconducting phase shifts utilizing π states have been realized. In this review, following a discussion of the basic physics of phase propagation and shifts in hybrid superconducting structures, interesting phenomena and device applications in phase-shifted superconducting systems are presented. In addition, various possibilities for developing electrically and magnetically controllable 0 and π junctions are presented; these possibilities are expected to be useful for future devices.
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.
The last several decades have seen an explosion in the use and size of computers for scientific applications. The US Department of Energy has set an ExaScale computing goal for high performance computing that is projected to be unattainable by current CMOS computing designs [1]. This has led to a renewed interest in superconducting computing as a means of beating these projections. One of the primary requirements of this thrust is the development of an efficient cryogenic memory. Estimates of power consumption of early Rapid Single Flux Quantum (RSFQ) memory designs are on the order of MW, far too steep for any real application [1]. Therefore, other memory concepts are required. S/F/S Josephson Junctions, a class of device in which two superconductors (S) are separated by one or more ferromagnetic layers (F) has shown promise as a memory element. Several different systems have been proposed utilizing either the spin-singlet or spin-triplet superconducting states [2]. This talk will discuss the concepts underpinning these devices, and the recent work done to demonstrate their feasibility. [1] Energy-Efficient Superconducting Computing Power Budgets and Requirements, D. Scott Holmes, Andrew L. Ripple, Marc A. Manheimer, IEEE Trans. Appl. Supercon., Vol 23, No. 3, June 2013 [2] B. M. Niedzielski et al., “Use of Pd-Fe and Ni-Fe-Nb as soft magnetic layers in ferromagnetic Josephson junctions for nonvolatile cryogenic memory,” IEEE Trans. Appl. Supercond., vol. 24, no. 4, Aug. 2014, Art. ID. 1800307.
The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F), is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunnelling in Josephson junctions with GdN ferromagnetic insulator barriers. We show a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions, which present clear signatures of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits.
The lack of dense, fast, energy efficient memory has been the main detractor for multiple superconducting digital projects in the past. Recently, fundamental physics research in superconductor-ferromagnet thin-film tunnel structures created a new opportunity to solve this long-standing problem. Superconductivity and ferromagnetism, two deeply antagonistic electronic properties, can co-exist in form of Magnetic Josephson Junctions (MJJs). The superconducting-ferromagnetic MJJs are electrically compatible with traditional superconductor-insulator-superconductor (SIS) Josephson junctions (JJs) used for digital energy-efficient single flux quantum (eSFQ/ERSFQ) circuits. Both MJJ and JJ circuits have similar fabrication process and can be integrated on a single chip. As a result, a combination of MJJs and JJs can be used to form addressable memory cells, energy-efficient memory periphery circuits and programmable logic elements. In this paper, we present the test results of superconductor-insulator-ferromagnet-superconductor (SIFS) MJJs showing their applicability for superconducting spintronic memory and digital circuits.