Satariano, Roberta (2023) Magnetic tunnel Josephson Junctions towards hybrid quantum architectures. [Tesi di dottorato]

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Abstract

The competition between superconducting and ferromagnetic orders in magnetic Josephson Junctions (MJJs) has paved the way for advances in superconducting digital technology, cryogenic memories, and potentially for quantum computing, where the possibility of switching between different critical currents states by means of magnetic pulses is a crucial advantage. Recently, a proof-of-concept of a hybrid ferromagnetic transmon qubit, namely the ferro-trasmon, has been proposed. In this layout, tunnel MJJ provide an alternative and intrinsically digital tuning of the qubit frequency by means of magnetic field pulses. This proposal may have a strong impact on the scalability of superconducting quantum systems since it may promote alternative control schemes with energy-efficient cryogenic digital electronics. The main aim of this thesis is to develop a tunnel MJJ suitable for the integration into the hybrid circuit mentioned above. In this pursuit, we have gained a deeper insight into the rich phenomenology of a special class of tunnel MJJs based on a Superconductor/ Insulator/ thin superconductor/ Ferromagnet/ Superconductor (SIsFS) structure. We have first demonstrated that the use a strong ferromagnet Permalloy (Py) as F-barrier in tunnel SIsFS Josephson Junctions based on Nb technology allows to scale the junctions’ dimensions down to a few μm2, and in principle down to submicron sizes, demonstrating their functionality as memory elements compatible in speed and power dissipation with standard single flux quantum (SFQ) circuitry. The achievement of a scalable energy-efficient memory is an important result for further developments of SFQ electronics, whose applications are still limited because of the lack of high-density RAM. In the framework of quantum computing, SFQ circuits can provide supporting functions, such as read-out and control, with the great advantage that can be located contiguously to qubit chips. The implementation of a cryogenic co-processor represents indeed a key approach for the realization of a scalable quantum computer. To date, the functionality of MJJs as magnetic switches for digital electronics and for spintronic devices has been demonstrated mostly at liquid-helium temperature. However, at the operating temperature of quantum circuits, novel phenomena at the S/F interface can emerge and lead to a significant modification of the operating regime of the overall device. Specifically, we have provided evidence of an unconventional behaviour of the magnetic dependence of the critical current in this junction and we have established a self-consisted method to ascribe these features to spin polarization phenomena at the S/F interface. Finally, we have transferred our knowledge and notions from the Nb-based to Al-based lithography process and demonstrated the hysteretic behavior of the magnetic field pattern in very low quasi-particle dissipation SISFS JJs with Al electrodes. Since superconducting quantum circuits rely almost exclusively on Al based JJs, we believe that it is a first fundamental step towards the actual integration of MJJs as active elements in quantum circuits.

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