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arXiv:2401.14597 · Communications Materials

Superconducting flux qubit with ferromagnetic Josephson π-junction operating at zero magnetic field

Sunmi Kim, Leonid Victorovich Abdurakhimov, Duong Pham, Wei Qiu, Hirotaka Terai, Sahel Ashhab, Shiro Saito, Taro Yamashita, Kouichi Semba

Published Oct 11, 2024• 20 citations
arXivHTMLPDF

Abstract

Conventional superconducting flux qubits require the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which complicates the on-chip integration of this type of device. It has been proposed that by inducing a π-phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as π-junction, it is possible to realize a flux qubit operating at zero magnetic flux. Here, we report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic π-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. Our results pave the way for developing quantum coherent devices, including qubits and sensors, that utilize the interplay between ferromagnetism and superconductivity. Conventional superconducting flux qubits require a finely tuned magnetic field to operate, hindering their on-chip integration. Here, ferromagnetic Josephson junctions with a π-phase shift in the superconducting order parameter allow the realization of a flux qubit operating at zero magnetic field.

DOI
10.1038/s43246-024-00659-1

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IEICE Transactions on Electronics•Feb 4, 2025•1 citation
Conventional superconducting flux qubits require the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which complicates the on-chip integration of this type of device. It has been proposed that by inducing a π-phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as π-junction, it is possible to realize a flux qubit operating at zero magnetic flux. Here, we report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic π-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. Our results pave the way for developing quantum coherent devices, including qubits and sensors, that utilize the interplay between ferromagnetism and superconductivity. Conventional superconducting flux qubits require a finely tuned magnetic field to operate, hindering their on-chip integration. Here, ferromagnetic Josephson junctions with a π-phase shift in the superconducting order parameter allow the realization of a flux qubit operating at zero magnetic field.
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.
In the field of superconducting electronics, a π phase shifter based on a ferromagnetic Josephson junction is expected to provide various advantages to classical and quantum superconducting devices. Here we report niobium nitride (NbN)-based ferromagnetic π junctions on a silicon (Si) substrate with a titanium nitride (TiN) buffer layer, which have applications to flux-bias-free flux quantum bits (qubits) and classical digital logic elements. We fabricated and characterized NbN/aluminum nitride (AlN)/NbN Josephson junctions, NbN/copper nickel (CuNi)/NbN ferromagnetic Josephson junctions, and superconducting quantum interference devices (SQUIDs) consisting of these junctions on the Si substrate. The fabricated NbN/AlN/NbN junctions showed a high junction quality suitable for qubit applications. Furthermore, the magnetic field dependence of the SQUID's critical current indicated that the NbN/CuNi/NbN junction worked as a π phase shifter on the Si substrate.
Half-flux-quantum (HFQ) logic is a superconductor logic family comprising conventional Josephson junctions (0-JJs) and π Josephson junctions (π-JJs). The energy scale of HFQ logic can be flexibly reduced by phase shifts owing to π-JJs; thus, HFQ circuits are a promising building block for qubit interface circuits. In this study, we demonstrate an NbN-based HFQ circuit element compatible with NbN-based superconducting qubits for all-NbN monolithic integration of qubits and HFQ circuits. The use of NbN is beneficial for both qubits and HFQ circuits owing to its high-quality junctions and high kinetic inductance. First, we developed a prototype of an NbN-based 0-JJ/π-JJ hybrid fabrication process for designing HFQ circuits. We evaluated 0-JJs and π-JJs fabricated through the hybrid process, which demonstrated that the 0-JJs had a small deviation with regard to critical current density and the π-JJs had a sufficiently high critical current density to work as π phase shifters. Furthermore, we fabricated an HFQ superconducting quantum interference device, which is one of the most fundamental elements in HFQ circuits, and observed clear HFQ-period modulation in the magnetic flux dependence of the maximum current at 4.2 K.
Abstract Storage of a single magnetic flux quantum in a superconducting loop containing a Josephson junction represents a promising unit cell configuration for construction of a cryogenic memory of superconducting digital circuits. However, application of a DC bias current is required for operation of such a memory cell to maintain trapping of the flux quantum in the storage loop. In this work, we present a superconducting memory cell that uses a superconducting-magnetic π junction. The cell characteristics show flux quantum hysteresis centering at the zero-bias current. We develop a fabrication process that combines superconductor–ferromagnet–superconductor (SFS) junctions with superconductor–normal metal–superconductor (SNS) junctions. The critical current density of the SFS junctions shows a 0–π oscillation as a function of the ferromagnetic layer thickness. The formation of the π junction is confirmed further by the flux modulation curves of a superconducting quantum interference device made from SNS junctions with an additional SFS junction.
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 Nb-based 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 π -junction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.
We report a hybrid fabrication process for superconductor large-scale integrated circuits (ICs) with ferromagnetic π-shifted Josephson junctions (π junctions) and conventional 0 junctions. π junctions have a Nb/PdNi/Nb sandwich structure and are formed on the reliable Nb four-layer structure used for Nb/AlOx/Nb 0-junction-based ICs. The additional process for making π junctions causes little damage to the 0 junctions. The π phase shift for the superconducting macroscopic wave function is confirmed by forming 0-0-π SQUIDs that have two 0 junctions and one π junction in a superconducting loop. The π junction serves as the π phase shifter because the critical currents of the π junctions are much larger than those of the 0 junctions. The modulation patterns of the 0-0-π SQUIDs show a clear shift by the magnetic field corresponding to the half flux quantum (HFQ). The critical currents of the 0-0-π SQUIDs without fields, which are referred to as the nominal critical currents, are reduced depending on the product of the loop inductance and the critical current of the 0 junction. The nominal critical current reaches 1/5 of the critical current of the single 0 junction. The small values of the nominal critical currents contribute to the reduction of the power consumption in the HFQ circuit directly.