The Spectrum Consortium has worked out a novel solution that is aimed at enhancing the control over superconducting Qubits while reducing the amount of lines inside the cryostat, which is in fact QueSt, i.e. a Quantum Superconducting Switch. QueSt is a nanofabricated network switch based on a superconducting high-frequency transistor, controlled by gate voltages.
Starting from the original product concept of fully metallic device, the team has been trying different solutions by varying materials and circuitry designs, achieving an inexpensive and easily scalable product.
Through an iterative trials and errors approach, different materials combinations realized through sputtering and other nanodeposition techniques.
In this way, a novel material platform was developed, based on InAsOI architecture. The approach leverages accurate epitaxial growth and selective doping techniques to precisely tune the electrical characteristics of the semiconductor layer. When combined with a conventional superconductive film (defining the drain and source) and a MOS-like gate structure, the team was able to address critical performance challenges in high-speed switching and quantum processor applications by leveraging Josephson Junction effect.
As a result of the above activity, the newly discovered and patented InAsOI platform is highlighted as a more robust alternative to the metallic approach, with structural similarities to silicon-on-insulator (SOI) systems. In particular, InAsOI features an InAs semiconductor layer atop a cryogenic insulator that is capable of providing several advantages.
Firstly, it enables stable superconductivity due to the proximity effect which, when coupled with superconducting leads, ensures a controlled, non-dissipative current flow, thereby reducing overheating risks and maintaining signal integrity. This translates into higher stability and performance, as InAsOI can operate at lower gate voltages and supports higher critical current densities, both essential for reliable superconducting operations. Furthermore, it provides predictable gate control, allowing for stable, low-voltage control over the conduction properties, which minimizes overheating issues and enhances compatibility with cryogenic conditions.
Within this framework, the CNR team has successfully fabricated various samples of JoFETs (Josephson Junction Field-Effect Transistor) utilizing this material platform. The findings indicate that in these JoFETs, gate-controlled modulation precisely manages superconducting pathways. This results in more stable gate control, as InAsOI FETs operate effectively under lower gate voltages, leading to improved reliability and energy efficiency. Moreover, the platform demonstrates scalability for quantum applications, given that its low energy dissipation and high critical current density make it highly suitable for integration into quantum computing systems, where superconducting efficiency is paramount.
Despite great promises, the workplan was not fulfilled because of the delays with delivery of some essential components.
As a consequence, the pilot study at CUT was performed in a partial manner.
Nevertheless, the results shown during the even partial pilot study has confirmed the ability of QueSt as a superconductive cryocomponent not generating any heat effect during functioning. This is a ground breaking achievement as all the state of the art switches produce some heat within the cryostat, thus creating noise and impeding smooth measurements of the Qubits.
