Superconducting Quarter-Wave Resonator for Quantum Information Systems

Computers based on quantum-mechanical phenomena, if realized, could disrupt many computationally intense fields of science, including high-energy physics. The building block element of a quantum computer is a quantum bit (qubit). The most promising technical solution to build a qubit with long lifetimes is to put a Josephson junction inside of a high Q-factor superconducting 3D cavity. During this project, we found that in addition to the long-term potential in quantum computing, there is a near term commercial opportunity from the development of niobium fabrication technologies, and in particular, in-house machining, welding and chemistry capabilities. In response to this problem, RadiaBeam has designed and built the superconducting (SRF) cavity for quantum computers based on a quarter-wave resonator (QWR) with optimized shape, operating at 6 GHz. We developed significant domestic in-house manufacturing and chemistry capabilities that have not only allowed us to build high quality factor niobium cavities for quantum computers, but also to machine niobium and niobium-tin parts for novel accelerators for National Laboratories. In Phase I, we optimized the shape of the QWR and improved its geometric factor (proportional to Q-factor) by at least a factor of two. In Phase II we have developed the machining and chemistry techniques in order to improve the surface quality of the resonators, and therefore, their Q-factors. The tests performed in Phase II have demonstrated the increase of Q-factors by about an order of magnitude, and we expect better results in the remaining tests. Our collaborators at the University of Chicago have also developed, built and tested the transmons that will be integrated into the niobium resonator. In this Phase IIB we propose to continue building-up the SRF capabilities gained through the quantum computer program, with a particular focus on developing and qualifying SRF component deep drawing and electron beam joining procedures, which are a critical need for DOE National Laboratories. In particular, we will revisit the fabrication technology of the cavity from the machining to forming and welding approach. We will make a niobium tube with a smooth interior, weld a cap with a machined inner conductor, and perform quality tests until the quality of the resonator meets industry standards. The results of this project will be the development of essential capabilities of SRF cavity fabrication in order for RadiaBeam to become a leading vendor of SRF products in the USA. As recently reported by DOE’s Snowmass Report, there is a need for high-quality, cost-competitive manufacturing of low to mid volume SRF products. In addition, the emerging discipline of quantum computing, although presently in its infancy, is rapidly growing and has an explosive commercial potential.