Fabrication and Test of a Cryogen-Free Bi-2212 Magnet Prototype for Solid-State Spin Qubit Applications

Abstract

Solid-state spin qubit experiments for quantum technology require highly stable and temporally homogeneous magnetic fields. While permanent magnets are commonly used, their performance is limited by sensitivity to temperature fluctuations and field strength. To address these challenges, this work explores the use of a cryogen-free Bi-2212 superconducting magnet as a viable alternative. The fabrication and test of a prototype magnet are presented. Two initial coils (Coil A and Coil B) were fabricated to investigate current lead assembly methods, revealing bonding and oxidation issues with Inconel 600 and copper components. A refined fabrication process was developed and validated using a third coil (Coil C), which successfully underwent heat treatment and current lead assembly. Coil C was cooled to below 12 K using a cryocooler and operated at currents up to 150 A. Nonlinear behaviors in terminal voltage and magnetic field were observed and attributed to the ferromagnetic response of the Inconel 600 bobbin at cryogenic temperatures. These effects were modeled using an empirically derived B-H curve, showing reasonable agreement with simulation. The results demonstrate the feasibility of cryogen-free Bi-2212 magnets for quantum applications and establish a reliable fabrication method for future scaling.