An ultra-stable 1.5 tesla permanent magnet assembly for cryogenic spin qubit experiments

The generation of static magnetic fields with flux densities of 0.3 – 1.5 T are key to performing many experiments from the characterisation of materials to providing an energy splitting and quantization axis for a spin qubit. We present a compact permanent magnet assembly capable of operating at mK and generating a magnetic field of up to ~1.5 T across a 7 mm airgap. The assembly, based on the Halbach array, is constructed from neodymium (NdFeB) permanent magnets and the soft magnetic material Supermendur. We demonstrate the operation of a silicon based single atom spin qubit device mounted to the assembly and use the qubit to characterise the assembly. In doing so, we measure an outstanding magnetic field stability of < 1.7 ppb/h.