An ultra-stable 1.5 T permanent magnet assembly for qubit experiments at cryogenic temperatures

Magnetic fields are a standard tool in the toolbox of every physicist, and required for characterization materials, as well polarization spins nuclear magnetic resonance or electron paramagnetic experiments. Quite often static field sufficiently large, but fixed magnitude is suitable these tasks. Here we present permanent magnet assembly that can achieve strengths up to 1.5T over an air gap length 7mm. The based on Halbach array neodymium (NdFeB) magnets, with inclusion soft material Supermendur boost strength inside gap. We design, simulation assembly, measuring outstanding stability drift rate |D| < 2.8 ppb/h. Our measurements demonstrate this be used spin qubit experiments dilution refrigerator, successfully replacing more expensive bulky superconducting solenoids.