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

Abstract

Magnetic fields are a standard tool in the toolbox of every physicist, and are required for the characterization of materials, as well as the polarization of spins in nuclear magnetic resonance or electron paramagnetic resonance experiments. Quite often a static magnetic field of sufficiently large, but fixed magnitude is suitable for these tasks. Here we present a permanent magnet assembly that can achieve magnetic field strengths of up to 1.5 T over an air gap length of 7 mm. The assembly is based on a Halbach array of neodymium (NdFeB) magnets, with the inclusion of the soft magnetic material Supermendur to boost the magnetic field strength inside the air gap. We present the design, simulation and characterization of the permanent magnet assembly, measuring an outstanding magnetic field stability with a drift rate of < 1.7 ppb/h. Our measurements demonstrate that this assembly can be used for spin qubit experiments inside a dilution refrigerator, successfully replacing the more expensive and bulky superconducting solenoids.

Precision tomography of a three-qubit donor quantum processor in silicon

“…The device was wire-bonded to a gold-plated printed circuit board and placed in a copper enclosure. The enclosure was placed in a permanent magnet array [48], producing a static magnetic field of 1.33 T at the device (see Extended Data Fig. 1 for field orientation).…”