Investigation of the stray magnetic field distribution in a magnetic refrigerator as a potential source of noise and decoherence of superconducting qubits

One important aspect to protect the properties of superconducting qubits is the isolation from external magnetic fields. This raises concerns on the suitability of magnetic refrigerators relying on Adiabatic Demagnetization Refrigeration (ADR) for the characterization of superconducting quantum processors.



We investigate the influence of stray magnetic fields on transmon qubits in a Kiutra L-Type Rapid fast characterization cryostat and show that all key qubit properties can be investigated using such an instrument. Foremost, we study the magnetic field distribution inside the cryostat using a 3D fluxgate and compare our results with numerical simulations. Then we investigate T1, T2*, and T2-echo on single qubit devices in different shielding scenarios. We consider three different setups: without any magnetic shielding; with cryogenic multilayer shield to minimize ADR contributions; adding dedicated on-puck shields.



We find the absolute magnetic field strength to vary between >50 µT in an unshielded scenario, to <100 nT in a fully shielded. Further, we show that already with an intermediate scale shielding reliable T1 times of up to 40 µs can be achieved. Our results shed light on an avenue to accelerate research with superconducting qubits as well as their characterization at different manufacturing stages, enabled by fast-turnaround cryostats based on magnetic cooling.