Stray magnetic fields in cryogenic environments as a source of decoherence of superconducting qubits

Noelia Fernandez¹, Oscar Gargiulo¹, Felix Rucker¹, Diana Joussetova^1*

¹kiutra GmbH, Munich, Germany

* Presenter:Diana Joussetova, email:diana.joussetova@kiutra.com

Exploring alternative circuit designs and materials for superconducting qubits is a key focus in the rapidly growing quantum computing industry. In this context, metrology is required to assess the reproducibility of qubit parameters, with benchmarking helping set realistic performance targets based on pre-defined standards. For a well understood quantum computing platform such as the superconducting transmon qubit, an
accepted benchmark is a measure of its decoherence, which defines the temporal stability of energy relaxation, dephasing and qubit transition frequency [1].
We investigate the influence of stray magnetic fields on the relaxation times (T₁) of pre-characterized transmon qubits (ConScience QiB0) in a kiutra L-Type Rapid (LTR) fast characterization cryostat and we show that all qubit properties can be investigated in 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 compare T₁ measurements of the qubit in the following scenarios: inside an LTR with no shield around the qubit case; with gold-plated Brass shield enclosing the qubit case; with gold-plated brass shield plus an IR absorption layer, instead.
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 50 μs can be achieved. Our results shed light on what
specific sources of decoherence stemming from the cryogenic environment affect qubit measurements by blocking them with layers of certain materials. Through this work, we also open 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.

References
[1] Burnett, J. J., Bengtsson, A., Scigliuzzo, M., Niepce, D., Kudra, M., Delsing, P., & Bylander, J. (2019), 54.

Keywords: quantum coherence and decoherence, superconducting quibits, cryogenic environment