Magnetic Shielding in Cryogenic Systems
Stray and time-varying magnetic fields disturb qubits and cryogenic sensors, so cold assemblies are wrapped in high-permeability and superconducting shields.
Why fields must be controlled
Many quantum devices are sensitive to magnetic fields. Flux noise dephases superconducting qubits, changing fields induce currents in moving conductors, and even the small ambient field of the Earth and its fluctuations from nearby equipment can shift device parameters. Controlling the magnetic environment of the cold stages is therefore part of building a quiet system, alongside thermal and radio-frequency shielding.
Two kinds of shield
High-permeability metal shields, such as mu-metal, redirect magnetic field lines around the protected volume by providing an easy path for flux. They are effective against static and low-frequency fields. Superconducting shields work differently: a superconductor expels magnetic flux by the Meissner effect and, once cold, locks in whatever field was present when it transitioned, so it both excludes external changes and freezes the internal field. Combining both gives strong static and dynamic shielding.
The cooling-in-field subtlety
Because a superconductor traps the field present at the moment it becomes superconducting, the sequence of cooling and field application matters. Cooling a superconducting shield in a low, well-controlled field, rather than in an uncontrolled ambient field, ensures the trapped field is small. Getting this wrong can leave a device sitting in a frozen-in field that degrades performance for the whole run until the next warm-up.
Interaction with the rest of the cryostat
Magnetic shields must coexist with thermal and vibration requirements. Shield materials add mass and heat capacity, slowing cooldown, and must themselves be heat-sunk. Any deliberate magnet for the experiment, such as a field for spin qubits, must be accommodated without letting its fringe field reach neighboring devices. Magnetic design is thus one more coupled constraint in the crowded volume of a cold system.
- Flux noise and stray fields disturb qubits and sensors
- Mu-metal redirects static and low-frequency fields
- Superconducting shields expel and trap flux via the Meissner effect
- Cool superconducting shields in a low, controlled field