Radiation and Infrared Shielding of Qubits
Qubits must be shielded from stray infrared photons, radio-frequency interference, and ionizing radiation, all of which degrade coherence.
An environment of unwanted photons
A qubit is a sensitive antenna. Photons it did not ask for, spanning infrared, microwave, and higher energies, can excite it out of its computational subspace, break Cooper pairs into quasiparticles, or add dephasing noise. Protecting the qubit from this photon bath is as important as cooling it, and the two goals overlap because both concern controlling energy reaching the device.
Infrared and blackbody photons
Even a surface at 4 kelvin radiates infrared photons energetic enough to disturb a millikelvin qubit. Cold sample enclosures with infrared-absorbing coatings, often a dark, lossy epoxy-and-powder mixture, line the innermost cavity to soak up stray infrared before it reaches the chip. Light-tight seams prevent room-temperature radiation from leaking down the wiring channels.
Radio-frequency and microwave hygiene
Control and readout lines must pass intended signals while blocking noise. Multi-stage filtering, including low-pass filters on DC lines and lossy filters on microwave lines, strips out-of-band interference. The whole cold assembly sits inside conductive enclosures acting as Faraday cages. Careful attention is paid to ground loops and to any gap that could act as a slot antenna.
Ionizing radiation
Cosmic rays and trace radioactivity in nearby materials deposit energy that creates bursts of quasiparticles, causing correlated errors across a chip. Mitigations include selecting low-radioactivity materials for components close to the qubits, adding lead or other shielding, and in research settings operating underground to cut the cosmic-ray flux. This concern grows with processor size, since a single event can affect many qubits at once, complicating error correction.
- Stray infrared, RF, and ionizing photons all degrade qubits
- Infrared-absorbing enclosures soak up blackbody photons
- Multi-stage filtering and Faraday cages block RF interference
- Low-radioactivity materials and shielding reduce ionizing events