Material sources of decoherence in solid-state qubits

5 problems

• Which interface dominates dielectric loss in millisecond transmons?

  Open

  Energy leaks from qubits into defects at several places: metal-air, metal-substrate and substrate-air surfaces, the substrate interior and the junction itself. Which of these limits the best current qubits is not settled.

• Source of residual nonequilibrium quasiparticles in superconducting qubits

  Partially resolved

  At the millikelvin temperatures where qubits run, essentially no electron pairs should be broken, yet broken pairs (quasiparticles) are found at densities vastly above thermal expectation, and they cause errors. Which sources remain once stray light and radiation are blocked is unknown.

• Microscopic identity of two-level defects in qubit oxides

  Open

  Superconducting qubits lose energy to defects in the thin oxide layers in and around the circuit, which behave like the tunneling defects of glasses. Which atoms or trapped charges form these defects is debated.

• Why does defect loss in superconducting resonators depend weakly on power?

  Open

  The standard tunneling model predicts that defect losses in superconducting resonators fall steeply once the microwave drive is strong enough to saturate the defects, but measured losses fall much more slowly. Whether interactions between the defects explain this is not established.

• Why do qubit energy-relaxation times fluctuate over hours?

  Open

  The energy-loss time of a given qubit can change several-fold from hour to hour, because defect frequencies wander in and out of resonance with it. Whether interactions between defects fully explain this wandering is not established.