What sets the residual surface resistance floor of niobium cavities
In plain words
Even far below the superconducting transition, cavities keep a small leftover energy loss whose sources are only partly identified.
Precise statement
At $T <\sim 1.5\,\mathrm{K}$, where the BCS surface resistance at 1.3 GHz is negligible, the quality factor of the best Nb cavities saturates at $Q_0$ of order 1e11 (residual resistance $\sim 1-3\,\mathrm{nOhm}$), set by a temperature-independent residual surface resistance. Determine the fractions contributed by trapped magnetic vortices, two-level systems in the surface Nb2O5 oxide, and subgap quasiparticle states, as functions of field, frequency and surface treatment.
What would settle it
Measurements of residual resistance versus trapped flux, frequency and field, combined with oxide removal or regrowth, that separate the three contributions.