Microscopic origin of anomalous motional heating in ion traps
In plain words
Ions held above metal electrodes heat up far faster than the electrical noise of the metal predicts, which spoils the shared motion that ion-qubit gates use. Which surface process produces this excess electric-field noise is not established.
Precise statement
Trapped ions at ion-electrode distance $d$ and secular frequency $\omega$ show electric-field noise spectral density $S_E(\omega)$ orders of magnitude above the Johnson-noise value, with approximate power laws $S_E \sim \omega^{-a} d^{-b}$ ($b$ reported roughly between 2 and 4), strong dependence on electrode temperature and large reductions after surface cleaning (exponents approximate). Identify the dominant microscopic mechanism (fluctuating adsorbate dipoles, two-level fluctuators in surface layers, patch-potential diffusion or other) and give a model that predicts $S_E$ as a function of $d$, $\omega$, $T$ and surface preparation.
What would settle it
A model that predicts measured $S_{E}(\omega, d, T)$ across traps of different materials and surface treatments, confirmed by surface-science characterization of the same electrodes.
Status in the literature
Surface cleaning and cryogenic operation lower the noise by orders of magnitude, but no single mechanism accounts for all data (review: Brownnutt, Kumph, Rabl and Blatt, Rev. Mod. Phys. 87, 1419, 2015, arXiv:1409.6572).