  Very high quality factor superconducting radio frequency cavities developed
for accelerators can enable fundamental physics searches with orders of
magnitude higher sensitivity, as well as offer a path to a 1000-fold increase
in the achievable coherence times for cavity-stored quantum states in the 3D
circuit QED architecture. Here we report the first measurements of multiple
accelerator cavities of $f_0=$1.3, 2.6, 5 GHz resonant frequencies down to
temperatures of about 10~mK and field levels down to a few photons, which
reveal record high photon lifetimes up to 2 seconds, while also further
exposing the role of the two level systems (TLS) in the niobium oxide. We also
demonstrate how the TLS contribution can be greatly suppressed by the vacuum
heat treatments at 340-450$^\circ$C.
