Very high quality factor superconducting radio frequency cavities developed for accelerators can 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 several accelerator cavities of f0 =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 special vacuum heat treatment.Superconducting radio frequency (SRF) cavities in particle accelerators routinely achieve [1, 2] very high quality factors Q > 10 10 − 10 11 corresponding to photon lifetimes T 1 as long as tens of seconds, much higher than highest reported Q ∼ 10 8 used in various quantum regime studies [3,4] with T 1 ∼ 1 msec. Thus, adopting SRF cavities for a 3D circuit QED architecture for quantum computing or memory appears to be a very promising approach due to the potential of a thousand-fold increase in the photon lifetime and therefore cavity-stored quantum state coherence times. Recent investigations [5] revealed that the two-level systems (TLS) residing inside the niobium oxide may play a significant role in the low field performance of SRF cavities, similarly to the 2D resonators [6,7]. Therefore, direct probing in the quantum regime is required to assess the performance of the "as-is" SRF cavities, as well as to guide any future Q improvement directions. Up to now, no such investigations have been performed.In this article, we report the first measurements of a selection of state-of-the-art SRF cavities down to very low temperatures (T < 20 mK) and very low fields ("quantum" regime). We achieve the highest reported photon lifetimes of more than 2 sec, and observe a Q decrease when going from previously explored temperatures of 1.4 K down to below 20 mK. Our results demonstrate that SRF cavities can serve as the longest coherence platform for e.g. 3D cQED and quantum memory [4,8] applications, as well as for various fundamental physics experiments, such as dark photon searches [9]. Furthermore, it is the first direct study of the TLS in the 3D Nb resonators in the quantum regime, as well as the demonstration of the drastic TLS-induced dissipation decrease associated with the oxide removal.We have used fine grain high residual resistivity ratio (RRR) > ∼ 200 bulk single cell niobium cavities of the TESLA shape [10] with resonant frequencies of the TM010 modes of 1.3, 2.6, and 5.0 GHz. One of the investigated 1.3 GHz cavities has been heat treated in vacuum at 340 • C in a custom designed furnace as the last step of the cavity preparation. This novel treatment [11] removes/modifies the niobium pentoxide and allows us to directly investigate the associated improvement in the TLS dissipation.The measurements have been performed first at the vertical t...

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