A microns-thick film of Nb3Sn on the inner surface of a superconducting radiofrequency (SRF) cavity has been demonstrated to substantially improve cryogenic efficiency compared to the standard niobium material, and its predicted superheating field is approximately twice as high. We review in detail the advantages of Nb3Sn coatings for SRF cavities. We describe the vapor diffusion process used to fabricate this material in the most successful experiments, and we compare the differences in the process used at different labs. We overview results of Nb3Sn SRF coatings, including CW and pulsed measurements of cavities as well as microscopic measurements. We discuss special considerations that must be practised when using Nb3Sn cavities in applications. Finally, we conclude by summarizing the state-of-the-art and describing the outlook for this alternative SRF material.

We report the finding of new surface treatments that permit to manipulate the niobium resonator nitrogen content in the first few nanometers in a controlled way, and the resonator fundamental Mattis-Bardeen surface resistance and residual resistance accordingly. In particular, we find surface "infusion" conditions that systematically a) increase the quality factor of these 1.3 GHz superconducting radio frequency (SRF) bulk niobium resonators, up to very high gradients; b) increase the achievable accelerating gradient of the cavity compared to its own baseline with state-of-the-art surface processing. Cavities subject to the new surface process have larger than two times the state of the art Q at 2K for accelerating fields > 35 MV/m. Moreover, very high accelerating gradients ~ 45 MV/m are repeatedly reached, which correspond to peak magnetic surface fields of 190 mT, among the highest measured for bulk niobium cavities. These findings open the opportunity to tailor the surface impurity content distribution to maximize performance in Q and gradients, and have therefore very important implications on future performance and cost of SRF based accelerators. They also help deepen the understanding of the physics of the RF niobium cavity surface.

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...

Previous work has demonstrated that the radio frequency surface resistance of niobium resonators is dramatically reduced when nitrogen impurities are dissolved as interstitial in the material. This effect is attributed to the lowering of the Mattis-Bardeen surface resistance with increasing accelerating field; however, the microscopic origin of this phenomenon is poorly understood. Meanwhile, an enhancement of the sensitivity to trapped magnetic field is typically observed for such cavities. In this paper, we conduct a systematic study on these different components contributing to the total surface resistance as a function of different levels of dissolved nitrogen, in comparison with standard surface treatments for niobium resonators. Adding these results together, we are able to show which is the optimum surface treatment that maximizes the Q-factor of superconducting niobium resonators as a function of expected trapped magnetic field in the cavity walls. These results also provide insights on the physics behind the change in the field dependence of the Mattis-Bardeen surface resistance, and of the trapped magnetic vortex induced losses in superconducting niobium resonators.

Theoretical limits to the performance of superconductors in high magnetic fields parallel to their surfaces are of key relevance to current and future accelerating cavities, especially those made of new higher-Tc materials such as Nb3Sn, NbN, and MgB2. Indeed, beyond the so-called superheating field H sh , flux will spontaneously penetrate even a perfect superconducting surface and ruin the performance. We present intuitive arguments and simple estimates for H sh , and combine them with our previous rigorous calculations, which we summarize. We briefly discuss experimental measurements of the superheating field, comparing to our estimates. We explore the effects of materials anisotropy and the danger of disorder in nucleating vortex entry. Will we need to control surface orientation in the layered compound MgB2? Can we estimate theoretically whether dirt and defects make these new materials fundamentally more challenging to optimize than niobium?Finally, we discuss and analyze recent proposals to use thin superconducting layers or laminates to enhance the performance of superconducting cavities. Flux entering a laminate can lead to so-called pancake vortices; we consider the physics of the dislocation motion and potential re-annihilation or stabilization of these vortices after their entry.