We present microwave-frequency NbTiN resonators on silicon, systematically achieving internal quality factors above 1 M in the quantum regime. We use two techniques to reduce losses associated with two-level systems: an additional substrate surface treatment prior to NbTiN deposition to optimize the metal-substrate interface, and deep reactive-ion etching of the substrate to displace the substrate-vacuum interfaces away from high electric fields. The temperature and power dependence of resonator behavior indicate that two-level systems still contribute significantly to energy dissipation, suggesting that more interface optimization could further improve performance.Superconducting coplanar waveguide (CPW) microwave resonators are crucial elements in photon detectors 1 , quantum-limited parametric amplifiers 2,3 and narrow-band filters 4 , as well as read-out, interconnect and memory elements in quantum processors based on circuit quantum electrodynamics 5 . They also play a critical role in hybrid devices, connecting superconducting circuits with micro-and nanomechanical resonators 6,7 and solid-state spins 8,9 . In many quantum science and technology applications, resonators must operate in the quantum regime, requiring low temperatures to reach the ground state (thermal energy k B T small compared to the photon energy at resonance, hf r ) and single-photon excitation levels. Under these conditions, however, internal quality factors (Q i s) are typically substantially lower than their high-temperature or high-power values.In the quantum regime, the dominant loss mechanism for high-Q superconducting resonators can be attributed to parasitic two-level systems (TLSs) in the dielectrics 10,11 . TLSs may reside in the bulk substrate 11 , as well as in the metal-substrate, metal-vacuum and substrate-vacuum interfaces 10,[12][13][14][15][16][17][18] where electric fields may be large (see Ref. 19 and references therein for a recent review of material-related loss in superconducting circuits). Interface TLSs are common by-products of the fabrication process, often introduced by impurities associated with substrate surfaces 20,21 and etching chemistry 22 . To our knowledge, the best resonators reported to date 15 (Q i = 1.72 M at 6 GHz) are fabricated by epitaxially growing aluminum on sapphire substrates following careful surface preparation (high-temperature annealing in an oxygen atmosphere). For CPW resonators on silicon (Si) substrates, achieving Q i > 1 M in the quantum regime has proven challenging, with the best resonators reported in Ref. 23.In this letter, we present silicon-based, gigahertzfrequency CPW resonators with Q i systematically above 1 M in the quantum regime, fabricated from niobium titanium nitride (NbTiN) superconducting films. This performance is reached by optimizing two aspects of the fabrication. First, the substrate surface is treated with hexamethyldisilazane (HMDS) immediately prior to metal deposition to reduce losses associated with the metal-substrate interface. Second, we employ highl...

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Nuclear spins are highly coherent quantum objects. In large ensembles, their control and detection via magnetic resonance is widely exploited, e.g. in chemistry, medicine, materials science and mining. Nuclear spins also featured in early ideas [1] and demonstrations [2] of quantum information processing. Scaling up these ideas requires controlling individual nuclei, which can be detected when coupled to an electron [3, 4, 5]. However, the need to address the nuclei via oscillating magnetic fields complicates their integration in multispin nanoscale devices, because the field cannot be localized or screened. Control via electric fields would resolve this problem, but previous methods [6, 7, 8] relied upon transducing electric signals into magnetic fields via the electron-nuclear hyperfine interaction, which severely affects the nuclear coherence. Here we demonstrate the coherent quantum control of a single antimony (spin-7/2) nucleus, using localized electric fields produced within a silicon nanoelectronic device. The method exploits an idea first proposed in 1961 [9] but never realized experimentally with a single nucleus. Our results are quantitatively supported by a microscopic theoretical model that reveals how the purely electrical modulation of the nuclear electric quadrupole interaction, in the presence of lat- † To whom correspondence should be addressed;