The superconducting transmon qubit is a leading platform for quantum computing and quantum science. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. Here, we fabricate two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. We have observed increased lifetimes for seventeen devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.

The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing 1, 2 . NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy 3-6 . However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects 7-10 . Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes [7][8][9][10][11][12] ; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. Here we demonstrate that surface spectroscopy methods and single spin measurements can be used as complementary diagnostics to understand sources of noise. We find that surface morphology is crucial for realizing reproducible chemical termination, and use these insights to achieve a highly ordered, oxygen-terminated surface with suppressed noise. We observe NV centers within 10 nm of the surface with coherence times extended by an order of magnitude.Although it is easy to place NV centers near the surface by low-energy ion implantation 8, 9 or delta-doping 7, 8 , the surface itself can host defects that lead to noise that obscures the sensing target ( Fig. 1a). We observe that coherence time degrades with proximity to the surface in numerous samples with different surface conditions ( Fig. 1b), consistent with prior studies 7, 10 , pointing to the need for new techniques to understand and control diamond surfaces. Gaining precise control over diamond surface chemistry is challenging because diamond is a chemically inert material, and also because it is hard to prepare uniform, flat diamond surfaces. Surface morphology is difficult 2 to control because diamond's hardness makes etching and polishing non-trivial. State-of-the-art diamond polishing can achieve surface roughness below 1 nm, but the resulting surface is highly strained. Plasma etching can remove this strained layer 13, 14 , but this process is highly anisotropic and therefore small differences in initial conditions can lead to dramatic differences in final morphology and termination 15, 16 (see Supplementary Information). Therefore, direct characterization of the surface is crucial for establishing that particular protocols reproducibly lead to specific, desired surface terminations.In this work, we characterize the diamond surface by correlating photoelectron spectroscopy, X-ray absorption, atomic force microscopy (AFM), and electron diffraction with measurements of NV spin decoherence and relaxation to identify and eliminate sources of noise at the surface. We find that surface roughness leads to poor NV coherence, and we observe that surface morphology changes the density of electronic defects observed with photoelectron spectroscopy, even for the same nominal chemical termination, implying that it ...

Quantum metrology enables some of the most precise measurements. In the life sciences, diamond-based quantum sensing has led to a new class of biophysical sensors and diagnostic devices that are being investigated as a platform for cancer screening and ultrasensitive immunoassays. However, a broader application in the life sciences based on nanoscale NMR spectroscopy has been hampered by the need to interface highly sensitive quantum bit (qubit) sensors with their biological targets. Here, we demonstrate an approach that combines quantum engineering with single-molecule biophysics to immobilize individual proteins and DNA molecules on the surface of a bulk diamond crystal that hosts coherent nitrogen vacancy qubit sensors. Our thin (sub–5 nm) functionalization architecture provides precise control over the biomolecule adsorption density and results in near-surface qubit coherence approaching 100 μs. The developed architecture remains chemically stable under physiological conditions for over 5 d, making our technique compatible with most biophysical and biomedical applications.

Nitrogen-vacancy (NV) centers in diamond can be used for nanoscale sensing with atomic resolution and sensitivity; however, it has been observed that their properties degrade as they approach the diamond surface. Here we report that in addition to degraded spin coherence, NV centers within nanometers of the surface can also exhibit decreased fluorescence contrast for optically detected electron spin resonance (OD-ESR). We demonstrate that this decreased OD-ESR contrast arises from charge state dynamics of the NV center, and that it is strongly surface-dependent, indicating that surface engineering will be critical for nanoscale sensing applications based on color centers in diamond.

Superconducting qubits are a leading system for realizing large scale quantum processors, but overall gate fidelities suffer from coherence times limited by microwave dielectric loss. Recently discovered tantalum-based qubits exhibit record lifetimes exceeding 0.3 ms. Here we perform systematic, detailed measurements of superconducting tantalum resonators in order to disentangle sources of loss that limit state-of-the-art tantalum devices. By studying the dependence of loss on temperature, microwave photon number, and device geometry, we quantify materials-related losses and observe that the losses are dominated by several types of saturable two level systems (TLSs), with evidence that both surface and bulk related TLSs contribute to loss. Moreover, we show that surface TLSs can be altered with chemical processing. With four different surface conditions, we quantitatively extract the linear absorption associated with different surface TLS sources. Finally, we quantify the impact of the chemical processing at single photon powers, the relevant conditions for qubit device performance. In this regime we measure resonators with internal quality factors ranging from 5 to 15 × 10 6 , comparable to the best qubits reported. In these devices the surface and bulk TLS contributions to loss are comparable, showing that systematic improvements in materials on both fronts will be necessary to improve qubit coherence further.