Measurement of the Low-temperature Loss Tangent of High-resistivity Silicon with a High Q-factor Superconducting Resonator

In this letter, we present the direct loss tangent measurement of a high-resistivity intrinsic (100) silicon wafer in the temperature range from ~ 70 mK to 1 K, approaching the quantum regime. The measurement was performed using a technique that takes advantage of a high quality factor superconducting niobium resonator and allows to directly measure the loss tangent of insulating materials with high level of accuracy and precision. We report silicon loss tangent values at the lowest temperature and for electric field amplitudes comparable to those found in planar transmon devices one order of magnitude larger than what was previously estimated. In addition, we discover a non-monotonic trend of the loss tangent as a function of temperature that we describe by means of a phenomenological model based on variable range hopping conduction between localized states around the Fermi energy. We also observe that the dissipation increases as a function of the electric field and that this behavior can be qualitatively described by the variable range hopping conduction mechanism as well. This study lays the foundations for a novel approach to investigate the loss mechanisms and accurately estimate the loss tangent in insulating materials in the quantum regime, leading to a better understanding of coherence in quantum devices

Fast ZZ-Free Entangling Gates for Superconducting Qubits Assisted by a Driven Resonator

Engineering high-fidelity two-qubit gates is an indispensable step toward practical quantum computing. For superconducting quantum platforms, one important setback is the stray interaction between qubits, which causes significant coherent errors. For transmon qubits, protocols for mitigating such errors usually involve fine-tuning the hardware parameters or introducing usually noisy flux-tunable couplers. In this work, we propose a simple scheme to cancel these stray interactions. The coupler used for such cancellation is a driven high-coherence resonator, where the amplitude and frequency of the drive serve as control knobs. Through the resonator-induced-phase (RIP) interaction, the static ZZ coupling can be entirely neutralized. We numerically show that such a scheme can enable short and high-fidelity entangling gates, including cross-resonance CNOT gates within 40 ns and adiabatic CZ gates within 140 ns. Our architecture is not only ZZ free but also contains no extra noisy components, such that it preserves the coherence times of fixed-frequency transmon qubits. With the state-of-the-art coherence times, the error of our cross-resonance CNOT gate can be reduced to below 1e-4

Millikelvin measurements of permittivity and loss tangent of lithium niobate

Lithium Niobate is an electro-optic material with many applications in microwave signal processing, communication, quantum sensing, and quantum computing. In this letter, we present findings on evaluating the complex electromagnetic permittivity of lithium niobate at millikelvin temperatures. Measurements are carried out using a resonant-type method with a superconducting radio-frequency (SRF) cavity operating at 7 GHz and designed to characterize anisotropic dielectrics. The relative permittivity tensor and loss tangent are measured at 50 mK with unprecedented accuracy.Comment: 5 pages, 4 figure

First Direct Observation of Nanometer size Hydride Precipitations on Superconducting Niobium

Superconducting niobium serves as a key enabling material for superconducting radio frequency (SRF) technology as well as quantum computing devices. At room temperature, hydrogen commonly occupies tetragonal sites in the Nb lattice as metal (M)-gas (H) phase. When the temperature is decreased, however, solid solution of Nb-H starts to be precipitated. In this study, we show the first identified topographical features associated with nanometer-size hydride phase (Nb1-xHx) precipitates on metallic superconducting niobium using cryogenic-atomic force microscopy (AFM). Further, high energy grazing incidence X-ray diffraction reveals information regarding the structure and stoichiometry that these precipitates exhibit. Finally, through time-of-flight secondary ion mass spectroscopy (ToF-SIMS), we are able to locate atomic hydrogen sources near the top surface. This systematic study further explains localized degradation of RF superconductivity by the proximity effect due to hydrogen clusters

Completely Positive Map for Noisy Driven Quantum Systems Derived by Keldysh Expansion

Accurate modeling of decoherence errors in quantum processors is crucial for analyzing and improving gate fidelities. To increase the accuracy beyond that of the Lindblad dynamical map, several generalizations have been proposed, and the exploration of simpler and more systematic frameworks is still ongoing. In this paper, we introduce a decoherence model based on the Keldysh formalism. This formalism allows us to include non-periodic drives and correlated quantum noise in our model. In addition to its wide range of applications, our method is also numerically simple, and yields a CPTP map. These features allow us to integrate the Keldysh map with quantum-optimal-control techniques. We demonstrate that this strategy generates pulses that mitigate correlated quantum noise in qubit state-transfer and gate operations

Quasiparticle spectroscopy in technologically-relevant niobium using London penetration depth measurements

London penetration depth was measured in niobium foils, thin films, single crystals, and superconducting radio-frequency (SRF) cavity pieces cut out from different places. The low-temperature (T<Tc/3) variation, sensitive to the low-energy quasiparticles with states inside the superconducting gap, differs dramatically between different types of samples. With the help of phenomenological modeling, we correlate these different behaviors with known pair-breaking mechanisms and show that such measurements may help distinguish between different pair-breaking mechanisms, such as niobium hydrides and two-level systems (TLS). The conclusions also apply to SRF cavities when tracking the temperature-dependent quality factor and the resonant frequency