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arXiv:1801.10204 · Journal of Physics Conference Series

Noise and loss of superconducting aluminium resonators at single photon energies

Jonathan Burnett, Andreas Bengtsson, David Niepce, Jonas Bylander

Published Mar 1, 2018• 53 citations
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Abstract

The loss and noise mechanisms of superconducting resonators are useful tools for understanding decoherence in superconducting circuits. While the loss mechanisms have been heavily studied, noise in superconducting resonators has only recently been investigated. In particular, there is an absence of literature on noise in the single photon limit. Here, we measure the loss and noise of an aluminium on silicon quarter-wavelength ( λ/ 4) resonator in the single photon regime.

DOI
10.1088/1742-6596/969/1/012131

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The loss and noise mechanisms of superconducting resonators are useful tools for understanding decoherence in superconducting circuits. While the loss mechanisms have been heavily studied, noise in superconducting resonators has only recently been investigated. In particular, there is an absence of literature on noise in the single photon limit. Here, we measure the loss and noise of an aluminium on silicon quarter-wavelength ( λ/ 4) resonator in the single photon regime.
Superconducting micro-resonators have application in sensors and quantum computing. Measurement of the resonator internal loss in the single-photon regime is a common tool to study the origins of dissipation, noise, and decoherence in quantum circuits, as well as characterization of materials used for quantum devices. However, such measurements are challenging and time-consuming with large uncertainties due to the poor signal-to-noise ratio when measured at single-photon power levels. We developed methods to extract a subset of the resonator fitting parameters at high power and fix them in the analysis of low power data, which reduce the parameter space in the regression analysis. By comparing the analysis with and without partial extraction, we show that these methods reduce the uncertainty of the results, while improving the robustness and efficiency of the loss measurement.
Quantum sensing and computation can be realized with superconducting microwave circuits. Qubits are engineered quantum systems of capacitors and inductors with non-linear Josephson junctions. They operate in the single-excitation quantum regime, photons of 27μ27 μ27μeV at 6.5 GHz. Quantum coherence is fundamentally limited by materials defects, in particular atomic-scale parasitic two-level systems (TLS) in amorphous dielectrics at circuit interfaces.[1] The electric fields driving oscillating charges in quantum circuits resonantly couple to TLS, producing phase noise and dissipation. We use coplanar niobium-on-silicon superconducting resonators to probe decoherence in quantum circuits. By selectively modifying interface dielectrics, we show that most TLS losses come from the silicon surface oxide, and most non-TLS losses are distributed throughout the niobium surface oxide. Through post-fabrication interface modification we reduced TLS losses by 85% and non-TLS losses by 72%, obtaining record single-photon resonator quality factors above 5 million and approaching a regime where non-TLS losses are dominant. [1]Müller, C., Cole, J. H. & Lisenfeld, J. Towards understanding two-level-systems in amorphous solids: insights from quantum circuits. Rep. Prog. Phys. 82, 124501 (2019)
Superconducting qubits, though promising for both near-term problem-solving as well as the development of large-scale quantum computing systems, are limited in performance largely by materials-induced decoherence channels which are maximized at millikelvin temperatures, single photon powers, and microwave frequencies. Superconducting microwave resonators are ideal tools for measuring these materials losses due to their simplicity, well-understood behavior, and sensitivity to these same losses. In order to better understand and mitigate dominant qubit decoherence channels, accurate and precise characterization of superconducting microwave resonator performance is critical. In this paper, current measurement techniques for superconducting microwave resonators are discussed, including experimental set-up, error analysis, and cryogenic calibration of a vector network analyzer.
Superconducting resonators are widely used in many applications such as qubit readout for quantum computing applications, and kinetic inductance detectors. These resonators are susceptible to numerous loss and noise mechanisms under microwave excitation, especially the dissipation due to non-equilibrium quasi-particles and two-level systems (TLS), which can result in a decrease of the superconducting intrinsic quality factor (QiQ_iQi​) in high quality superconducting resonators. Particularly in the few-photon and low temperature (T) regime, TLS losses can become a dominant loss mechanism. In this study, novel aluminum half-wavelength resonators are investigated, focusing on the loss properties at extra-low power and low temperature. An unusual increase of Qi(T)Q_i(T)Qi​(T) with deceasing temperature is observed. This behavior is attributed to the increase of TLS coherence time (T2T_2T2​) at ultra-low temperatures and powers. This T2T_2T2​ increase is consistent with other work on resonant frequency noise in resonators and measurements of individual TLS, and likely arises from interacting TLS in the aluminum half-wavelength resonators.
Solid-state devices used for quantum computation and quantum sensing applications are adversely affected by loss and noise caused by spurious, charged two-level systems (TLS) and stray paramagnetic spins. These two sources of noise are interconnected, exacerbating the impact on circuit performance. We use an on-chip electron spin resonance (ESR) technique, with niobium nitride ( Nb N ) superconducting resonators, to study surface spins on silicon and the effect of postfabrication surface treatments. We identify two distinct spin species that are characterized by different spin-relaxation times and respond selectively to various surface treatments (annealing and hydrofluoric acid). Only one of the two spin species has a significant impact on the TLS-limited resonator quality factor at low-power (near-single-photon) excitation. We observe a three- to fivefold reduction in the total density of spins after surface treatments and demonstrate the efficacy of ESR spectroscopy in developing strategies to mitigate loss and decoherence in quantum systems. Published by the American Physical Society 2024
Two-level systems (TLSs) are known to be a dominant source of dissipation and decoherence in superconducting qubits. Superconducting resonators provide a convenient way to study TLS-induced loss due to easier design and fabrication in comparison with devices that include nonlinear elements such as Josephson junctions. However, precisely measuring TLS-induced loss in a resonator in the quantum regime is challenging due to low signal-to-noise ratio (SNR) and the temporal fluctuations of the TLS, leading to uncertainties of 20% or more in the loss measurement. To address these limitations, we develop a multiwavelength resonator device that extends the resonator length from a standard quarter wavelength λ / 4 to N λ / 4 , where N = 37 , at 6 GHz. This design provides two key advantages: the TLS-induced fluctuations are reduced by a factor of N due to spatial averaging over an increased number of independent TLSs, and the measurement SNR for a given intraresonator energy density increases by a factor of N . The multiwavelength resonator also has fundamental and harmonic resonances spanning a wide frequency range, allowing one to study the frequency dependence of TLS-induced loss. In this work we fabricate both multiwavelength and quarter-wave coplanar waveguide resonators formed from thin-film aluminum on a silicon substrate, and characterize their TLS properties at 10 and 200 mK. Our results show that the power-dependent TLS-induced loss measured from the N λ / 4 resonator agrees well with that measured from the λ / 4 resonator, while achieving a fivefold reduction in measurement uncertainty due to TLS fluctuations, down to 5 % . The N λ / 4 resonator also provides a measure of the fully unsaturated TLS-induced loss due to the increased measurement SNR at low intraresonator energy densities. Finally, measurements across seven harmonic resonances of the N λ / 4 resonator between 4 and 6.5 GHz reveal no frequency dependence in the TLS-induced loss over this range. These results highlight the benefits of the multiwavelength resonator approach for material development in superconducting quantum circuits.