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Investigating a challenge of building a superconducting quantum computer.

N. Tubsrinuan, Gustav Andersson, Per Delsing

Published Dec 31, 2020• 0 citations
Publisher

Abstract

Parasitic two-level systems (TLS) coupling to a superconducting quantum circuit causes information loss. To mitigate this problem, a better physical understanding of the coupling mechanism is crucial. Phase noise caused by TLS interaction is investigated by using a surface acoustic wave resonator. The results show a significant correlation of resonance frequency fluctuations between different resonance modes, supporting the prediction of the TLS model.

DOI
10.5281/zenodo.4383309

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Parasitic two-level systems (TLS) coupling to a superconducting quantum circuit causes information loss. To mitigate this problem, a better physical understanding of the coupling mechanism is crucial. Phase noise caused by TLS interaction is investigated by using a surface acoustic wave resonator. The results show a significant correlation of resonance frequency fluctuations between different resonance modes, supporting the prediction of the TLS model.
Frequency instabilities are a major source of errors in quantum devices. This Letter investigates frequency fluctuations in a surface acoustic wave (SAW) resonator, where reflection coefficients of 14 SAW modes are measured simultaneously for more than 7 h. We report two distinct noise characteristics. Multimode frequency noise caused by interactions with two-level system (TLS) defects shows significant degrees of correlation that diminish with increased detuning. This finding agrees with the current understanding of the parasitic TLS behavior as one of the dominant noise sources in quantum devices. In addition to the TLS-induced noise, we observe strong anomalous frequency fluctuations with slow, anticorrelated dynamics. These noise bursts resemble signatures of cosmic radiation observed in superconducting quantum systems. Published by the American Physical Society 2024
Superconducting qubits are the most prevalent way to design qubits due to their relatively easy manufacturing, tunability and fast gates. However, one of their major problems is the short coherence time, which is influenced by various factors, such as charge noise, flux noise and thermal noise. Several working solutions have been found to mitigate these factors. In contrast, there is no general solution to mitigating the formation of parasitic two-level systems (TLS), which is one of the most dominant causes of error in superconducting circuits. There is no general theory for parasitic TLS, as they form randomly and their physical form varies. Due to this, it is important to be able to characterize parasitic TLSs, so that information can be obtained about their formation and effects on superconducting qubits to improve qubit design. This thesis demonstrates ways to characterize parasitic TLSs in a superconducting qubit. The frequency and the coupling strength of a TLS can be extracted by simulating the spectrum of the system and comparing it to experimental data. The states of a qubit and a TLS are possible to distinguish by utilizing an iSWAP gate to be able to characterize them separately. In this thesis a sample consisting of two fixed-frequency transmon qubits and one tunable transmon coupler was used. The cryostat measurements began by characterizing the sample and calibrating control pulses. After this, data of Rabi oscillations of the system were collected, from which simulations were made and relevant values were obtained through fitting. Then, the iSWAP gate was calibrated and used to swap qubit populations to distinguish the TLS from the qubits. The frequency and the coupling strength of the TLS were obtained from the simulations. At first, the TLS was nearly resonant with the qubit, but then its frequency had moved further away. The separation done with the iSWAP succeeded, which allowed a clear distinction of the separate modes. The results showed that these methods can be utilized to characterize parasitic TLSs. Research can be improved, for example, by measuring TLS coherence times to characterize its properties more accurately.
Microscopic two-level system (TLS) defects at dielectric surfaces and interfaces are among the dominant sources of loss in superconducting quantum circuits, and their properties have been extensively probed using superconducting resonators and qubits. We report on spectroscopy of TLSs coupling to the strain field in a surface acoustic wave (SAW) resonator. The narrow free spectral range of the resonator allows for two-tone spectroscopy where a strong pump is applied at one resonance, while a weak signal is used to probe a different mode. We map the spectral hole burnt by the pump tone as a function of frequency and extract parameters of the TLS ensemble. Our results suggest that detuned acoustic pumping can be used to enhance the coherence of superconducting devices by saturating TLSs.
Noise within solid-state systems at low temperatures can typically be traced back to material defects. In amorphous materials, these defects are broadly described by the tunneling two-level systems (TLSs) model. TLS have recently taken on further relevance in quantum computing because they dominate the coherence limit of superconducting quantum circuits. Efforts to mitigate TLS impacts have thus far focused on circuit design, material selection, and surface treatments. Our work takes an approach that directly modifies TLS properties. This is achieved by creating an acoustic bandgap that suppresses all microwave-frequency phonons around the operating frequency of a transmon qubit. For embedded TLS strongly coupled to the transmon qubit, we measure a pronounced increase in relaxation time by two orders of magnitude, with the longestT1time exceeding 5 milliseconds. Our work opens avenues for studying the physics of highly coherent TLS and methods for mitigating noise within solid-state quantum devices.
Abstract Microscopic two-level system (TLS) defects at dielectric surfaces and interfaces are among the dominant sources of loss in superconducting quantum circuits, and their properties have been extensively probed using superconducting resonators and qubits. We report on spectroscopy of TLSs coupling to the strain field in a surface acoustic wave (SAW) resonator. The narrow free spectral range of the resonator allows for two-tone spectroscopy where a strong pump is applied at one resonance, while a weak signal is used to probe a different mode. We map the spectral hole burnt by the pump tone as a function of frequency and extract parameters of the TLS ensemble. Our results suggest that detuned acoustic pumping can be used to enhance the coherence of superconducting devices by saturating TLSs.
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)