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arXiv:2103.07970 · npj Quantum Information

Effects of surface treatments on flux tunable transmon qubits

Matthias Mergenthaler, Clemens Müller, Marc Ganzhorn, Stephan Paredes, P. Müller, Gian Salis, Vivekananda P. Adiga, Markus Brink, Martin Sandberg, Jared Hertzberg, Stefan Filipp, Andreas Fuhrer

Published Oct 29, 2021• 16 citations
arXivHTMLPDF

Abstract

Abstract One of the main limitations in state-of-the art solid-state quantum processors is qubit decoherence and relaxation due to noise from adsorbates on surfaces, impurities at interfaces, and material defects. For the field to advance towards full fault-tolerant quantum computing, a better understanding of these microscopic noise sources is therefore needed. Here, we use an ultra-high vacuum package to study the impact of vacuum loading, UV-light exposure, and ion irradiation treatments on relaxation and coherence times, as well as slow parameter fluctuations of flux tunable superconducting transmon qubits. The treatments studied do not significantly impact the relaxation rate Γ 1 and the echo decay rate $${{{\Gamma }}}_{2,{{{\rm{SS}}}}}^{{{{\rm{e}}}}}$$ Γ 2 , SS e at the sweet spot, except for Ne ion bombardment which reduces Γ 1 . In contrast, flux noise parameters are improved by removing magnetic adsorbates from the chip surfaces with UV-light and NH 3 treatments. Additionally, we demonstrate that SF 6 ion bombardment can be used to adjust qubit frequencies in situ and post-fabrication without affecting qubit relaxation and coherence times at the sweet spot.

DOI
10.1038/s41534-021-00491-2

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•Sep 16, 2014•0 citations
Abstract One of the main limitations in state-of-the art solid-state quantum processors is qubit decoherence and relaxation due to noise from adsorbates on surfaces, impurities at interfaces, and material defects. For the field to advance towards full fault-tolerant quantum computing, a better understanding of these microscopic noise sources is therefore needed. Here, we use an ultra-high vacuum package to study the impact of vacuum loading, UV-light exposure, and ion irradiation treatments on relaxation and coherence times, as well as slow parameter fluctuations of flux tunable superconducting transmon qubits. The treatments studied do not significantly impact the relaxation rate Γ 1 and the echo decay rate Γ2,SSe{{{\Gamma }}}_{2,{{{\rm{SS}}}}}^{{{{\rm{e}}}}}Γ2,SSe​ Γ 2 , SS e at the sweet spot, except for Ne ion bombardment which reduces Γ 1 . In contrast, flux noise parameters are improved by removing magnetic adsorbates from the chip surfaces with UV-light and NH 3 treatments. Additionally, we demonstrate that SF 6 ion bombardment can be used to adjust qubit frequencies in situ and post-fabrication without affecting qubit relaxation and coherence times at the sweet spot.
Superconducting qubits are prone to noise induced by material defects, which degrades their coherence and performance of gate operations. In this work, by thermally annealing a chip with 20 tunable transmon qubits in vacuum, we realize an approximate 30% enhancement in the average energy relaxation time T¯1 from 162 to 210 μs. After annealing, T¯1 also shows smaller fluctuations in time, and the aging of the qubits, when kept under ambient conditions, is remarkably decelerated. Our work demonstrates a simple yet highly effective way to improve the coherence and stability of superconducting qubits for scalable superconducting quantum processors.
We describe design, implementation, and performance of an ultra-high vacuum (UHV) package for superconducting qubit chips or other surface sensitive quantum devices. The UHV loading procedure allows for annealing, ultra-violet light irradiation, ion milling, and surface passivation of quantum devices before sealing them into a measurement package. The package retains vacuum during the transfer to cryogenic temperatures by active pumping with a titanium getter layer. We characterize the treatment capabilities of the system and present measurements of flux tunable qubits with an average T1 = 84 µs and T2echo=134 μs after vacuum-loading these samples into a bottom loading dilution refrigerator in the UHV-package.
Superconducting qubits are considered as a promising platform for implementing a fault tolerant quantum computing. However, surface defects of superconductors and the substrate leading to qubit state decoherence and fluctuations in qubit parameters constitute a significant problem. The amount and type of defects depend both on the chip materials and fabrication procedure. In this work, transmons produced by two different methods of aluminum etching: wet etching in a solution of weak acids and dry etching using a chlorine-based plasma are experimentally studied. The relaxation and coherence times for dry-etched qubits are more than twice as long as those for wet-etched ones. Additionally, the analysis of time fluctuations of qubit frequencies and relaxation times, which is an effective method to identify the dominant dielectric loss mechanisms, i-ndicates a significantly lower impact of two-level systems in the dry-etched qubits compared to the wet-etched ones.
Decoherence and parameter fluctuations are two of the mayor obstacles for solid-state quantum computing. In this work, decoherence in superconducting qubits of the transmon type is investigated. For this purpose, a time-multiplexed measurement protocol was developed and applied in long-term measurements. The resulting simultaneous measurement of the qubit's relaxation and dephasing rate, as well as its resonance frequency enables analysis of correlations between these parameters. A spectral noise analysis complements these measurements. Together, the results agree well with the interacting defect model of two-level-systems and yield information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits. Our measurements show inherent correlations between dephasing and fluctuations in qubit frequency on the timescale of seconds to days, which is attributed to the influence of individual defects, located close to conductor edges. Cross-correlation and spectral noise analysis confirm this interpretation and ascribe the source of fluctuation to interactions between thermal fluctuators and surface defects. Single defects reducing the coherence of qubits by up to one order of magnitude are a major challenge for future quantum computers. Non-tunable qubits are intrinsically insensitive to some decoherence channels and thus ideal for this fundamental analysis. However, to widen the focus and contrast the results of different material systems, we pursue the fabrication of voltage controlled gatemon qubits. In the course of this work, the theoretical foundation and technical implementation of transmon qubits based on regular Josephson weak links, and semiconducting nanowires is given. The experimental design and measurement setup are explained in detail. Our findings make continuous re-calibration a necessity in today's solid-state qubits, although new materials or processing techniques might mitigate the problem. However, the results of this work imply that fundamental improvements of qubit parameter stability are necessary in order to realize scalable and coherent qubit circuits.
Scaling up superconducting quantum processors with optimized performance requires a sufficient flexibility in the choice of operating points for single and two qubit gates to maximize their fidelity and cope with imperfections. Flux control is an efficient technique to manipulate the parameters of tunable qubits, in particular to activate entangling gates. At flux sensitive points of operation, the ubiquitous presence of 1/f flux noise however gives rise to dephasing by inducing fluctuations of the qubit frequency. We show how two-tone modulation of the flux bias, a bichromatic modulation, gives rise to a continuum of dynamical sweet spots where dephasing due to slow flux noise is suppressed to first order for a wide range of time-averaged qubit frequencies. The qubits can be operated at these dynamical sweet spots to realize protected entangling gates and to avoid collisions with two-level-system defects.
Quantum computing is a very interesting field, due to the ability of quantum computers to solve many problems much faster than a classical computer. Superconducting qubits are electronic circuits composed of superconducting capacitors, inductors and Josephson junctions, which can implement a physical qubit. However they need improvements in their coherence time to create a viable quantum computing. In this work I study the effect on decoherence caused by two level systems in the native oxide that forms on the surface of qubit. To this end I fabricate 3D transmon qubits using materials grown my molecular beam epitaxy, to which a variety of different surface treatments had been applied. I began by fabricating qubits from niobium/aluminum oxide/niobium trilayers. To this end I developed a self-aligned process for fabricating sub-micron Josephson junctions. This process presented many challenges. During the development of this process, it became clear that niobium was an inferior material for fabricating qubits compared to aluminum. I then switched to making qubits from Aluminum. I began by studying the growth of aluminum on sapphire, and was able to achieve aluminum films with an RMS roughness of 0.2 nm by growing on c-plane sapphire that had been annealed in oxygen at 1100 °C and dipped in BOE. Next I fabricated 3D transmon qubits by adapting the standard shadowmask process for use with MBE. I fabricated qubits with a long in situ oxidation to fully passivate the surface before exposure to air. I also passivated the surface by means of growing co deposited aluminum oxide, and by grown aluminum nitride using a nitrogen plasma source. The coherence times of these qubits were compared to those a control sample that had been exposed to air immediately after growth. Overall it doesn’t appear that cladding the surface changes the coherence time much, however it is difficult to form conclusions with this small sample size. The coated samples appeared to have slightly longer coherence times than the untreated samples, with diffusive oxidation and coating with aluminum oxide providing the largest increase in T1 and diffusive oxidation providing the largest increase in T2.