Correlating Decoherence in Transmon Qubits: Low Frequency Noise by Single Fluctuators

Schlör, Steffen; Lisenfeld, Jürgen; Müller, Clemens; Bilmes, Alexander; Schneider, Andre; Pappas, David P.; Ustinov, A. V.; Weides, Martin

doi:10.1103/physrevlett.123.190502

Cited by 176 publications

(149 citation statements)

References 55 publications

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“…As a consequence of significantly enhanced coherence times, qubits became sensitive also to weakly coupling defects residing on the surfaces and interfaces of circuit electrodes [9]. Since these are limiting the performance of state-of-the-art circuits [10][11][12], further progress towards scaled-up quantum processors requires strong efforts to prevent the appearance of defects, e.g. by using better materials, improved fabrication procedures [13][14][15][16], and surface treatment to avoid contamination and parasitic adsorbates [17,18].…”

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confidence: 99%

Resolving the positions of defects in superconducting quantum bits

Bilmes

Megrant

Klimov

et al. 2020

Sci Rep

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Solid-state quantum coherent devices are quickly progressing. Superconducting circuits, for instance, have already been used to demonstrate prototype quantum processors comprising a few tens of quantum bits. This development also revealed that a major part of decoherence and energy loss in such devices originates from a bath of parasitic material defects. However, neither the microscopic structure of defects nor the mechanisms by which they emerge during sample fabrication are understood. Here, we present a technique to obtain information on locations of defects relative to the thin film edge of the qubit circuit. Resonance frequencies of defects are tuned by exposing the qubit sample to electric fields generated by electrodes surrounding the chip. By determining the defect's coupling strength to each electrode and comparing it to a simulation of the field distribution, we obtain the probability at which location and at which interface the defect resides. This method is applicable to already existing samples of various qubit types, without further on-chip design changes. It provides a valuable tool for improving the material quality and nano-fabrication procedures towards more coherent quantum circuits.Material defects have a manifold of microscopic origins such as impurities in solids or adsorbates hosted on surfaces [1]. Their detrimental role was identified already in first experiments with superconducting quantum bits (qubits) [2]. Strong interaction with a long-known defect type, charged two-level systems (TLS) [3,4], residing in the tunnel barrier of a qubit's Josephson junction gives rise to avoided level crossings and resonant energy absorption [5]. This form of dielectric loss could be mitigated by reducing the amount of lossy dielectrics, e.g. by incorporating smaller Josephson junctions [6] and by avoiding insulating layers. Another strategy is to enlarge the footprint of device capacitors in order to dilute the electric field induced by the qubit, which excites defects by coupling to their electric dipole moments [7,8]. As a consequence of significantly enhanced coherence times, qubits became sensitive also to weakly coupling defects residing on the surfaces and interfaces of circuit electrodes [9]. Since these are limiting the performance of state-of-the-art circuits [10][11][12], further progress towards scaled-up quantum processors requires strong efforts to prevent the appearance of defects, e.g. by using better materials, improved fabrication procedures [13][14][15][16], and surface treatment to avoid contamination and parasitic adsorbates [17,18]. This endeavor needs to be guided by careful analysis of defect properties such as densities, electric dipole moments, and positions, in order to identify and improve the manufacturing steps that reduce defect formation, and to analyze the microscopic structure of defects.In this Letter, we present a method to extract information on the spatial positions of defects at the profile of the film edge in a qubit circuit. For doing this, we exploit the tunabi...

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confidence: 99%

Resolving the positions of defects in superconducting quantum bits

Bilmes

Megrant

Klimov

et al. 2020

Sci Rep

Self Cite

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“…Due to the material's random structure, TLS resonance frequencies are widely distributed, and those that are near resonance with qubits can dominate qubit energy relaxation [15]. Moreover, the thermal activation of low-energy TLS causes resonance frequency fluctuations of high-energy TLS, resonators, and qubits, which occur on time-scales spanning from milliseconds to hours and days [16][17][18][19][20][21]. For quantum processors, this implies that each qubit needs to be frequently recalibrated, while individual qubits can also become completely unusable due to randomly occurring resonant interaction with fluctuating TLS.…”

Section: Introductionmentioning

confidence: 99%

Electric field spectroscopy of material defects in transmon qubits

Lisenfeld¹,

Bilmes²,

Megrant³

et al. 2019

npj Quantum Inf

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116

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Superconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a spectroscopy method to extract the defects' energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only ≈ 3% are identified as strongly coupled defects which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.

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“…The structural and nanochemical properties of AlOx layers have a significant influence on the performance of these devices. For example, AlOx-layer thickness variations and structural defects in AlOx-tunnel barriers of JJs cause noise and limit the detection sensitivity of superconducting interference devices and coherence times in quantum bits [14][15][16][17][18][19][20].…”

Section: Introductionmentioning

confidence: 99%

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

Fritz

Radtke

Schneider

et al. 2019

Phys. Rev. Materials

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The structural and nanochemical properties of thin AlOx layers are decisive for the performance of advanced electronic devices. For example, they are frequently used as tunnel barriers in Josephson junction-based superconducting devices. However, systematic studies of the influence of oxidation parameters on structural and nanochemical properties are rare up to now, as most studies focus on the electrical properties of AlOx layers. This study aims to close this gap by applying transmission electron microscopy in combination with electron energy loss spectroscopy to analyze the structural and nanochemical properties of differently fabricated AlOx layers and correlate them with fabrication parameters. With respect to the application of AlOx as tunnel barrier in superconducting Josephson junctions, Al/AlOx/Al-layer systems were deposited on Si substrates. We will show that the oxygen content and structure of amorphous AlOx layers is strongly dependent on the fabrication process and oxidation parameters.Dynamic and static oxidation of Al yields oxygen-deficient amorphous AlOx layers, where the oxygen content ranges from x = 0.5 to x = 1.3 depending on oxygen pressure and substrate temperature. Thicker layers of stoichiometric crystalline -Al2O3 layers were grown by electron-beam evaporation of Al2O3 and reactive sputter deposition.

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Correlating Decoherence in Transmon Qubits: Low Frequency Noise by Single Fluctuators

Cited by 176 publications

References 55 publications

Resolving the positions of defects in superconducting quantum bits

Resolving the positions of defects in superconducting quantum bits

Electric field spectroscopy of material defects in transmon qubits

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

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