Identifying materials-level sources of performance variation in superconducting transmon qubits
Akshay A. Murthy, Mustafa Bal, Michael J. Bedzyk, Hilal Cansizoglu, Randall K. Chan, Venkat Chandrasekhar, Francesco Crisa, Amlan Datta, Yanpei Deng, Celeo D. Matute Diaz, Vinayak P. Dravid, David A. Garcia Wetten, Sabrina Garattoni, Sunil Ghimire, Dominic P. Goronzy, Sebastian E. de Graaf, Samuel Haeuser, Mark C. Hersam, P. F. Hopkins, Dieter Isheim, Kamal R. Joshi, Richard H. J. Kim, Saagar Kolachina, Cameron Kopas, Matthew Joseph Kramer, Ella Lachman, Jaeyel Lee, Peter G. Lim, Andrei Lunin, William Mah, Jayss Marshall, J. Mutus, Jin‐Su Oh, David I. Olaya, David P. Pappas, J.-M. Park, R. Prozorov, Roberto dos Reis, David N. Seidman, Zuhawn Sung, Makariy A. Tanatar, Mitchell J. Walker, Jigang Wang, Maxwell Wisne, Haotian Wu, L.W. Zhou, Shaojiang Zhu, Anna Grassellino, Alexander Romanenko
The Superconducting Quantum Materials and Systems Center, a U. S. Department of Energy National Quantum Information Science Research Center, has conducted a comprehensive and coordinated study using superconducting transmon qubit chips with known performance metrics to identify the underlying materials-level sources of device-to-device performance variation. Following qubit coherence measurements, these qubits of varying base superconducting metals and substrates have been examined with various non-destructive and invasive material characterization techniques at Northwestern University, Ames National Laboratory, and Fermilab as part of a blind study. We find trends in variations of the depth of the etched substrate trench, the thickness of the surface oxide, and the geometry of the sidewall, which when combined, lead to correlations with the T1 lifetime across different qubits on the same chip. In addition, we provide a list of features that varied from device to device, for which the impact on performance requires further studies. Finally, we identify two low-temperature characterization techniques that may potentially serve as proxy tools for qubit measurements. These insights provide materials-oriented solutions to not only reduce performance variations across neighboring devices but also to engineer and fabricate devices with optimal geometries to achieve performance metrics beyond the state-of-the-art values.
The Superconducting Quantum Materials and Systems Center, a U. S. Department of Energy National Quantum Information Science Research Center, has conducted a comprehensive and coordinated study using superconducting transmon qubit chips with known performance metrics to identify the underlying materials-level sources of device-to-device performance variation. Following qubit coherence measurements, these qubits of varying base superconducting metals and substrates have been examined with various non-destructive and invasive material characterization techniques at Northwestern University, Ames National Laboratory, and Fermilab as part of a blind study. We find trends in variations of the depth of the etched substrate trench, the thickness of the surface oxide, and the geometry of the sidewall, which when combined, lead to correlations with the T1 lifetime across different qubits on the same chip. In addition, we provide a list of features that varied from device to device, for which the impact on performance requires further studies. Finally, we identify two low-temperature characterization techniques that may potentially serve as proxy tools for qubit measurements. These insights provide materials-oriented solutions to not only reduce performance variations across neighboring devices but also to engineer and fabricate devices with optimal geometries to achieve performance metrics beyond the state-of-the-art values.
While significant strides have been made to increase the coherence time of superconducting qubits, further advancements are essential for realizing scalable quantum computing. Decoherence is often a result of loss and noise stemming from two-level systems and excess quasiparticles, arising due to material defects, fabrication processes, and ambient exposure, particularly at surfaces and interfaces. Our recent efforts to mitigate these decoherence mechanisms have employed a variety of strategies, including low-loss surface encapsulation materials, advanced substrate preparation techniques, modifications to metal film growth, and the development of novel fabrication processes. The structural and chemical properties of materials, surfaces, and interfaces are studied using scanning probe microscopy, electron microscopy, photoelectron spectroscopy, mass spectrometry, and X-ray diffraction, which is correlated to device performance metrics, including superconducting resonator internal quality factor and qubit T1 time. This information is used to identify and understand material sources of loss and their origins in the device fabrication process. Through multi-institution efforts within SQMS we have identified the loss mechanism of interstitial hydrogen in niobium-based devices and shown how standard fabrication processes introduce these hydrides, developing strategies to mitigate their formation.1 Furthermore, we have characterized the metal-substrate interface, including the loss of niobium-silicides formed at that interface, and developed silicon surface treatments that reduce atomic scale roughness and oxygen content at the metal-substrate and Josephson junction interfaces.2-4 By developing the connection between materials properties and the overall performance of superconducting quantum circuitry, we can develop fabrication strategies to mitigate material losses, thus supporting the ongoing efforts to enhance coherence time in superconducting quantum devices. 1. Torres-Castanedo, C. G.*, Goronzy, D. P.*, et al., Adv. Funct. Mater., 2401365 (2024) 2. Lu, X., et al., Phys. Rev. Materials 6, 064402 (2022) 3. Berti, G., Appl. Phys. Lett. 122, 192605 (2023) 4. Kopas, C. J., Goronzy, D. P., et al., arXiv:2408.02863 (2024)
In superconducting transmon qubits, macroscopic characterization is the primary method for probing performance metrics, such as relaxation T1 and dephasing T2 times, resonance frequency, and quality factor. While these measurements can aid our understanding of the dielectric loss of a transmon, they do not allow us to characterize the structural and chemical inhomogeneity of the interfaces which directly impacts this energy loss. These fundamental materials problems must be studied using transmission electron microscopy (TEM) to provide insight into these defects across different qubit interfaces at an atomic scale. In this contribution, we will examine qubits, such as those shown in Figure 1, fabricated at Fermi National Accelerator Laboratory (SQMS Center) [1]. The qubit devices consist of superconducting metal base layers, typically sputter deposited Nb or Ta, and Al/AlOx/Al junctions, where the AlOx barrier is roughly 3nm in thickness and prepared through in-situ oxidation [2]. Characterizing this system prior to its integration within a superconducting radio frequency cavity establishes a baseline for the system, allowing us to isolate fabrication-induced defects before they manifest in performance degradation during quantum measurements. With the samples and fabrication process well known on a macroscopic scale, we study the atomic structure and chemistry of different heterointerfaces within the Josephson junction (JJ) of the transmon qubit and between the sapphire substrate and Nb resonator pads to identify potential defects contributing to the decoherence for qubits constructed with particular geometries. To resolve these defects, atomic-resolution imaging, energy-dispersive X-ray spectroscopy (EDS), and electron-energy loss spectroscopy (EELS) are performed using the JEOL-ARM200CF, an aberration-corrected scanning transmission electron microscope (STEM). Figure 2 provides a clear image of the stacked layers formed as the JJ, but with the addition of a platinum and carbon protective coating added during the focus ion beam (FIB) milling process to preserve pristine Al/AlOx/Al interfaces. 4D-STEM approaches, such as ptychography, orientation mapping and nanoscale diffraction will be employed to further study the layered interfaces and defects in real space and reciprocal space. These techniques can also provide insight into any damage that incurs during lamella preparation using the FIB [3]. Magnetic field-free atomic resolution imaging, differential phase contrast mapping, and monochromated EELS can be achieved using the new MARS-MONO STEM (JEM-Z200MF) without inducing current fluctuations or magnetic field related distortions [4]. By integrating this work across the functional interfaces of the transmon qubits and their different geometries, we will identify possible structural and chemical defects which limit device performance using our multi-modal approach [5]. SQMS fabricated transmon qubits fabricated on HEMEX sapphire substrate imaged via a) ZEISS optical microscope, b) SEM and c) EDS. TEM image of Josephson Junction layered interface at on JEOL-ARM200CF on the ClearView camera with a 200kV accelerating voltage.
Abstract We present a transmon qubit fabrication technique that yields systematic improvements in T 1 relaxation times. We encapsulate the surface of niobium and prevent the formation of its lossy surface oxide. By maintaining the same superconducting metal and only varying the surface, this comparative investigation examining different capping materials, such as tantalum, aluminum, titanium nitride, and gold, as well as substrates across different qubit foundries demonstrates the detrimental impact that niobium oxides have on coherence times of superconducting qubits, compared to native oxides of tantalum, aluminum or titanium nitride. Our surface-encapsulated niobium qubit devices exhibit T 1 relaxation times 2–5 times longer than baseline qubit devices with native niobium oxides. When capping niobium with tantalum, we obtain median qubit lifetimes above 300 μs, with maximum values up to 600 μs. Our comparative structural and chemical analysis provides insight into why amorphous niobium oxides may induce higher losses compared to other amorphous oxides.
With massive improvements in device coherence times and gate fidelity over the past two decades, superconducting quantum devices have emerged as a leading technology platform for next generation quantum computing. While many of these improvements have been driven through optimized device designs and geometries, the presence of defects, impurities, interfaces, and surfaces in the constituent materials continue to limit performance and present a critical barrier in achieving scalable quantum systems with long coherence times. As part of the Superconducting Quantum Materials and Systems (SQMS) center, we have applied state-of-the-art materials characterization techniques, such as scanning/transmission electron microscopy, secondary ion mass spectrometry, and atom probe tomography, to identify structural defects and chemical inhomogeneities in superconducting qubits. In this review, we discuss the findings of these efforts as they relate to the inherent surfaces, interfaces, impurities, and defects in superconducting qubits. Through this comprehensive investigation, we provide insight into possible sources of dissipation in these systems. Further, we supplement these findings with potential strategies for mitigating these undesired features in an effort to systematically and intelligently improve the coherence times of superconducting qubits.
Abstract Decoherence of superconducting transmon qubits is purported to be consistent with surface loss from two-level systems on the substrate surface. Here, we present a study of surface loss in transmon devices, explicitly designed to have varying sensitivities to different surface loss contributors. Our experiments also encompass two particular different sapphire substrates, which reveal the onset of a yet unknown additional loss mechanism outside of surface loss for one of the substrates. Tests across different wafers and devices demonstrate substantial variation, and we emphasize the importance of testing large numbers of devices for disentangling different sources of decoherence.
Over the past decades, superconducting qubits have emerged as one of the leading hardware platforms for realizing a quantum processor. Consequently, researchers have made significant effort to understand the loss channels that limit the coherence times of superconducting qubits. A major source of loss has been attributed to two level systems that are present at the material interfaces. It is recently shown that replacing the metal in the capacitor of a transmon with tantalum yields record relaxation and coherence times for superconducting qubits, motivating a detailed study of the tantalum surface. In this work, the chemical profile of the surface of tantalum films grown on c-plane sapphire using variable energy X-ray photoelectron spectroscopy (VEXPS) is studied. The different oxidation states of tantalum that are present in the native oxide resulting from exposure to air are identified, and their distribution through the depth of the film is measured. Furthermore, it is shown how the volume and depth distribution of these tantalum oxidation states can be altered by various chemical treatments. Correlating these measurements with detailed measurements of quantum devices may elucidate the underlying microscopic sources of loss.