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.
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.
Superconducting qubits are sensitive to a variety of loss mechanisms including dielectric loss from interfaces. By changing the physical footprint of the qubit it is possible to modulate sensitivity to surface loss. Here we show a systematic study of planar superconducting transmons of differing physical footprints to optimize the qubit design for maximum coherence. We find that qubits with small footprints are limited by surface loss and that qubits with large footprints are limited by other loss mechanisms which are currently not understood.
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.
Superconducting qubits are a leading system for realizing large-scale quantum processors, but overall gate fidelities suffer from coherence times limited by microwave dielectric loss.Recently discovered tantalum-based qubits exhibit record lifetimes exceeding 0.3 ms.Here, we perform systematic, detailed measurements of superconducting tantalum resonators in order to disentangle sources of loss that limit state-of-the-art tantalum devices.By studying the dependence of loss on temperature, microwave photon number, and device geometry, we quantify materials-related losses and observe that the losses are dominated by several types of saturable two-level systems (TLSs), with evidence that both surface and bulk related TLSs contribute to loss.Moreover, we show that surface TLSs can be altered with chemical processing.With four different surface conditions, we quantitatively extract the linear absorption associated with different surface TLS sources.Finally, we quantify the impact of the chemical processing at singlephoton powers, the relevant conditions for qubit device performance.In this regime, we measure resonators with internal quality factors ranging from 5 to 15 × 10 6 , comparable to the best qubits reported.In these devices, the surface and bulk TLS contributions to loss are comparable, showing that systematic improvements in materials on both fronts are necessary to improve qubit coherence further.
Abstract Surface dielectric loss from superconducting transmon qubits is believed to be one of the dominant sources of decoherence. Reducing surface dielectric loss is crucial for achieving a high quality factor and a long relaxation time (T 1), which can be engineered by carefully changing the geometry of a transmon. In this paper, we present a shape-optimization approach to reduce surface dielectric loss in a transmon qubit. The capacitor pads and junction wires of transmons are shaped as spline curves and optimized through the finite-element method and a global optimization algorithm. We find that the participation ratio of capacitor pads and junction wires can be reduced by 15 % − 18 % and 22 % − 26 % compared to previous designs, while the overall footprint and anharmonicity are maintained at acceptable values. As a result, the two-level system-limited quality factor and the corresponding T 1 were increased by 21.6%.
Dielectric loss is one of the major decoherence sources of superconducting qubits. Contemporary high-coherence superconducting qubits are formed by material systems mostly consisting of superconducting films on substrate with low dielectric loss, where the loss mainly originates from the surfaces and interfaces. Among the multiple candidates for material systems, a combination of titanium nitride (TiN) film and sapphire substrate has good potential because of its chemical stability against oxidization and its high quality at interfaces. In this work, we report a TiN film deposited onto sapphire substrate achieving low dielectric loss at the material interface. Through the systematic characterizations of a series of transmon qubits fabricated with identical batches of TiN base layers but different geometries of qubit shunting capacitors with various participation ratios of the material interface, we quantitatively extract the loss-tangent value at the substrate-metal interface, which is smaller than $8.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ in a 1-nm disordered layer, and a limiting quality factor of about 7.3 million. By optimizing the interface participation ratio of the two-dimensional (2D) transmon qubit, we reproducibly achieve a quality factor of 5.7 million on average. The best 2D qubits show lifetimes of up to $300\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$ and quality factors achieving 8.1 million. We demonstrate that TiN film on sapphire substrate is an ideal material system for high-coherence superconducting qubits. Our analyses further suggest that the interface dielectric loss around the Josephson-junction part of the circuit could be the dominant limitation of lifetimes for state-of-the-art transmon qubits.
The Josephson junction and shunt capacitor form a transmon qubit, which is the cornerstone of modern quantum computing platforms. For reliable quantum computing, it is important how long a qubit can remain in a superposition of quantum states, which is determined by the coherence time (T1). The coherence time of a qubit effectively sets the "lifetime" of usable quantum information, determining how long quantum computations can be performed before errors occur and information is lost. There are several sources of decoherence in transmon qubits, but the predominant one is generally considered to be dielectric losses in the natural oxide layer formed on the surface of the superconductor. In this paper, we present a numerical study of microwave surface losses in planar superconducting antennas of different transmon qubit designs. An asymptotic method for estimating the energy participation ratio in ultrathin films of nanometer scales is proposed, and estimates are given for the limits of achievable minimum RF losses depending on the electrical properties of the surface oxide and the interface of the qubit with the substrate material.