Quantum devices such as qubits are critical for storing and manipulating quantum information. The qubit lifespan, known as coherence time, determines how long data can be stored and processed before an error occurs. This phenomenon, called quantum decoherence, is a critical obstacle to operating quantum processors and sensors.

At the Superconducting Quantum Materials and Systems Center, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, we are focusing on solving the decoherence challenge. By tackling the problem at the material level, in the work presented in this paper we have achieved a significant result: reproducible improvements in coherence times, up to record values of 0.6 milliseconds in superconducting transmon qubits. The results were achieved by exploring innovative materials that eliminated a major source of losses in these devices. 

Our researchers worked across academic and commercial qubit foundries, achieving this new milestone by developing a technique called ‘surface encapsulation.’ The novel process protects key layers of the qubit during fabrication and prevents the formation of lossy materials at the surfaces and interfaces of these devices. By carefully investigating and comparing various materials and deposition techniques, SQMS researchers have studied alternative oxides that lead to higher qubit lifetimes and lower “two-level-system losses (TLS).” 

SQMS scientists’ systematic, in-depth study of the hierarchy of losses in transmon qubits pointed to the niobium surface as the primary culprit in these qubits. In the presence of air these qubits oxidize, resulting in a few nanometer thick niobium native oxide, which limits performance by shortening coherence times.

Prior measurements from our group had indicated that niobium is the best superconductor for these qubits. While the metal losses are near zero, the niobium surface oxide is problematic and the main driver of TLS losses in these circuits.

SQMS scientists had the idea of encapsulating the niobium during the qubit fabrication and before being exposed to air, to prevent oxide formation. While we had a hypothesis on which materials would work best for encapsulation, determining the optimal material required a detailed study. So, we systematically tested this technique with different materials, including aluminum, tantalum, titanium nitride, and gold. 

With each capping layer attempt, we analyzed the materials with several advanced characterization techniques such as time-of-flight secondary ion mass spectrometry (ToF-SIMS), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) at material science labs at Fermilab, Ames National Lab, Northwestern University and Temple University. Qubit performance was measured at dilution fridge temperatures, and the results demonstrated that we could prepare qubits with 2 to 5 times coherence improvement compared to samples prepared without a capping layer (containing the niobium native oxide layer). Our team found that the material encapsulation process systematically improved coherence times for all materials explored in the study.