Shielding Niobium from Contamination Boosts Quantum Computer Coherence Times

Researchers are increasingly focused on improving the coherence of superconducting qubits, and a new study by Datta, Joshi, and Ghimire, along with colleagues from Ames National Laboratory and Fermi National Accelerator Laboratory, investigates the impact of metal encapsulation on the bulk superconducting properties of niobium thin films used in these devices. Niobium constitutes the vast majority of transmon qubits, and defects within the material can significantly limit coherence. This work presents a comprehensive analysis of niobium films encapsulated with gold and palladium/gold, revealing substantial differences in their superconducting behaviour compared to bare niobium. The findings demonstrate that gold capping not only passivates the surface but also improves the overall film quality, reducing defect scattering and suggesting that bulk disorder, not just surface effects, is a key source of decoherence in superconducting qubits.

Niobium film coherence enhancement via gold and palladium encapsulation

Scientists have demonstrated a substantial improvement in the coherence of niobium thin films, critical components in superconducting quantum circuits, through metal encapsulation. Niobium currently comprises nearly 100% of the volume in many 2D transmon devices, yet imperfections within the material itself can limit quantum coherence.
This work reveals that disorder throughout the niobium film, not solely surface issues, significantly contributes to decoherence, a major obstacle in building stable qubits. Researchers performed a comprehensive study of niobium films encapsulated with gold and palladium/gold, comparing their superconducting properties to bare niobium films to understand the impact of encapsulation on the material’s macroscopic behaviour.

Magneto-optical imaging, magnetization measurements, resistivity analysis, and precise London and Campbell penetration depth determinations all revealed marked differences between encapsulated and unencapsulated samples. Both sputtered and epitaxially grown gold-capped films exhibited the highest residual resistivity ratio and superconducting transition temperature, alongside the lowest upper critical field, London penetration depth, and critical current.
These findings align with theoretical models describing the anisotropic normal and superconducting states of niobium, confirming a deeper understanding of the material’s behaviour. The study conclusively demonstrates that pair-breaking within the bulk of niobium films, stemming from disorder throughout the material, is a significant source of decoherence in transmons.

Furthermore, gold capping not only passivates the surface, preventing oxidation, but also fundamentally alters the film’s properties, substantially reducing the scattering rate caused by defects. This reduction is likely due to the prevention of surface diffusion, which introduces defects if the film is not immediately protected after fabrication.

Specifically, Au-capped films achieved the highest residual resistivity ratio, indicating significantly lower disorder compared to other films, with the epitaxial Nb/Au film exhibiting the highest value. The lowest residual resistivity was observed in these films, suggesting a reduced density of scattering centres, although a slightly smeared superconducting transition curve was also noted. Conversely, the lowest values for the upper critical field, London penetration depth, and critical current were also consistently observed in the gold-capped samples, demonstrating a clear correlation between encapsulation and improved superconducting characteristics.

Fabrication of niobium thin films and low-temperature magneto-optical characterisation

Niobium thin films were fabricated on sapphire substrates using either sputtering or molecular beam epitaxy to investigate the impact of gold encapsulation on superconducting properties. Six distinct samples were prepared including bare sputtered niobium, sputtered niobium with palladium/gold, sputtered niobium with gold deposited after argon ion cleaning, sputtered niobium with gold following annealing, molecular beam epitaxy-deposited bare niobium, and molecular beam epitaxy-deposited niobium with in-situ gold encapsulation.

Sputtered PdAu-capped and Au-capped films were created in separate sputtering chambers, acknowledging potential variations in residual impurities such as niobium nitride. Film thicknesses ranged from 110nm to 163nm for niobium, with capping layers varying from 5nm to 10nm. Low-temperature magneto-optical imaging was performed using a closed-cycle optical cryo-station, specifically a Model-s50 from Montana Instruments, coupled with an Olympus BX3M optical system.

Samples were mounted on a gold-plated cold stage and observed through optical windows, allowing for controlled measurements between room temperature and 3.8 K. Real-time mapping of magnetic induction was achieved via magneto-optical imaging, utilising the Faraday effect in bismuth-doped iron-garnet indicators placed on the sample surface.

Linearly polarized light, reflected from a mirror layer, revealed spatial variations in magnetic induction, with brighter areas indicating higher magnetic induction and darker areas representing complete magnetic screening. Magnetic penetration depth measurements employed a highly sensitive self-oscillating tunnel diode resonator.

Samples were positioned within a single-layer inductor coil generating a small AC magnetic field of approximately 14MHz. A tunnel diode, biased to its negative differential resistance region, enabled resonance when impedances matched, achieving a resolution better than one part per billion. Frequency shifts, proportional to magnetic susceptibility, were converted into magnetic penetration depth using the relation (1 −N) χ = λ/R tanh (R/λ)−1, where N represents the generalised demagnetizing factor and R is the effective sample dimension.

Gold encapsulation enhances superconducting properties in niobium films

Researchers demonstrate that gold encapsulation significantly alters the bulk superconducting properties of niobium films, not just the surface. Resistivity measurements reveal that both sputtered and epitaxially grown gold-capped films exhibit the highest residual resistivity ratio, a key indicator of film quality, and superconducting transition temperature.

Specifically, the highest residual resistivity ratio was observed in a niobium/gold epitaxial film, though this film also displayed a more gradual superconducting transition. The lowest resistivity at the critical temperature, ρ(Tc), was measured in Au-capped films, suggesting a reduced density of scattering centres within the material.

Analysis of temperature-dependent resistivity, ρ(T), from 300K down to 7K, established that the ratio of ρ(300K) to ρ(T) provides a reliable approximation of the residual resistivity ratio, RRR. Among the films studied, Au-capped films consistently showed the highest RRR values, indicating significantly lower disorder compared to bare niobium films.

The cleanest bulk niobium samples achieve an RRR of approximately 90000, while typical thin films range from 5 to 50; the Au-capped films in this study surpassed these values. Further investigation into the superconducting transition near Tc revealed that the offset temperature, where ρ(T) equals zero, remained consistent across Au-capped films and was notably higher than in other samples.

Films ranged in thickness from 110nm to 163nm, and the contribution of the capping layer to the overall resistivity was found to be negligible due to its thickness of less than 10nm. These findings suggest that gold capping not only passivates the surface but also reduces the scattering rate caused by defects throughout the niobium film.

Disorder-mediated suppression of superconducting properties in gold-protected niobium films

Niobium films, essential components of superconducting transmon devices, are susceptible to decoherence caused by pair-breaking effects stemming from disorder within the material itself. Investigations into the superconducting properties of niobium thin films encapsulated with gold and palladium/gold, compared to bare niobium films, reveal substantial differences in their macroscopic behaviour.

Magneto-optical imaging, magnetization measurements, and assessments of London and Campbell penetration depths demonstrate that both sputtered and epitaxially grown gold-capped films exhibit elevated residual resistivity ratios and superconducting transition temperatures, alongside reduced upper critical fields, London penetration depths, and critical currents. These findings align with theoretical models describing the anisotropic normal and superconducting states of niobium.

The research establishes that disorder throughout the niobium film, rather than solely at the surface, significantly contributes to decoherence in transmons. Gold encapsulation not only passivates the surface but also modifies the bulk properties of the film, diminishing defect-induced scattering. The highest residual resistivity ratio was observed in the epitaxial gold-capped film, indicating the cleanest material, with bare niobium exhibiting the lowest ratio and transition temperature.

Furthermore, the study identified thermo-magnetic instabilities in sputtered bare niobium and palladium/gold-capped films, absent in gold-capped films, suggesting insufficient heat conduction to the substrate which could be problematic for high-frequency qubit operation. Time-of-flight secondary ion mass spectrometry confirmed that bare niobium films contain significantly higher oxygen concentrations, directly correlating with increased disorder scattering and reduced transition temperatures.

The authors acknowledge that disorder must range in scale from atomic defects to approximately 20 nanometres to effectively suppress the transition temperature and enhance the critical current density. Future research could focus on optimising encapsulation techniques to further minimise disorder and improve thermal conductivity, potentially extending qubit coherence times and performance. These results highlight the importance of considering bulk material properties, alongside surface passivation, in the development of high-coherence superconducting devices.