Niobium Films Reveal Hidden Flaws Threatening Future Quantum Computer Stability

Researchers are increasingly focused on understanding decoherence mechanisms that limit the performance of superconducting qubits. Amlan Datta, Kamal R. Joshi, and Sunil Ghimire, from Ames National Laboratory and Iowa State University, alongside Makariy A. Tanatar from Ames National Laboratory, Cameron J. Kopas and Jayss Marshall from Rigetti Computing, present a magneto-optical study investigating spatial inhomogeneities and magnetic-flux vortices within niobium thin films. This research is significant because niobium is a key material in transmon qubits, and identifying sources of dissipation within it is crucial for improving quantum coherence. Their quantitative imaging reveals how varying fabrication conditions impact flux penetration and critical current density, demonstrating that the niobium/silicon interface can substantially contribute to decoherence and requires careful optimisation.

Researchers have now utilised quantitative magneto-optical imaging to reveal previously unexplored spatial inhomogeneities within niobium films and their direct correlation to qubit decoherence.

Characterizing superconducting films and critical current density

This work focuses on characterising the superconducting state and critical current density, jc, in niobium films created using varying sputtering techniques. The imaging technique detected distinct flux-penetration regimes, ranging from nearly ideal behaviour to strongly nonuniform thermo-magnetic dendritic avalanches, providing a detailed map of the film’s superconducting properties.
By meticulously fitting the measured magnetic-induction profiles, the team extracted values for jc and established a clear link between this parameter, the physical properties of the films, and the internal quality factors of fabricated qubits. The study demonstrates that the niobium/silicon interlayer can act as a substantial contributor to decoherence, highlighting its importance as a key optimisation target.

This discovery stems from an investigation into the dynamics of the vortex lattice, employed as a sensitive “litmus test” for the superconducting state’s homogeneity. The research involved fabricating three niobium samples, designated A, B, and C, on high-resistivity silicon wafers using different deposition methods: HiPIMS, a low-high power DC sputtering sequence, and high-power DC sputtering, respectively.
All films were nominally 175nm thick and underwent identical post-deposition processing. Magneto-optical imaging, performed at temperatures down to 3.5K, enabled real-time mapping of the magnetic induction with sub-micron resolution. This technique relies on the Faraday effect, utilising a bismuth-doped ferrimagnetic iron-garnet indicator film to visualize the local magnetic field distribution.

The findings suggest that the atomic structure of the niobium-silicon interface, which can form an amorphous or crystalline niobium silicide layer, plays a crucial role in controlling thermal transport and mitigating decoherence. This detailed understanding of the interface, coupled with the ability to correlate film properties with qubit performance, offers a rapid feedback mechanism for optimising fabrication processes and advancing the development of more robust superconducting qubits.

Niobium film fabrication via HiPIMS and pulsed DC magnetron sputtering

Methods for niobium deposition and sample fabrication process

Niobium films were deposited on high-resistivity silicon wafers, possessing resistivity greater than 10 kΩ·cm, using physical vapor deposition (PVD) techniques. Three distinct samples, labelled A, B, and C, were fabricated employing variations in the sputtering process. Sample A utilised high-power impulse magnetron sputtering (HiPIMS), while Samples B and C were created using DC sputtering with differing power sequences.

All samples underwent a 5:1 buffered oxide etch (BOE) dip and identical pre-treatment, followed by an in situ bake prior to niobium deposition. The HiPIMS process for Sample A applied 850V voltage pulses with a 1% duty cycle, achieving a deposition rate of approximately 5.1nm/min. DC sputtering for Samples B and C involved a low-high power sequence, initially depositing approximately 30nm at 75W before switching to 350W to complete the 175nm nominal film thickness, matching the HiPIMS deposition rate.

Sample C was fabricated using DC sputtering at a constant high power setting. Magneto-optical imaging was performed at low temperatures using a closed-cycle optical cryogenic station, integrated with an Olympus BX3M polarized light microscope and long working distance objectives. This microscope-cryostat combination ensured stable optical access and mechanical/thermal stability for high-resolution imaging, enabling continuous measurements from room temperature down to 3.5 K.

Mapping magnetic induction for quantum stability assessment

Samples were rigidly mounted on a gold-plated copper stage within a vacuum chamber. Real-time spatial mapping of the normal component of magnetic induction was achieved via magneto-optical (MO) imaging, leveraging the Faraday effect. A multilayer indicator film, comprising bismuth-doped ferrimagnetic iron-garnet with in-plane magnetization, a mirror, and a protective layer, was placed on the sample surface, mirror-side down.

Linearly polarized light propagating through the garnet experienced a double Faraday rotation, with the resulting intensity proportional to the local Bz, allowing sub-μm spatial and temporal resolution. Brighter areas in the images correspond to higher amplitudes of Bz. Zero-field cooled (ZFC) and field-cooled (FC) imaging protocols were employed, applying magnetic fields of 40 Oe and 80 Oe at 4 K and 6 K to characterise flux penetration.

Critical current density and flux penetration regimes in sputtered niobium films

Niobium films exhibited critical current densities, jc, strongly influenced by fabrication conditions and correlated with qubit internal quality factors. Magneto-optical imaging revealed distinct flux-penetration regimes, ranging from nearly ideal Bean critical states to strongly nonuniform thermo-magnetic dendritic avalanches within the superconducting material.

Analysis of magnetic-induction profiles allowed extraction of jc, providing a direct link between film properties and quantum coherence. Sample A, fabricated using high-power impulse magnetron sputtering, demonstrated a deposition rate of 5.1 nanometers per minute. Sample B, created with a low-high power sequence using DC sputtering, initially deposited approximately 30 nanometers at 75 watts before transitioning to 350 watts to achieve the target film thickness.

Sample C utilised DC sputtering at a consistent high power, resulting in a deposition rate of 25 nanometers per minute. These varying deposition parameters significantly impacted the resulting superconducting film characteristics. The research utilised a closed-cycle 4He refrigeration system enabling continuous measurements from room temperature down to 3.5 Kelvin.

Real-time spatial mapping of the normal component of magnetic induction was achieved through magneto-optical imaging, leveraging the Faraday effect and a bismuth-doped ferrimagnetic iron-garnet indicator film. This technique provided a sensitive probe of the vortex lattice dynamics and the homogeneity of the superconducting state within the niobium films. The Nb/Si interface was identified as a crucial factor influencing decoherence and requiring optimisation for improved qubit performance.

Niobium Film Properties and Their Impact on Transmon Qubit Coherence

Scientists have demonstrated that the spatial homogeneity of the superconducting state and effective magnetic field screening are important determinants of quantum coherence in niobium transmon qubits. Quantitative magneto-optical imaging was used to characterise the distribution of magnetic flux and critical current density within niobium films produced using varying sputtering techniques.

The imaging revealed differing behaviours, ranging from near-ideal magnetic field exclusion to substantial thermo-magnetic instabilities manifesting as avalanches. The study identified that a balanced thermal link to the substrate, alongside a thin and spatially uniform silicide layer, is crucial for achieving optimal quantum performance. Furthermore, the research highlights the importance of optimising flux pinning alongside the superconducting transition temperature and interface quality.

The authors acknowledge that multiple decoherence sources, including surface oxides, impurities, and device geometry, contribute to overall performance limitations. While this work focuses on the niobium-silicon interface, achieving peak device performance will likely require systematic mitigation of all contributing loss channels. Future research should concentrate on optimising flux pinning specifically at the operating frequencies of the quantum device, alongside continued efforts to refine the interfacial layer and superconducting properties of the materials used.