Terahertz Imaging Reveals Coherence Limits in Superconducting Qubit Devices.

Terahertz nano-imaging and spectroscopy non-invasively characterise niobium transmon qubits, revealing sidewall scattering linked to coherence. Hyperspectral analysis probes dielectric responses at aluminium junctions. These findings establish terahertz near-field methods as a rapid characterisation tool for optimising qubit fabrication and performance.

The pursuit of stable quantum computation necessitates a detailed understanding of the factors limiting qubit coherence – the duration for which a qubit maintains its quantum state. Identifying and mitigating these limitations currently relies on techniques often incompatible with high-throughput analysis or which require destructive sample preparation. Researchers from Ames National Laboratory, Iowa State University, Fermi National Accelerator Laboratory, and the National Institute of Standards and Technology report a non-invasive method utilising terahertz (THz) nanophotonics to characterise losses in superconducting qubits. Their work, detailed in ‘Correlating Superconducting Qubit Performance Losses to Sidewall Near-Field Scattering via Terahertz Nanophotonics’, demonstrates a correlation between sidewall scattering observed via THz imaging and qubit coherence, offering a potential pathway for rapid, non-destructive assessment of qubit quality and optimisation of fabrication processes. The collaborative team includes Richard H. J. Kim, Samuel J. Haeuser, Joong-Mok Park, Randall K. Chan, Jin-Su Oh, Thomas Koschny, Lin Zhou, Matthew J. Kramer, Akshay A. Murthy, Mustafa Bal, Francesco Crisa, Sabrina Garattoni, Shaojiang Zhu, Andrei Lunin, David Olaya, Peter Hopkins, Alex Romanenko, Anna Grassellino, and Jigang Wang.

Terahertz Nano-Imaging Reveals Coherence-Limiting Factors in Superconducting Qubits

Researchers have developed a non-invasive method utilising terahertz (THz) nano-imaging and spectroscopy to characterise niobium transmon qubits, addressing a critical need for high-throughput materials analysis in quantum computing. The study focuses on identifying factors limiting qubit coherence – the duration for which a qubit maintains its quantum state – without resorting to destructive testing methods or complex low-temperature measurements. This innovative approach promises to accelerate the development of more stable and reliable quantum computers by providing a rapid means of assessing qubit quality and identifying potential sources of decoherence, paving the way for improved fabrication processes and device selection.

The investigation demonstrates that THz nano-imaging effectively visualises near-field scattering originating from qubit sidewalls, revealing subtle structural imperfections that contribute to energy dissipation. This scattering correlates directly with observed reductions in qubit coherence, establishing a clear link between physical defects and quantum performance. Researchers meticulously analysed the scattering patterns to map the distribution of these defects, providing valuable insights into the mechanisms driving decoherence and allowing for targeted improvements in fabrication techniques to enhance qubit stability.

Further analysis employs THz hyperspectral line scans, which probe the dielectric response – a material’s ability to store electrical energy in an electric field – and field participation at the aluminium-niobium junctions within the qubit. These scans reveal variations in the dielectric properties and field distribution at these interfaces, indicating the presence of interface imperfections and variations in oxide thickness. Researchers believe these imperfections act as loss centres, degrading qubit coherence and providing a powerful tool for optimising junction quality and overall qubit performance.

The findings demonstrate the potential of THz near-field methods as a high-throughput characterisation tool for superconducting qubits, offering a significant advantage over traditional, time-consuming techniques. Unlike destructive methods, THz imaging and spectroscopy are non-destructive and can be performed relatively quickly, allowing for the screening of large numbers of qubits, which is crucial for optimising fabrication processes and identifying qubits with superior performance characteristics, accelerating the development of quantum computing technology. By providing a direct correlation between structural features, dielectric properties, and qubit coherence, this research paves the way for a more efficient and targeted approach to improving qubit quality.

This research extends the application of near-field THz techniques beyond material characterisation to the realm of quantum circuit diagnostics, opening up new possibilities for understanding and improving qubit performance. Researchers successfully demonstrated the ability to map dielectric loss and structural variations at the nanoscale, providing valuable insights into the mechanisms limiting qubit coherence. This detailed characterisation enables targeted improvements in fabrication techniques and material selection, paving the way for more robust and reliable quantum computers.