Metrological assessment of NIS tunnel junctions for cryogenic thermometry

Solid-state quantum computers and sensors often require ultra-low temperatures (<1 K) to function. Reliably achieving sub-kelvin temperatures is highly non-trivial and yet is critical for understanding exotic electronic states of matter and for superconducting qubit development. Even at such extreme temperatures, device performance is often strongly temperature dependent (e.g., decoherence in qubits and quantization in topological insulators). To maintain quality and equivalence in benchmarking results, temperature traceability to the International System of Units (SI) is essential. 

Thin film normal metal-insulator-superconductor (NIS) junctions are commonly used as secondary thermometers for local temperature measurements below 1 K. Their feasibility as primary thermometers, however, is less established. Here, we show the results of a metrological evaluation of Al-based NIS thermometers for primary thermometry from 30 mK to 800 mK. The devices were fabricated using standard optical lithography and their current-voltage characteristics were measured with a simple DC biasing scheme. Preliminary results indicate excellent stability over time, good agreement between devices from different cleanrooms, and between NIS thermometers and independently calibrated resistance thermometers.