High-quality electrical contacts to van der Waals superconductors

Layered Van der Waals (VdW) superconductors are a great promise for high frequency devices such as qubits, due to their highly crystalline nature and potentially defect free interfaces. A major obstacle toward this goal is the requirement for extremely high-performance contacts, which is challenging due to the air-sensitivity of common VdW superconductors such as NbSe₂ .We have developed and tested a reliable lift-off recipe for making superconducting contact between multi-layer crystals of NbSe₂ and aluminum films, using argon ion milling to strip away degraded layers of NbSe₂ before in situ aluminum deposition. Because performance at the level required for quantum applications cannot be tested by low-frequency techniques, we have fabricated microwave resonators with embedded NbSe₂ flakes and characterized these at cryogenic temperature. We find that the contacts achieved have low enough losses to be used in high-quality quantum devices.