Trimming the frequency of superconducting qubits on sapphire substrates by a mode-locked laser

As superconducting quantum processors build up in complexity, how to address the challenge of frequency crowding proves formidable. By utilizing continuous-wave lasers, the recently developed method of laser-annealing has been successfully exploited to realize frequency collision-free superconducting quantum processors synthesized on silicon substrates. Due to its very low microwave loss tangent and chemical inertness, sapphire provides an excellent substrate on which superconducting qubits with long coherence time can be fabricated. To confront the problem of frequency crowding for sapphire-based qubits, we developed a post-fabrication laser-annealing approach based on a Ti:sapphire mode-locked laser. The characterization of the annealed qubits at low temperatures demonstrated the preservation of high coherence after laser treatment. While the unannealed qubits showed about 3.5% variation in their resistances, the variation of the resistance of the annealed qubits was improved to be around 1%. This work demonstrates the effectiveness of employing the mode-locked laser to trim the frequency of sapphire-based superconducting qubits, and therefore paves the way to engineer the frequency collision-free lattice with > 100 qubits on such substrates.