Among the factors that limit the energy relaxation time T1 of superconducting qubits, which is most closely related to fabrication processes such as thin-film deposition and surface treatment?

Answer: C. Dielectric loss from defects called two-level systems (TLS) in amorphous oxides and other layers at the surfaces and interfaces of the substrate and metal

The electric field of a superconducting qubit passes through the substrate–metal interface, the surfaces of the metal and substrate, the oxide of the Josephson junction and so on. The amorphous materials there contain many two-level-system (TLS) defects, in which atoms fluctuate between two configurations, and they absorb the qubit's energy and shorten T1. Substrate cleaning, treatments that reduce surface oxides, the choice of film material (such as tantalum) and electrode shapes that keep the field from concentrating at surfaces therefore all improve coherence. This is a materials and process challenge much like reducing interface defects in the gate insulator of a MOSFET.

• A. A larger anharmonicity makes it easier to address only the transition between |0⟩ and |1⟩. It is not a factor that shortens T1.
• B. Aluminum becomes superconducting at about 1.2 K, well above the operating temperature of about 10 mK, so it is fully superconducting.
• C. Correct. Materials and process improvements that reduce surface and interface defects improve T1.