Erasure Qubits: Overcoming the T1 Limit in Superconducting Circuits

The amplitude-damping time T1 has long stood as the major factor limiting quantum fidelity in superconducting circuits, prompting concerted efforts in the material science and design of qubits aimed at increasing T1. In contrast, the dephasing time Tφ can usually be extended above T1 (via, e.g., dynamical decoupling) to the point where it does not limit fidelity. In this article, we propose a scheme for overcoming the conventional T1 limit on fidelity by designing qubits in a way that amplitude-damping errors can be detected and converted into erasure errors. Compared to standard qubit implementations, our scheme improves the performance of fault-tolerant protocols, as numerically demonstrated by the circuit-noise simulations of the surface code. We describe two simple qubit implementations with superconducting circuits and discuss procedures for detecting amplitude-damping errors, performing entangling gates, and extending Tφ. Our results suggest that engineering efforts should focus on improving Tφ and the quality of quantum coherent control, as they effectively become the limiting factor on the performance of fault-tolerant protocols.