Erasure qubits: Overcoming the $T_1$ limit in superconducting circuits

The amplitude damping time, $T_1$, 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 $T_1$. In contrast, the dephasing time, $T_{\phi}$, can usually be extended above $T_1$ (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 $T_1$ 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 $T_\phi$. Our results suggest that engineering efforts should focus on improving $T_\phi$ and the quality of quantum coherent control, as they effectively become the limiting factor on the performance of fault-tolerant protocols.