Erasure Qubits: Overcoming the T₁ Limit in Superconducting Circuits

Abstract

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 T_ϕ 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 T_ϕ. Our results suggest that engineering efforts should focus on improving T_ϕ and the quality of quantum coherent control, as they effectively become the limiting factor on the performance of fault-tolerant protocols.

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