Erasure Qubits: Overcoming the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>T</mml:mi><mml:mn>1</mml:mn></mml:msub></mml:math> Limit in Superconducting Circuits
Aleksander Kubica, Arbel Haim, Yotam Vaknin, Harry Levine, Fernando G. S. L. Brandão, Alex Retzker
Abstract
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_{ϕ} 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_{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.