The Arm Qubit: A Superconducting Qubit Co-Designed for Coherence and Coupling

TL;DR

This paper introduces a superconducting qubit design with dual modes for improved coherence and coupling, enabling faster, high-fidelity quantum gates and readout, advancing scalable quantum computing.

Contribution

The arm qubit design uniquely combines data storage and coupling modes, achieving superior gate and readout fidelities through capacitive coupling, with minimal linear coupling.

Findings

CZ gate infidelity of 8.6e-5 in 17 ns

Readout error of 1e-4 in 27 ns

Single-qubit gate infidelity below 1e-5

Abstract

We present a superconducting qubit which consists of two strongly coupled modes: one for data storage and one for coupling, allowing faster, higher-fidelity entangling gates and readout. The use of a dedicated coupling mode allows nonlinear couplings of several hundred MHz between the data mode and other elements, with minimal linear coupling to the data mode. Including decoherence, simulations show that this architecture enables microwave-only CZ gates with an infidelity of $8.6\times10^{-5}$ in 17 ns and always-on ZZ interaction less than 0.4 kHz. Numerical simulations also show readout with state assignment error of $1\times10^{-4}$ in 27 ns (assuming quantum efficiency $\eta=0.5$), Purcell-limited lifetime of 167 ms without a Purcell filter, and a mechanism to suppress shot-noise dephasing ($1/\Gamma_{\phi}=15.8$ ms). Single-qubit gate infidelities are below $1\times10^{-5}$ including decoherence. These beyond experimental state-of-the-art gate and readout fidelities rely only on capacitive coupling between arm qubits, making the arm qubit a promising scalable building block for fault-tolerant quantum computers.