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 $8.7\times {10}^{-5}$ in 17 ns and always-on $ZZ$ interaction less than 0.4 kHz. Numerical simulations also show readout with state assignment error of $1\times {10}^{-3}$ in 38 ns (assuming quantum efficiency $\eta =0.5$). Our readout scheme naturally reduces Purcell decay to negligible levels without a Purcell filter and provides a mechanism to suppress shot noise dephasing times to the 10 ms level when not performing readout. Single-qubit gate infidelities are below $1\times {10}^{-5}$ 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.

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• Optoelectronics
• Quantum computation
• Quantum gates
• Quantum information with solid state qubits
• Superconducting qubits
• Microwave techniques