Demonstration of an All-Microwave Controlled-Phase Gate between Far Detuned Qubits

TL;DR

This paper demonstrates an all-microwave controlled-phase gate between far-detuned transmon qubits, achieving high fidelity without additional control lines, which could simplify scaling in quantum processors.

Contribution

The authors realize a novel all-microwave controlled-phase gate between far-detuned transmon qubits, avoiding the need for tunable couplers or extra drive lines, and model it with Floquet theory.

Findings

Gate fidelity of 97.5% at 126 ns

Operates with a single microwave tone

Potential for scalable quantum processor architecture

Abstract

A challenge in building large-scale superconducting quantum processors is to find the right balance between coherence, qubit addressability, qubit-qubit coupling strength, circuit complexity and the number of required control lines. Leading all-microwave approaches for coupling two qubits require comparatively few control lines and benefit from high coherence but suffer from frequency crowding and limited addressability in multi-qubit settings. Here, we overcome these limitations by realizing an all-microwave controlled-phase gate between two transversely coupled transmon qubits which are far detuned compared to the qubit anharmonicity. The gate is activated by applying a single, strong microwave tone to one of the qubits, inducing a coupling between the two-qubit $|f,g\rangle$ and $|g,e\rangle$ states, with $|g\rangle$, $|e\rangle$, and $|f\rangle$ denoting the lowest energy states of a transmon qubit. Interleaved randomized benchmarking yields a gate fidelity of $97.5\pm 0.3 \%$ at a gate duration of $126\,\rm{ns}$, with the dominant error source being decoherence. We model the gate in presence of the strong drive field using Floquet theory and find good agreement with our data. Our gate constitutes a promising alternative to present two-qubit gates and could have hardware scaling advantages in large-scale quantum processors as it neither requires additional drive lines nor tunable couplers.