Ultrafast Single Qubit Gates through Multi-Photon Transition Removal

TL;DR

This paper introduces a novel method called R2D to perform ultrafast single qubit gates with minimal leakage, achieving high fidelity and low error rates in superconducting transmon qubits by removing multi-photon transitions.

Contribution

The paper presents the R2D method that effectively suppresses multi-photon transitions, enabling faster and more accurate single qubit gates with minimal leakage in superconducting qubits.

Findings

Achieved leakage errors below 2.0×10⁻⁵ with fidelities above 99.98%.

Demonstrated gate durations as short as 6.8 ns.

Identified higher order transitions as the main error source at high speeds.

Abstract

One of the main enablers in quantum computing is having qubit control that is precise and fast. However, qubits typically have multilevel structures making them prone to unwanted transitions from fast gates. This leakage out of the computational subspace is especially detrimental to algorithms as it has been observed to cause long-lived errors, such as in quantum error correction. This forces a choice between either achieving fast gates or having low leakage. Previous works focus on suppressing leakage by mitigating the first to second excited state transition, overlooking multi-photon transitions, and achieving faster gates with further reductions in leakage has remained elusive. Here, we demonstrate single qubit gates with a total leakage error consistently below $2.0\times10^{-5}$, and obtain fidelities above $99.98\%$ for pulse durations down to 6.8 ns for both $X$ and $X/2$ gates. This is achieved by removing direct transitions beyond nearest-neighbor levels using a double recursive implementation of the Derivative Removal by Adiabatic Gate (DRAG) method, which we name the R2D method. Moreover, we find that at such short gate durations and strong driving strengths the main error source is from these higher order transitions. This is all shown in the widely-used superconducting transmon qubit, which has a weakly anharmonic level structure and suffers from higher order transitions significantly. We also introduce an approach for amplifying leakage error that can precisely quantify leakage rates below $10^{-6}$. The presented approach can be readily applied to other qubit types as well.