Fast Ion Gates Outside the Lamb-Dicke Regime by Robust Quantum Optimal Control

TL;DR

This paper introduces a robust quantum optimal control method for fast ion-trap entangling gates that operate beyond the Lamb-Dicke regime without requiring weak-coupling approximations, enabling faster and more reliable quantum operations.

Contribution

The authors develop a gradient-based optimal control framework that finds laser pulse protocols for fast, robust entangling gates outside the Lamb-Dicke approximation, advancing quantum gate speed and fidelity.

Findings

Achieved microsecond-scale entangling gates comparable to trap frequencies.

Demonstrated robustness against ion temperature and phase imperfections.

Enabled gate protocols beyond the Lamb-Dicke regime without perturbative assumptions.

Abstract

We present a robust quantum optimal control framework for implementing fast entangling gates on ion-trap quantum processors. The framework leverages tailored laser pulses to drive the multiple vibrational sidebands of the ions to create phonon-mediated entangling gates and, unlike the state of the art, requires neither weak-coupling Lamb-Dicke approximation nor perturbation treatment. With the application of gradient-based optimal control, it enables finding amplitude- and phase-modulated laser control protocols that work beyond the Lamb-Dicke regime, promising gate speed at the order of microseconds comparable to the characteristic trap frequencies. Also, robustness requirements on the temperature of the ions and initial optical phase can be conveniently included to pursue high-quality fast gates against experimental imperfections. Our approach represents a step in speeding up quantum gates to achieve larger quantum circuits for quantum computation and simulation, and thus can find applications in near-future experiments.