Controlled-Z gates with giant atoms in structured waveguides

TL;DR

This paper proposes a protocol for implementing high-fidelity controlled-Z gates with giant atoms in structured waveguides, overcoming non-Markovian effects that limit previous designs, thus enabling universal quantum simulation.

Contribution

It introduces an extended coupling scheme that suppresses non-Markovian effects, allowing reliable CZ gate operation with fidelities up to 97.7%.

Findings

A minimal two-point coupling supports decoherence-free interactions but suffers from non-Markovian degradation.

Adding a third coupling point suppresses non-Markovian effects and improves gate fidelity.

The proposed scheme enables both iSWAP and CZ gates in structured waveguides for scalable quantum computing.

Abstract

Giant atoms are quantum emitters coupled to waveguides at multiple, spatially separated points, enabling interference effects that fundamentally change their light-matter interactions. A notable consequence of the interference is the emergence of decoherence-free interaction (DFI), which allows coherent excitation exchange between giant atoms via the waveguide without radiative loss. Leveraging DFI offers a promising route to implementing two-qubit quantum gates without the need for additional resources, positioning giant atoms as a versatile platform for scalable universal quantum simulators. However, existing work has focused primarily on continuous, Markovian waveguides; in structured waveguides, where non-Markovian effects become significant, only iSWAP gates have been explored. To address this gap, we introduce and analyze a protocol for implementing controlled-Z (CZ) gates with giant atoms in structured waveguides. We first show that while a minimal two-point coupling scheme supports DFI, it also exhibits strong non-Markovian effects that substantially degrade gate fidelity. To overcome this limitation, we propose an extended design featuring a third coupling point. This configuration suppresses non-Markovian effects and enables CZ gates with fidelities up to $97.7\%$ (assuming typical values for experimental imperfections). Our results broaden the accessible gate set for giant atoms in structured waveguides to include both iSWAP and CZ gates, advancing these systems as a pathway toward universal quantum simulators operating in non-Markovian environments.