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Controlled-Z gates with giant atoms in structured waveguides

Walter Rieck1,2, Ariadna Soro1, Anton Frisk Kockum1,*, and Guangze Chen1,†

  • *Contact author: anton.frisk.kockum@chalmers.se
  • †Contact author: guangze@chalmers.se
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Phys. Rev. A 114, 043719 – Published 14 October, 2026

DOI: https://doi.org/10.1103/yfm3-qmqq

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 average gate 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.

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Phys. Rev. A 114, 043719– Published 14 October, 2026
  • Received 2 April 2026
  • Revised 1 September 2026
  • Accepted 24 September 2026
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