Efficient three-qubit gates with giant atoms

TL;DR

The paper tackles the challenge of implementing high-fidelity native three-qubit gates in quantum processors, which are hard due to crosstalk and the overhead of tunable couplers. It introduces giant artificial atoms coupled to a waveguide as a platform where interference from multiple coupling points creates decoherence-free operating frequencies, enabling native CCZS and DIV gates through simple frequency tuning without parametric modulation. The authors provide analytic and numerical analyses showing gate fidelities exceeding $0.995$ with realistic parameters and sub-100 ns gate times, and demonstrate fast three- and five-qubit GHZ state preparation with minimal gate depth. The results highlight giant atoms as a scalable, hardware-efficient route to low-depth quantum circuits and open-system dynamics simulations for near-term quantum computing.

Abstract

Three-qubit gates are highly beneficial operations in quantum computing, enabling compact implementations of quantum algorithms and efficient generation of multipartite entangled states. However, realizing such gates with high fidelity remains challenging due to crosstalk, complex control requirements, and the overhead of parametric or tunable couplers. In this work, we propose and analyze the implementation of fast, high-fidelity three-qubit gates using giant atoms--artificial atoms coupled to a waveguide at multiple spatially separated points. By leveraging interference effects intrinsic to the giant-atom architecture, we demonstrate that native three-qubit gates, such as the controlled-CZ-SWAP (CCZS) and the dual-iSWAP (DIV), can be realized through simple frequency tuning, without the need for complex pulse shaping or additional hardware. We evaluate gate performance under realistic decoherence and show that fidelities exceeding 99.5% are achievable with current experimental parameters in superconducting circuits. As an application, we present a scalable protocol for preparing three- and five-qubit GHZ states using minimal gate depth, achieving high state fidelity within sub-300ns timescales. Our results position giant-atom systems as a promising platform for entangled-state preparation and low-depth quantum circuit design in near-term quantum computers and quantum simulators.

Figures (4)

• Figure 1: Three-giant-atom setup for implementing CCZS and DIV gates. (a) Schematic of the system. Three giant atoms with transition frequencies $\omega_1$, $\omega_2$, and $\omega_3$, and anharmonicities $\chi_1$, $\chi_2$, and $\chi_3$, are coupled to a waveguide (black line) at multiple spatially separated points with coupling strengths $\gamma$. The coupling points are organized in a braided fashion. (b) Frequency dependence of the individual decay rates $\Gamma_\text{ind}$ and inter-atomic coupling strengths $g_{12}$ and $g_{13}$, showing selective activation of desired interactions. Due to the symmetry of the setup, $g_{12} = g_{23}$. (c,d) Frequency configurations used to perform (c) a CCZS gate and (d) a DIV gate, exploiting interference effects at decoherence-free points.
• Figure 2: Average process fidelity of three-qubit gates implemented with the giant-atom setup shown in Fig. \ref{['fig1']}. (a) Fidelity of the CCZS gate as a function of qubit decay rate $\Gamma_\text{ex}$ and (b) dephasing rate $\Gamma_\phi$. (c,d) Fidelity of the DIV gate under the same noise conditions. In all cases, the qubit–qubit coupling strength is $g=\gamma$, set by the waveguide-mediated interaction.
• Figure 3: Preparation of a three-qubit GHZ state using giant-atom gates. (a) Quantum circuit for generating the GHZ state using a CCZS gate and single-qubit gates. (b) Frequency-tuning protocol used to activate the required gate interactions of the circuit in (a) via decoherence-free points in the giant-atom setup shown in Fig. \ref{['fig1']}. (c) Fidelity of the resulting GHZ state as a function of qubit decay rate $\Gamma_\text{ex}$ and (d) $\Gamma_\phi$, assuming ideal single-qubit operations.
• Figure 4: Preparation of a five-qubit GHZ state using giant-atom gates. (a) Schematic of the setup [an extension of that in Fig. \ref{['fig1']}]. (b) Quantum circuit for generating the GHZ state using two CCZS gates, one iSWAP gate, and single-qubit gates. (c) Frequency tuning protocol used to activate the required gate interactions of the circuit in (a) via decoherence-free points in the giant-atom setup. (d) Fidelity of the resulting GHZ state as a function of qubit decay rate $\Gamma_\text{ex}$ and (e) $\Gamma_\phi$, assuming ideal single-qubit operations.