Scalable quantum simulator with an extended gate set in giant atoms

Quantum computation and quantum simulation require a versatile gate set to optimize circuit compilation for practical applications. However, existing platforms are often limited to specific gate types or rely on parametric couplers to extend their gate set, which compromises scalability. Here, we propose a scalable quantum simulator with an extended gate set based on giant-atom three-level systems, which can be implemented with superconducting circuits. Unlike conventional small atoms, giant atoms couple to the environment at multiple points, introducing interference effects that allow exceptional tunability of their interactions. By leveraging this tunability, our setup supports both CZ and iSWAP gates through simple frequency adjustments, eliminating the need for parametric couplers. This dual-gate capability enhances circuit efficiency, reducing the overhead for quantum simulation. As a demonstration, we showcase the simulation of spin dynamics in dissipative Heisenberg XXZ spin chains, highlighting the setup’s ability to tackle complex open quantum many-body dynamics. Finally, we discuss how a two-dimensional extension of our system could enable fault-tolerant quantum computation, paving the way for a universal quantum processor.

💡 Research Summary

The paper proposes a scalable quantum simulator architecture that achieves an extended two‑qubit gate set—both controlled‑phase (CZ) and iSWAP—without the need for parametric couplers, by exploiting the unique properties of giant‑atom superconducting circuits. A giant atom is a three‑level artificial atom (Ξ‑type transmon) that couples to a waveguide at several spatially separated points. The multiple coupling points generate interference effects that make the individual decay rate Γ_ind(ω), the collective decay Γ_coll(ω), and the exchange interaction g_12(ω) strongly frequency‑dependent and periodic with period ω₀ = 2πv/Δx, where Δx is the spacing between coupling points and v is the waveguide propagation speed. At specific “decoherence‑free” frequencies ω_DF = (n + m/8)·ω₀ (with m = 1,2,3,5,6,7) the individual decay vanishes while the exchange interaction remains finite, providing a broadband window where qubits experience negligible loss but retain a sizable coupling.

Using a pair of such giant atoms arranged in a braided geometry, the authors show how simple frequency tuning implements two distinct gates:

1. iSWAP – By setting both atoms to the same decoherence‑free frequency, the XY exchange interaction swaps the |01⟩ and |10⟩ states and adds a phase i, leaving |00⟩ and |11⟩ unchanged. The interaction strength is g ≈ 2γ, where γ is the single‑point coupling strength, leading to a gate time τ_iSWAP = π/(2g). With realistic parameters (γ/2π = 2 MHz, external decay Γ_ex ≈ 0.02 MHz, dephasing Γ_φ ≈ 0.05 MHz) the average process fidelity exceeds 99.7 %; increasing γ to 4 MHz pushes fidelity above 99.8 %.

2. CZ and CZ_φ – A CZ gate is realized by resonantly coupling the |11⟩ state to the higher‑excited |20⟩ (or |02⟩) manifold. This is achieved by detuning one atom’s frequency by the transmon anharmonicity χ (chosen as –ω₀/8) so that ω₂ = ω₁ + χ₁ (or vice‑versa). The coupling between |11⟩ and |20⟩ is √2 g, and the gate time is τ_CZ = π/(√2 g). Under the same decay/dephasing rates, the average fidelity reaches ≈99.4 %, improving to ≈99.7 % for stronger coupling. A generalized controlled‑phase gate CZ_φ is obtained by adding a small detuning Δ to the resonance condition, yielding a Hamiltonian H = (Δ/2)σ_z + √2 g σ_x in the {|11⟩,|20⟩} subspace. The resulting phase φ = π /