Scalable quantum simulator with an extended gate set in giant atoms

TL;DR

The paper introduces a scalable quantum simulator built from giant, three-level atoms coupled to a common waveguide, enabling an extended gate set that includes iSWAP and CZ_\varphi gates through simple frequency tuning without parametric couplers. This approach leverages decoherence-free interactions arising from multiple coupling points to achieve high-fidelity two-qubit operations and efficient open-system simulations, demonstrated via a dissipative XXZ spin-chain model. A one-dimensional scalable architecture is proposed, with a detailed protocol for implementing nearest-neighbor gates and a clear path to a two-dimensional, fault-tolerant universal quantum processor using surface-code concepts. The work also discusses practical realization in superconducting circuits, non-Markovian considerations, and provides an error-analysis framework showing the extended gate set reduces circuit depth and improves simulation accuracy.

Abstract

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.

Figures (9)

• Figure 1: A two-giant-atom setup for performing both iSWAP and CZ gates. (a) Sketch of the setup. The two giant atoms, with frequencies $\omega_{1,2}$ and detunings $\chi_{1,2}$, are coupled to the waveguide (black line) at multiple points with different coupling strengths $\gamma_{kn}$ and spacings $\Delta x_n$. The coupling points are organized in a braided fashion. (b) Frequency dependence of the individual decay rates $\Gamma_{1,2}$, inter-atomic coupling strength $g_{12}$, and collective decay rates $\Gamma_{\rm coll,12}$ of the giant atoms. (c,d) The protocol to perform (c) an iSWAP gate and (d) a CZ gate in this setup.
• Figure 2: Average process fidelity of two-qubit (a,b) iSWAP (c,d) CZ and (e) CZ$_\varphi$ gates performed with the setup in Fig. \ref{['fig1']}, as a function of qubit decay rate $\Gamma_\text{ex}$ and dephasing rate $\Gamma_\phi$. Here, $g\approx2.1\gamma$ is the qubit-qubit coupling strength used in the gates.
• Figure 3: Scalable giant-atom-based quantum simulator. (a) The architecture of the quantum simulator, where neighboring giant atoms are coupled to the waveguide in a braided configuration. (b,c) The frequency dependence of individual decay rates $\Gamma_\text{ind}$, coupling strength $g$, and collective decay rates $\Gamma_\text{coll}$ for (b) neighboring and (c) non-neighboring giant atoms.
• Figure 4: Protocol to perform different two-qubit operations on the giant-atom-based simulator. The qubits' frequencies are tuned to achieve: (a) $R_\text{XY}(\theta)$ between qubits $2k-1$ and $2k$, (b) $R_\text{XY}(\theta)$ between qubits $2k$ and $2k+1$, (c) CZ between qubits $2k-1$ and $2k$ and (d) CZ between qubits $2k$ and $2k+1$.
• Figure 5: Protocol to simulate the dynamics of the dissipative XXZ spin chain using the quantum simulator in Fig. \ref{['fig3']}. (a) The operations that need to be performed within a single Trotter step to simulate the dynamics. (b) The protocol to tune the frequencies of the giant atoms to achieve the operations in (a).