$\mathbb{Z}_2$ lattice gauge theories and Kitaev's toric code: A scheme for analog quantum simulation

TL;DR

This paper proposes a practical scheme for simulating $ ext{Z}_2$ lattice gauge theories and Kitaev's toric code using superconducting qubits, enabling the study of topological order and anyonic excitations in quantum simulators.

Contribution

It introduces a novel building block for implementing the toric code and topological excitations in superconducting qubits, with a pathway for preparing non-trivial states and detecting anyons.

Findings

Proposed a superconducting qubit implementation of the toric code.

Verified the feasibility of preparing topologically non-trivial states with a large energy gap.

Outlined experimental signatures and a braiding protocol for detecting anyonic excitations.

Abstract

Kitaev's toric code is an exactly solvable model with $\mathbb{Z}_2$-topological order, which has potential applications in quantum computation and error correction. However, a direct experimental realization remains an open challenge. Here, we propose a building block for $\mathbb{Z}_2$ lattice gauge theories coupled to dynamical matter and demonstrate how it allows for an implementation of the toric-code ground state and its topological excitations. This is achieved by introducing separate matter excitations on individual plaquettes, whose motion induce the required plaquette terms. The proposed building block is realized in the second-order coupling regime and is well suited for implementations with superconducting qubits. Furthermore, we propose a pathway to prepare topologically non-trivial initial states during which a large gap on the order of the underlying coupling strength is present. This is verified by both analytical arguments and numerical studies. Moreover, we outline experimental signatures of the ground-state wavefunction and introduce a minimal braiding protocol. Detecting a $\pi$-phase shift between Ramsey fringes in this protocol reveals the anyonic excitations of the toric-code Hamiltonian in a system with only three triangular plaquettes. Our work paves the way for realizing non-Abelian anyons in analog quantum simulators.