Approximate Error Correction for Quantum Simulations of SU(2) Lattice Gauge Theories

Abstract: We present a protocol for actively suppressing Gauss law violations in quantum simulations of SU(2) lattice gauge theory. The protocol uses mid-circuit measurements to extract a characterization of the gauge-violation sector at each lattice vertex, resolving both the total angular momentum and magnetic quantum numbers of the violation via a group quantum Fourier transform. Syndrome-conditional recovery operations map the state back to the gauge-invariant subspace through an iterative sweep over vertices, a procedure we call gauge cooling. We show that while the Knill-Laflamme conditions are not generically satisfied at vertices with nontrivial singlet multiplicity, every single-qubit error is detected by the gauge syndrome. We demonstrate gauge cooling on a single-plaquette simulation of the Kogut-Susskind Hamiltonian truncated to the spin-$1/2$ representation under depolarizing and amplitude damping noise, showing that the protocol restores gauge invariance and improves fidelity at noise rates representative of current superconducting hardware.