Curator's Take
AI Commentary
This article introduces “completion” qubits that turn non‑commuting gauge checks into a commuting set while preserving the code distance and low‑density parity‑check structure, offering a constant‑factor overhead solution to a long‑standing obstacle in fault‑tolerant logical measurement. By guaranteeing global commutation, the authors achieve a logical measurement whose circuit fault distance scales linearly with the parent code distance using only O(d) measurement rounds—a clear improvement over earlier lattice‑surgery schemes that required more complex ancilla handling or higher overhead. The result opens a practical pathway for scalable magic‑state preparation and other fault‑tolerant operations, though its current demonstration is limited to toric‑code examples and prime‑dimensional qudits, so broader code families will need further validation.
— Mark Eatherly
Summary
Gauging turns a logical operator into a product of local checks, providing a route to logical measurement and lattice surgery. These checks can fail to commute outside the code space, so measuring one can randomize the outcome of another. In this work, we introduce completion qubits that make the checks commute for Clifford $\mathbb{Z}_2$ symmetries of qubit stabilizer codes. Adding one completion qubit per noncommuting pair preserves the gauged code space and its distance while keeping the checks Clifford. Completion also preserves the low-density parity-check property with a constant-factor qubit overhead. We generalize completion to qudits and establish bounds on the Clifford-hierarchy level of the completed checks in prime dimensions. Global commutation ensures that a fault-free round fixes the syndrome until the next fault and allows us to bound fault propagation independently of the number of measurement rounds. For qubit codes, we show that gauging and ungauging a distance-$d$ parent code implements a logical measurement with circuit fault distance $Ω(d)$ in $O(d)$ rounds. We demonstrate the construction by gauging two toric codes into the $D(D_4)$ quantum double and back, with an explicit circuit whose circuit fault distance equals the parent code distance. Our results provide a framework for fault-tolerant logical measurements using commuting checks, with applications to magic-state preparation by lattice surgery.