The surface code with a twist

TL;DR

This paper introduces triangle codes, a new surface code variant with weight-four checks, fewer qubits, and efficient Clifford gate implementation via lattice surgery, enhancing fault-tolerance and simplifying quantum error correction.

Contribution

It proposes triangle codes with simplified syndrome extraction, reduced qubit overhead, and full Clifford gate implementation without state distillation, advancing topological quantum error correction.

Findings

Triangle codes have weight-four checks and high thresholds.

They use 25% fewer qubits than traditional surface codes.

They enable all Clifford gates via lattice surgery without distillation.

Abstract

The surface code is one of the most successful approaches to topological quantum error-correction. It boasts the smallest known syndrome extraction circuits and correspondingly largest thresholds. Defect-based logical encodings of a new variety called twists have made it possible to implement the full Clifford group without state distillation. Here we investigate a patch-based encoding involving a modified twist. In our modified formulation, the resulting codes, called triangle codes for the shape of their planar layout, have only weight-four checks and relatively simple syndrome extraction circuits that maintain a high, near surface-code-level threshold. They also use 25% fewer physical qubits per logical qubit than the surface code. Moreover, benefiting from the twist, we can implement all Clifford gates by lattice surgery without the need for state distillation. By a surgical transformation to the surface code, we also develop a scheme of doing all Clifford gates on surface code patches in an atypical planar layout, though with less qubit efficiency than the triangle code. Finally, we remark that logical qubits encoded in triangle codes are naturally amenable to logical tomography, and the smallest triangle code can demonstrate high-pseudothreshold fault-tolerance to depolarizing noise using just 13 physical qubits.