Heterogeneous quantum error-correcting codes

TL;DR

This paper introduces heterogeneous quantum error-correcting codes with distinct qubit types, demonstrating significant improvements in error thresholds and logical error rates through strategic qubit placement and bias management, supported by tensor network decoding.

Contribution

It presents the first detailed analysis of heterogeneous quantum codes with mixed qubit types, revealing optimal placement strategies and a bias-inversion phenomenon, along with an information-theoretic explanation.

Findings

Thresholds exceeding 0.4 with certain placements

Logical error rate improvements over three orders of magnitude

Bias-inversion property in logical error channels

Abstract

We introduce heterogeneous quantum error-correcting codes composed of qubit types with distinct error channels and study their performance in the code-capacity regime using maximum-likelihood tensor network decoding. In the regime where both qubit types share the same noise bias but differ in physical error rate, placing noisier qubits in the bulk -- where each error triggers more syndrome bits -- and cleaner qubits on the boundary yields thresholds exceeding 0.4 (compared to ~0.2 for the reverse placement) and improvements exceeding three orders of magnitude in logical error rate at high bias, with the advantage growing exponentially with code distance. In the regime where both types share the same error rate but differ in bias, the optimal strategy reverses: placing high-bias (more predictable) qubits on the boundary increases the threshold from 0.292(5) to 0.360(9) at a bias ratio of 100, and from 0.29(1) to 0.398(4) at a bias ratio of 1000. We also observe a striking bias-inversion property: the logical error channel becomes strongly XX X- and YY Y-biased despite the physical noise being ZZ Z-biased. We propose a stabilizer-ratio hypothesis that provides a unified information-theoretic explanation for both placement rules and predicts even larger advantages for code families such as color codes.