# Mitigating Coherent Noise Using Pauli Conjugation

## TL;DR

This paper introduces Pauli conjugation as a method to improve quantum error correction by leveraging noise coherence, outperforming traditional twirling in logical fidelity and threshold improvements across various quantum codes.

## Contribution

It demonstrates that optimal Pauli conjugation can enhance logical fidelity over twirling, providing a new noise mitigation technique in quantum error correction.

## Key findings

- Optimal Pauli conjugation improves logical fidelity.
- Conjugation schemes outperform twirling in thresholds.
- Method is robust against gate errors.

## Abstract

Coherent noise can be much more damaging than incoherent (probabilistic) noise in the context of quantum error correction. One solution is to use twirling to turn coherent noise into incoherent Pauli channels. In this Article, we show that some of the coherence of the noise channel can actually be used to improve its logical fidelity by simply sandwiching the noise with a chosen pair of Pauli gates, which we call Pauli conjugation. Using the optimal Pauli conjugation, we can achieve a higher logical fidelity than using twirling and doing nothing. We devise a way to search for the optimal Pauli conjugation scheme and apply it to Steane code, 9-qubit Shor code and distance-3 surface code under global coherent $Z$ noise. The optimal conjugation schemes show improvement in logical fidelity over twirling while the weights of the conjugation gates we need to apply are lower than the average weight of the twirling gates. In our example noise and codes, the concatenated threshold obtained using conjugation is consistently higher than the twirling threshold and can be up to 1.5 times higher than the original threshold where no mitigation is applied. Our simulations show that Pauli conjugation can be robust against gate errors. With the help of logical twirling, the undesirable coherence in the noise channel can be removed and the advantages of conjugation over twirling can persist as we go to multiple rounds of quantum error correction.