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Paper 1

Splitting decoders for correcting hypergraph faults

Nicolas Delfosse, Adam Paetznick, Jeongwan Haah, Matthew B. Hastings

Year

2023

Journal

arXiv preprint

DOI

arXiv:2309.15354

arXiv

2309.15354

The surface code is one of the most popular quantum error correction codes. It comes with efficient decoders, such as the Minimum Weight Perfect Matching (MWPM) decoder and the Union-Find (UF) decoder, allowing for fast quantum error correction. For a general linear code or stabilizer code, the decoding problem is NP-hard. What makes it tractable for the surface code is the special structure of faults and checks: Each X and Z fault triggers at most two checks. As a result, faults can be interpreted as edges in a graph whose vertices are the checks, and the decoding problem can be solved using standard graph algorithms such as Edmonds' minimum-weight perfect matching algorithm. For general codes, this decoding graph is replaced by a hypergraph making the decoding problem more challenging. In this work, we propose two heuristic algorithms for splitting the hyperedges of a decoding hypergraph into edges. After splitting, hypergraph faults can be decoded using any surface code decoder. Due to the complexity of the decoding problem, we do not expect this strategy to achieve a good error correction performance for a general code. However, we empirically show that this strategy leads to a good performance for some classes of LDPC codes because they are defined by low weight checks. We apply this splitting decoder to Floquet codes for which some faults trigger up to four checks and verify numerically that this decoder achieves the maximum code distance for two instances of Floquet codes.

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Paper 2

Qubit-oscillator concatenated codes: decoding formalism & code comparison

Yijia Xu, Yixu Wang, En-Jui Kuo, Victor V. Albert

Year

2022

Journal

arXiv preprint

DOI

arXiv:2209.04573

arXiv

2209.04573

Concatenating bosonic error-correcting codes with qubit codes can substantially boost the error-correcting power of the original qubit codes. It is not clear how to concatenate optimally, given there are several bosonic codes and concatenation schemes to choose from, including the recently discovered GKP-stabilizer codes [Phys. Rev. Lett. 125, 080503 (2020)}] that allow protection of a logical bosonic mode from fluctuations of the mode's conjugate variables. We develop efficient maximum-likelihood decoders for and analyze the performance of three different concatenations of codes taken from the following set: qubit stabilizer codes, analog/Gaussian stabilizer codes, GKP codes, and GKP-stabilizer codes. We benchmark decoder performance against additive Gaussian white noise, corroborating our numerics with analytical calculations. We observe that the concatenation involving GKP-stabilizer codes outperforms the more conventional concatenation of a qubit stabilizer code with a GKP code in some cases. We also propose a GKP-stabilizer code that suppresses fluctuations in both conjugate variables without extra quadrature squeezing, and formulate qudit versions of GKP-stabilizer codes.

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