## Description

Also called a gauge stabilizer code. Modular-qudit generalization of a subsystem qubit stabilizer code. Can be obtained by taking a modular qudit stabilizer code and assigning some of its logical qubits to be gauge qubits. For composite qudit dimensions, such codes need not encode an integer number of qudits.

Subsystem stabilizer codes are defined by a gauge group \(\mathsf{G}\) and a stabilizer group \(\mathsf{S}\), both subgroups of the \(n\)-qudit Pauli group \(\mathsf{P}_n\) that satisfy \(\mathsf{Z}(\mathsf{G})=\mathsf{S}\), where \(\mathsf{Z}\) denotes taking the center of a group.

A code can be constructed by starting with either group. Given an \(\mathsf{S}\), one can pick any \(\mathsf{G}\) satisfying \(\mathsf{S}\subseteq\mathsf{G}\subseteq\mathsf{N(S)}\), where \(\mathsf{N(S)}\) is the normalizer of the stabilizer group within \(\mathsf{P}_n\). Alternatively, given a \(\mathsf{G}\), one defines \(\mathsf{S}\) to be the center of the gauge group.

The logical Pauli group is \(\mathsf{N(G)/S}\). As such, the case when \(\mathsf{G}=\mathsf{S}\) reduces to an ordinary stabilizer code, while the case \(\mathsf{G}=\mathsf{N(S)}\) reduces to a trivial code.

One can gauge fix [1] an Abelian subgroup of the gauge group by adding it to the stabilizer group.

Gauge fixing: Gauge fixing is a map between subsystem codes that is done using an abelian subgroup \(\mathsf{F}\subseteq\mathsf{G}\), \begin{align} \begin{split} \mathsf{S}&\to\left\langle \mathsf{S},\mathsf{F}\right\rangle \\ \mathsf{G}&\to\mathsf{N}_{\mathsf{G}}\left(\mathsf{F}\right)~, \end{split} \tag*{(1)}\end{align} where \(\mathsf{N}_{\mathsf{G}}\left(\mathsf{F}\right)\) is the normalizer of the stabilizer group within \(\mathsf{G}\).

Gauge fixing can be used to switch between different stabilizer codes that yield different gauge sets in a process known as gauge switching. Gauge fixing also encompasses lattice surgery and code deformation [2].

One can also gauge out a subgroup \(\mathsf{F}\) of the Pauli group by adding it to the gauge group.

Gauging out: Gauging out is a map between subsystem codes that is done using a subgroup \(\mathsf{F}\subseteq\mathsf{P}_n\), \begin{align} \begin{split} \mathsf{S}&\to\mathsf{Z}\left(\left\langle \mathsf{G},\mathsf{F}\right\rangle \right)\\ \mathsf{G}&\to\left\langle \mathsf{G},\mathsf{F}\right\rangle ~. \end{split} \tag*{(2)}\end{align} When \(\mathsf{F}\) is a subgroup of the logical Pauli group, this is also called gauging. If \(\mathsf{F}\) is itself a Pauli group of \(m\) logical qubits of the original subsystem code, then gauging those qubits is equivalent to treating them as gauge qubits.

## Decoding

## Parents

- Modular-qudit stabilizer code — Subsystem modular-qudit stabilizer codes reduce to modular-qudit stabilizer codes when there are no gauge qudits.
- Subsystem quantum error-correcting code

## Children

- Subsystem qubit stabilizer code — Subsystem modular-qudit stabilizer codes reduce to subsystem qubit stabilizer codes for qudit dimension \(q=2\).
- Abelian topological code — All Abelian topological orders can be realized as modular-qudit subsystem stabilizer codes by starting with an abelian quantum double model (slightly different from that of Ref. [4]) along with a family of Abelian TQDs that generalize the double semion anyon theory and gauging out certain bosonic anyons [5]. The stabilizer generators of the new subsystem code may no longer be geometrically local.

## References

- [1]
- D. Poulin, “Stabilizer Formalism for Operator Quantum Error Correction”, Physical Review Letters 95, (2005) arXiv:quant-ph/0508131 DOI
- [2]
- C. Vuillot et al., “Code deformation and lattice surgery are gauge fixing”, New Journal of Physics 21, 033028 (2019) arXiv:1810.10037 DOI
- [3]
- M. Suchara, S. Bravyi, and B. Terhal, “Constructions and noise threshold of topological subsystem codes”, Journal of Physics A: Mathematical and Theoretical 44, 155301 (2011) arXiv:1012.0425 DOI
- [4]
- T. D. Ellison et al., “Pauli Stabilizer Models of Twisted Quantum Doubles”, PRX Quantum 3, (2022) arXiv:2112.11394 DOI
- [5]
- T. D. Ellison et al., “Pauli topological subsystem codes from Abelian anyon theories”, (2022) arXiv:2211.03798

## Page edit log

- Victor V. Albert (2022-11-09) — most recent

## Cite as:

“Subsystem modular-qudit stabilizer code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/qudit_subsystem_stabilizer