qiskit.ignis.verification.GraphDecoder

class GraphDecoder(code, S=None)

Class to construct the graph corresponding to the possible syndromes of a quantum error correction code, and then run suitable decoders.

Parameters

• code (RepitionCode) – The QEC Code object for which this decoder will be used.

• S (networkx.Graph) – Graph describing connectivity between syndrome elements. Will be generated automatically if not supplied.

Additional information:

The decoder for the supplied code is initialized by running _make_syndrome_graph(). Since this process can take some time, it is also possible to load in a premade S. However, if this was created for a differently defined code, it won’t work properly.

__init__(code, S=None)

Parameters

• code (RepitionCode) – The QEC Code object for which this decoder will be used.

• S (networkx.Graph) – Graph describing connectivity between syndrome elements. Will be generated automatically if not supplied.

Additional information:

The decoder for the supplied code is initialized by running _make_syndrome_graph(). Since this process can take some time, it is also possible to load in a premade S. However, if this was created for a differently defined code, it won’t work properly.

Methods

__init__(code[, S])	param code The QEC Code object for which this decoder
get_logical_prob(results[, algorithm])	param results A results dictionary, as produced by the
make_error_graph(string[, subgraphs])	param string A string describing the output from the code.
matching(string)	param string A string describing the output from the code.
weight_syndrome_graph(results)	Generate weighted syndrome graph from result counts.

get_logical_prob(results, algorithm='matching')

Parameters

• results (dict) – A results dictionary, as produced by the

• method of the code. (process_results) –

• algorithm (str) – Choice of which decoder to use.

Returns

Dictionary of logical error probabilities for each of the encoded logical states whose results were given in the input.

Return type

dict

make_error_graph(string, subgraphs=None)

Parameters

• string (str) – A string describing the output from the code.

• subgraphs (list) – Used when multiple, semi-independent graphs need

• to created. (need) –

Returns

The subgraph(s) of S which corresponds to the non-trivial syndrome elements in the given string.

Return type

E

matching(string)

Parameters

string (str) – A string describing the output from the code.

Returns

A string with corrected logical values,

computed using minimum weight perfect matching.

Return type

str

Additional information:

This function can be run directly, or used indirectly to calculate a logical error probability with get_logical_prob

weight_syndrome_graph(results)

Generate weighted syndrome graph from result counts.

Parameters

• results (dict) – A results dictionary, as produced by the

• method of the code. (process_results) –

Additional information:

Uses results to estimate the probability of the errors that create the pairs of nodes in S. The edge weights are then replaced with the corresponding -log(p/(1-p).