---
title: LegacySolve (latest version)
description: API reference for qiskit_noise_learning.analysis.LegacySolve in the latest version of qiskit-noise-learning
source: https://quantum.cloud.ibm.com/docs/en/api/qiskit-noise-learning/generated/analysis-legacy-solve
---

# qiskit\_noise\_learning.analysis.LegacySolve

*class* `qiskit_noise_learning.analysis.LegacySolve`

[GitHub](https://github.com/Qiskit/qiskit-noise-learning/tree/stable/0.1/qiskit_noise_learning/analysis/legacy.py)

Bases: [`AnalysisStage`](/docs/api/qiskit-noise-learning/generated/analysis-analysis-stage "qiskit_noise_learning.analysis.analysis_pipeline.AnalysisStage")

Solves for the [`ModelData`](/docs/api/qiskit-noise-learning/generated/data-model-data "qiskit_noise_learning.data.ModelData") using the legacy pair-fidelity method.

This solver assumes that the gate set only has a single unitary gate, and that the paths are of a vanilla-learning type (i.e. even depth with no single-qubit Cliffords required).

Delegates to `fit_noise_model_legacy()` with `noise_assumption="symmetric_fidelities"`, `optimizer_name="nnls"`, and `constrained=True`.

### \_\_init\_\_

`__init__()`

## Methods

|                                                                                                                            |                                                                                                                                                                       |
| -------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [`__init__`](#qiskit_noise_learning.analysis.LegacySolve.__init__ "qiskit_noise_learning.analysis.LegacySolve.__init__")() |                                                                                                                                                                       |
| `run`(fit)                                                                                                                 | Run this stage, returning a new [`Fit`](/docs/api/qiskit-noise-learning/generated/analysis-fit "qiskit_noise_learning.analysis.Fit") with the output level populated. |

### input\_level

alias of [`AggregatedObservableData`](/docs/api/qiskit-noise-learning/generated/data-aggregated-observable-data "qiskit_noise_learning.data.aggregated_observable_data.AggregatedObservableData")

### output\_level

alias of [`ModelData`](/docs/api/qiskit-noise-learning/generated/data-model-data "qiskit_noise_learning.data.model_data.ModelData")