qiskit-documentation/docs/api/qiskit/0.26/qiskit.ignis.verification.StateTomographyFitter.mdx

---
title: StateTomographyFitter
description: API reference for qiskit.ignis.verification.StateTomographyFitter
in_page_toc_min_heading_level: 1
python_api_type: class
python_api_name: qiskit.ignis.verification.StateTomographyFitter
---

# qiskit.ignis.verification.StateTomographyFitter

<Class id="qiskit.ignis.verification.StateTomographyFitter" isDedicatedPage={true} github="https://github.com/qiskit-community/qiskit-ignis/tree/stable/0.6/qiskit/ignis/verification/tomography/fitters/state_fitter.py" signature="StateTomographyFitter(result, circuits, meas_basis='Pauli')" modifiers="class">
  Maximum-Likelihood estimation state tomography fitter.

  Initialize state tomography fitter with experimental data.

  **Parameters**

  *   **result** (`Result`) – a Qiskit Result object obtained from executing tomography circuits.
  *   **circuits** (`List`\[`QuantumCircuit`]) – a list of circuits or circuit names to extract count information from the result object.
  *   **meas\_basis** (`Union`\[`TomographyBasis`, `str`]) – (default: ‘Pauli’) A function to return measurement operators corresponding to measurement outcomes. See Additional Information (default: ‘Pauli’)

  ### \_\_init\_\_

  <Function id="qiskit.ignis.verification.StateTomographyFitter.__init__" signature="__init__(result, circuits, meas_basis='Pauli')">
    Initialize state tomography fitter with experimental data.

    **Parameters**

    *   **result** (`Result`) – a Qiskit Result object obtained from executing tomography circuits.
    *   **circuits** (`List`\[`QuantumCircuit`]) – a list of circuits or circuit names to extract count information from the result object.
    *   **meas\_basis** (`Union`\[`TomographyBasis`, `str`]) – (default: ‘Pauli’) A function to return measurement operators corresponding to measurement outcomes. See Additional Information (default: ‘Pauli’)
  </Function>

  ## Methods

  |                                                                                                                                                                                  |                                                              |
  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------ |
  | [`__init__`](#qiskit.ignis.verification.StateTomographyFitter.__init__ "qiskit.ignis.verification.StateTomographyFitter.__init__")(result, circuits\[, meas\_basis])             | Initialize state tomography fitter with experimental data.   |
  | [`add_data`](#qiskit.ignis.verification.StateTomographyFitter.add_data "qiskit.ignis.verification.StateTomographyFitter.add_data")(results, circuits)                            | Add tomography data from a Qiskit Result object.             |
  | [`fit`](#qiskit.ignis.verification.StateTomographyFitter.fit "qiskit.ignis.verification.StateTomographyFitter.fit")(\[method, standard\_weights, beta])                          | Reconstruct a quantum state using CVXPY convex optimization. |
  | [`set_measure_basis`](#qiskit.ignis.verification.StateTomographyFitter.set_measure_basis "qiskit.ignis.verification.StateTomographyFitter.set_measure_basis")(basis)             | Set the measurement basis                                    |
  | [`set_preparation_basis`](#qiskit.ignis.verification.StateTomographyFitter.set_preparation_basis "qiskit.ignis.verification.StateTomographyFitter.set_preparation_basis")(basis) | Set the preparation basis function                           |

  ## Attributes

  |                                                                                                                                                               |                                          |
  | ------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------- |
  | [`data`](#qiskit.ignis.verification.StateTomographyFitter.data "qiskit.ignis.verification.StateTomographyFitter.data")                                        | Return tomography data                   |
  | [`measure_basis`](#qiskit.ignis.verification.StateTomographyFitter.measure_basis "qiskit.ignis.verification.StateTomographyFitter.measure_basis")             | Return the tomography measurement basis. |
  | [`preparation_basis`](#qiskit.ignis.verification.StateTomographyFitter.preparation_basis "qiskit.ignis.verification.StateTomographyFitter.preparation_basis") | Return the tomography preparation basis. |

  ### add\_data

  <Function id="qiskit.ignis.verification.StateTomographyFitter.add_data" signature="add_data(results, circuits)">
    Add tomography data from a Qiskit Result object.

    **Parameters**

    *   **results** (`List`\[`Result`]) – The results obtained from executing tomography circuits.
    *   **circuits** (`List`\[`Union`\[`QuantumCircuit`, `str`]]) – circuits or circuit names to extract count information from the result object.

    **Raises**

    **QiskitError** – In case some of the tomography data is not found in the results
  </Function>

  ### data

  <Attribute id="qiskit.ignis.verification.StateTomographyFitter.data">
    Return tomography data
  </Attribute>

  ### fit

  <Function id="qiskit.ignis.verification.StateTomographyFitter.fit" signature="fit(method='auto', standard_weights=True, beta=0.5, **kwargs)">
    Reconstruct a quantum state using CVXPY convex optimization.

    **Fitter method**

    The `cvx` fitter method used CVXPY convex optimization package. The `lstsq` method uses least-squares fitting (linear inversion). The `auto` method will use ‘cvx’ if the CVXPY package is found on the system, otherwise it will default to ‘lstsq’.

    **Objective function**

    This fitter solves the constrained least-squares minimization: $minimize: ||a \cdot x - b ||_2$

    subject to:

    > *   $x >> 0$
    > *   $\text{trace}(x) = 1$

    where:

    > *   a is the matrix of measurement operators $a[i] = \text{vec}(M_i).H$
    > *   b is the vector of expectation value data for each projector $b[i] \sim \text{Tr}[M_i.H \cdot x] = (a \cdot x)[i]$
    > *   x is the vectorized density matrix to be fitted

    **PSD constraint**

    The PSD keyword constrains the fitted matrix to be postive-semidefinite. For the `lstsq` fitter method the fitted matrix is rescaled using the method proposed in Reference \[1]. For the `cvx` fitter method the convex constraint makes the optimization problem a SDP. If PSD=False the fitted matrix will still be constrained to be Hermitian, but not PSD. In this case the optimization problem becomes a SOCP.

    **Trace constraint**

    The trace keyword constrains the trace of the fitted matrix. If trace=None there will be no trace constraint on the fitted matrix. This constraint should not be used for process tomography and the trace preserving constraint should be used instead.

    **CVXPY Solvers:**

    Various solvers can be called in CVXPY using the solver keyword argument. See the [CVXPY documentation](https://www.cvxpy.org/tutorial/advanced/index.html#solve-method-options) for more information on solvers.

    References:

    **\[1] J Smolin, JM Gambetta, G Smith, Phys. Rev. Lett. 108, 070502**

    (2012). Open access: arXiv:1106.5458 \[quant-ph].

    **Parameters**

    *   **method** (`str`) – The fitter method ‘auto’, ‘cvx’ or ‘lstsq’.
    *   **standard\_weights** (`bool`) – (default: True) Apply weights to tomography data based on count probability
    *   **beta** (`float`) – (default: 0.5) hedging parameter for converting counts to probabilities
    *   **\*\*kwargs** – kwargs for fitter method.

    **Raises**

    **QiskitError** – In case the fitting method is unrecognized.

    **Return type**

    `array`

    **Returns**

    The fitted matrix rho that minimizes $||\text{basis_matrix} \cdot \text{vec}(\text{rho}) - \text{data}||_2$.
  </Function>

  ### measure\_basis

  <Attribute id="qiskit.ignis.verification.StateTomographyFitter.measure_basis">
    Return the tomography measurement basis.
  </Attribute>

  ### preparation\_basis

  <Attribute id="qiskit.ignis.verification.StateTomographyFitter.preparation_basis">
    Return the tomography preparation basis.
  </Attribute>

  ### set\_measure\_basis

  <Function id="qiskit.ignis.verification.StateTomographyFitter.set_measure_basis" signature="set_measure_basis(basis)">
    Set the measurement basis

    **Parameters**

    **basis** (`Union`\[`TomographyBasis`, `str`]) – measurement basis

    **Raises**

    **QiskitError** – In case of invalid measurement or preparation basis.
  </Function>

  ### set\_preparation\_basis

  <Function id="qiskit.ignis.verification.StateTomographyFitter.set_preparation_basis" signature="set_preparation_basis(basis)">
    Set the preparation basis function

    **Parameters**

    **basis** (`Union`\[`TomographyBasis`, `str`]) – preparation basis

    **Raises**

    **QiskitError** – in case the basis has no preperation data
  </Function>
</Class>