Migrate measure mitigation from ignis for QuantumInstance

docs/apidocs/terra.rst

@@ -33,5 +33,6 @@ Qiskit Terra API Reference
    transpiler_preset
    transpiler_plugins
    utils
+   utils_mitigation
    opflow
    algorithms

docs/apidocs/utils_mitigation.rst

@@ -0,0 +1,6 @@
+.. _qiskit-utils-mitigation:
+
+.. automodule:: qiskit.utils.mitigation
+   :no-members:
+   :no-inherited-members:
+   :no-special-members:

qiskit/utils/measurement_error_mitigation.py

@@ -14,12 +14,19 @@
 
 import copy
 from typing import List, Optional, Tuple, Dict, Callable
+
 from qiskit import compiler
 from qiskit.providers import BaseBackend
 from qiskit.circuit import QuantumCircuit
 from qiskit.qobj import QasmQobj
 from qiskit.assembler.run_config import RunConfig
-from ..exceptions import QiskitError, MissingOptionalLibraryError
+from qiskit.exceptions import QiskitError
+from qiskit.utils.mitigation import (
+    complete_meas_cal,
+    tensored_meas_cal,
+    CompleteMeasFitter,
+    TensoredMeasFitter,
+)
 
 # pylint: disable=invalid-name
 
@@ -135,37 +142,43 @@ def build_measurement_error_mitigation_circuits(
         the labels of the calibration circuits
     Raises:
         QiskitError: when the fitter_cls is not recognizable.
-        MissingOptionalLibraryError: Qiskit-Ignis not installed
     """
-    try:
-        from qiskit.ignis.mitigation.measurement import (
-            complete_meas_cal,
-            tensored_meas_cal,
-            CompleteMeasFitter,
-            TensoredMeasFitter,
-        )
-    except ImportError as ex:
-        raise MissingOptionalLibraryError(
-            libname="qiskit-ignis",
-            name="build_measurement_error_mitigation_qobj",
-            pip_install="pip install qiskit-ignis",
-        ) from ex
-
     circlabel = "mcal"
 
     if not qubit_list:
         raise QiskitError("The measured qubit list can not be [].")
 
+    run = False
     if fitter_cls == CompleteMeasFitter:
         meas_calibs_circuits, state_labels = complete_meas_cal(
             qubit_list=range(len(qubit_list)), circlabel=circlabel
         )
+        run = True
     elif fitter_cls == TensoredMeasFitter:
         meas_calibs_circuits, state_labels = tensored_meas_cal(
             mit_pattern=mit_pattern, circlabel=circlabel
         )
-    else:
-        raise QiskitError(f"Unknown fitter {fitter_cls}")
+        run = True
+    if not run:
+        try:
+            from qiskit.ignis.mitigation.measurement import (
+                CompleteMeasFitter as CompleteMeasFitter_IG,
+                TensoredMeasFitter as TensoredMeasFitter_IG,
+            )
+        except ImportError as ex:
+            # If ignis can't be imported we don't have a valid fitter
+            # class so just fail here with an appropriate error message
+            raise QiskitError(f"Unknown fitter {fitter_cls}") from ex
+        if fitter_cls == CompleteMeasFitter_IG:
+            meas_calibs_circuits, state_labels = complete_meas_cal(
+                qubit_list=range(len(qubit_list)), circlabel=circlabel
+            )
+        elif fitter_cls == TensoredMeasFitter_IG:
+            meas_calibs_circuits, state_labels = tensored_meas_cal(
+                mit_pattern=mit_pattern, circlabel=circlabel
+            )
+        else:
+            raise QiskitError(f"Unknown fitter {fitter_cls}")
 
     # the provided `qubit_list` would be used as the initial layout to
     # assure the consistent qubit mapping used in the main circuits.
@@ -209,19 +222,6 @@ def build_measurement_error_mitigation_qobj(
         QiskitError: when the fitter_cls is not recognizable.
         MissingOptionalLibraryError: Qiskit-Ignis not installed
     """
-    try:
-        from qiskit.ignis.mitigation.measurement import (
-            complete_meas_cal,
-            tensored_meas_cal,
-            CompleteMeasFitter,
-            TensoredMeasFitter,
-        )
-    except ImportError as ex:
-        raise MissingOptionalLibraryError(
-            libname="qiskit-ignis",
-            name="build_measurement_error_mitigation_qobj",
-            pip_install="pip install qiskit-ignis",
-        ) from ex
 
     circlabel = "mcal"
 

qiskit/utils/mitigation/__init__.py

@@ -0,0 +1,53 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2019.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+# This code was originally copied from the qiskit-ignis repsoitory see:
+# https://github.com/Qiskit/qiskit-ignis/blob/b91066c72171bcd55a70e6e8993b813ec763cf41/qiskit/ignis/mitigation/measurement/__init__.py
+# it was migrated as qiskit-ignis is being deprecated
+
+"""
+=============================================================
+Measurement Mitigation Utils (:mod:`qiskit.utils.mitigation`)
+=============================================================
+
+.. currentmodule:: qiskit.utils.mitigation
+
+.. warning::
+
+    The user-facing API stability of this module is not guaranteed except for
+    its use with the :class:`~qiskit.utils.QuantumInstance` (i.e. using the
+    :class:`~qiskit.utils.mitigation.CompleteMeasFitter` or
+    :class:`~qiskit.utils.mitigation.TensoredMeasFitter` classes as values for the
+    ``meas_error_mitigation_cls``). The rest of this module should be treated as
+    an internal private API that can not be relied upon.
+
+Measurement correction
+======================
+
+The measurement calibration is used to mitigate measurement errors.
+The main idea is to prepare all :math:`2^n` basis input states and compute
+the probability of measuring counts in the other basis states.
+From these calibrations, it is possible to correct the average results
+of another experiment of interest. These tools are intended for use solely
+with the :class:`~qiskit.utils.QuantumInstance` class as part of
+:mod:`qiskit.algorithms` and :mod:`qiskit.opflow`.
+
+.. autosummary::
+   :toctree: ../stubs/
+
+   CompleteMeasFitter
+   TensoredMeasFitter
+"""
+
+# Measurement correction functions
+from .circuits import complete_meas_cal, tensored_meas_cal
+from .fitters import CompleteMeasFitter, TensoredMeasFitter