Changes to readout mitigator tests (#7482)

* Bugfix in local mitigator and a relevant test

* Name fix

* Refactoring mitigation tests

* Cleanup

* Linting

* Linting

* Small bugfix and a test for it

* Fix to rng usage

* Linting

* Changing test sensitivity

* Added settings property to mitigator to allow JSON encoding when given as experiment result

* Linting

* Linting

* Linting

* Release note relating to the bugfix

* Get the assignment matrices implicitly

* Using more specific assertions

* Removing magic constants

* Make initialization conform to settings

* Linting

* Removing testcase which might fail on some machines

* Removing testcase which might fail on some machines

* Linting

* Reword release note

* Reword release note

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit ff267b5)

# Conflicts:
#	test/python/result/test_mitigators.py

qiskit/result/mitigation/correlated_readout_mitigator.py

@@ -13,7 +13,7 @@
 Readout mitigator class based on the A-matrix inversion method
 """
 
-from typing import Optional, List, Tuple, Iterable, Callable, Union
+from typing import Optional, List, Tuple, Iterable, Callable, Union, Dict
 import numpy as np
 
 from qiskit.exceptions import QiskitError
@@ -33,20 +33,20 @@ class CorrelatedReadoutMitigator(BaseReadoutMitigator):
     :math:`2^N x 2^N` so the mitigation complexity is :math:`O(4^N)`.
     """
 
-    def __init__(self, amat: np.ndarray, qubits: Optional[Iterable[int]] = None):
+    def __init__(self, assignment_matrix: np.ndarray, qubits: Optional[Iterable[int]] = None):
         """Initialize a CorrelatedReadoutMitigator
 
         Args:
-            amat: readout error assignment matrix.
+            assignment_matrix: readout error assignment matrix.
             qubits: Optional, the measured physical qubits for mitigation.
 
         Raises:
             QiskitError: matrix size does not agree with number of qubits
         """
-        if np.any(amat < 0) or not np.allclose(np.sum(amat, axis=0), 1):
+        if np.any(assignment_matrix < 0) or not np.allclose(np.sum(assignment_matrix, axis=0), 1):
             raise QiskitError("Assignment matrix columns must be valid probability distributions")
-        amat = np.asarray(amat, dtype=float)
-        matrix_qubits_num = int(np.log2(amat.shape[0]))
+        assignment_matrix = np.asarray(assignment_matrix, dtype=float)
+        matrix_qubits_num = int(np.log2(assignment_matrix.shape[0]))
         if qubits is None:
             self._num_qubits = matrix_qubits_num
             self._qubits = range(self._num_qubits)
@@ -59,9 +59,14 @@ def __init__(self, amat: np.ndarray, qubits: Optional[Iterable[int]] = None):
             self._qubits = qubits
             self._num_qubits = len(self._qubits)
         self._qubit_index = dict(zip(self._qubits, range(self._num_qubits)))
-        self._assignment_mat = amat
+        self._assignment_mat = assignment_matrix
         self._mitigation_mats = {}
 
+    @property
+    def settings(self) -> Dict:
+        """Return settings."""
+        return {"assignment_matrix": self._assignment_mat, "qubits": self._qubits}
+
     def expectation_value(
         self,
         data: Counts,

qiskit/result/mitigation/local_readout_mitigator.py

@@ -14,7 +14,7 @@
 """
 
 
-from typing import Optional, List, Tuple, Iterable, Callable, Union
+from typing import Optional, List, Tuple, Iterable, Callable, Union, Dict
 import numpy as np
 
 from qiskit.exceptions import QiskitError
@@ -38,43 +38,43 @@ class LocalReadoutMitigator(BaseReadoutMitigator):
 
     def __init__(
         self,
-        amats: Optional[List[np.ndarray]] = None,
+        assignment_matrices: Optional[List[np.ndarray]] = None,
         qubits: Optional[Iterable[int]] = None,
         backend=None,
     ):
         """Initialize a LocalReadoutMitigator
 
         Args:
-            amats: Optional, list of single-qubit readout error assignment matrices.
+            assignment_matrices: Optional, list of single-qubit readout error assignment matrices.
             qubits: Optional, the measured physical qubits for mitigation.
             backend: Optional, backend name.
 
         Raises:
             QiskitError: matrices sizes do not agree with number of qubits
         """
-        if amats is None:
-            amats = self._from_backend(backend, qubits)
+        if assignment_matrices is None:
+            assignment_matrices = self._from_backend(backend, qubits)
         else:
-            amats = [np.asarray(amat, dtype=float) for amat in amats]
-        for amat in amats:
+            assignment_matrices = [np.asarray(amat, dtype=float) for amat in assignment_matrices]
+        for amat in assignment_matrices:
             if np.any(amat < 0) or not np.allclose(np.sum(amat, axis=0), 1):
                 raise QiskitError(
                     "Assignment matrix columns must be valid probability distributions"
                 )
         if qubits is None:
-            self._num_qubits = len(amats)
+            self._num_qubits = len(assignment_matrices)
             self._qubits = range(self._num_qubits)
         else:
-            if len(qubits) != len(amats):
+            if len(qubits) != len(assignment_matrices):
                 raise QiskitError(
                     "The number of given qubits ({}) is different than the number of qubits "
-                    "inferred from the matrices ({})".format(len(qubits), len(amats))
+                    "inferred from the matrices ({})".format(len(qubits), len(assignment_matrices))
                 )
             self._qubits = qubits
             self._num_qubits = len(self._qubits)
 
         self._qubit_index = dict(zip(self._qubits, range(self._num_qubits)))
-        self._assignment_mats = amats
+        self._assignment_mats = assignment_matrices
         self._mitigation_mats = np.zeros([self._num_qubits, 2, 2], dtype=float)
         self._gammas = np.zeros(self._num_qubits, dtype=float)
 
@@ -91,6 +91,11 @@ def __init__(
                 ainv = np.linalg.pinv(mat)
             self._mitigation_mats[i] = ainv
 
+    @property
+    def settings(self) -> Dict:
+        """Return settings."""
+        return {"assignment_matrices": self._assignment_mats, "qubits": self._qubits}
+
     def expectation_value(
         self,
         data: Counts,
@@ -231,8 +236,9 @@ def mitigation_matrix(self, qubits: Optional[Union[List[int], int]] = None) -> n
             qubits = self._qubits
         if isinstance(qubits, int):
             qubits = [self._qubits[qubits]]
-        mat = self._mitigation_mats[qubits[0]]
-        for i in qubits[1:]:
+        qubit_indices = [self._qubit_index[qubit] for qubit in qubits]
+        mat = self._mitigation_mats[qubit_indices[0]]
+        for i in qubit_indices[1:]:
             mat = np.kron(self._mitigation_mats[i], mat)
         return mat
 
@@ -253,9 +259,10 @@ def assignment_matrix(self, qubits: List[int] = None) -> np.ndarray:
             qubits = self._qubits
         if isinstance(qubits, int):
             qubits = [qubits]
-        mat = self._assignment_mats[qubits[0]]
-        for i in qubits[1:]:
-            mat = np.kron(self._assignment_mats[qubits[i]], mat)
+        qubit_indices = [self._qubit_index[qubit] for qubit in qubits]
+        mat = self._assignment_mats[qubit_indices[0]]
+        for i in qubit_indices[1:]:
+            mat = np.kron(self._assignment_mats[i], mat)
         return mat
 
     def _compute_gamma(self, qubits=None):

releasenotes/notes/fix_local_readout_mitigator_assignment_matrix-8bd4229a5159a7fe.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    :meth:`.LocalReadoutMitigator.assignment_matrix` will now accept lists of
+    qubits beyond the trivial ``[0, 1, 2, ..., n-1]``.