Fix merge conflict

qiskit/quantum_info/__init__.py

@@ -84,6 +84,9 @@
 
    partial_trace
    shannon_entropy
+   commutator
+   anti_commutator
+   double_commutator
 
 Random
 ======
@@ -127,45 +130,55 @@
    XXDecomposer
 """
 
-from .operators import Operator, ScalarOp, Pauli, Clifford, SparsePauliOp
-from .operators import PauliList, PauliTable, StabilizerTable, pauli_basis
-from .operators.channel import Choi, SuperOp, Kraus, Stinespring, Chi, PTM
-from .operators.measures import process_fidelity, average_gate_fidelity, gate_error, diamond_norm
-from .operators.dihedral import CNOTDihedral
-
-from .states import Statevector, DensityMatrix, StabilizerState
-from .states import (
-    partial_trace,
-    state_fidelity,
-    purity,
-    entropy,
-    concurrence,
-    entanglement_of_formation,
-    mutual_information,
-    shannon_entropy,
+from .analysis import hellinger_distance, hellinger_fidelity, Z2Symmetries
+from .operators import (
+    Clifford,
+    Operator,
+    Pauli,
+    PauliList,
+    PauliTable,
+    ScalarOp,
+    SparsePauliOp,
+    StabilizerTable,
+    anti_commutator,
+    commutator,
+    double_commutator,
+    pauli_basis,
 )
-
+from .operators.channel import PTM, Chi, Choi, Kraus, Stinespring, SuperOp
+from .operators.dihedral import CNOTDihedral
+from .operators.measures import average_gate_fidelity, diamond_norm, gate_error, process_fidelity
 from .random import (
-    random_quantum_channel,
-    random_unitary,
     random_clifford,
+    random_cnotdihedral,
+    random_density_matrix,
+    random_hermitian,
     random_pauli,
-    random_pauli_table,
     random_pauli_list,
+    random_pauli_table,
+    random_quantum_channel,
     random_stabilizer_table,
-    random_hermitian,
     random_statevector,
-    random_density_matrix,
-    random_cnotdihedral,
+    random_unitary,
+)
+from .states import (
+    DensityMatrix,
+    StabilizerState,
+    Statevector,
+    concurrence,
+    entanglement_of_formation,
+    entropy,
+    mutual_information,
+    partial_trace,
+    purity,
+    shannon_entropy,
+    state_fidelity,
 )
-
 from .synthesis import (
     OneQubitEulerDecomposer,
-    TwoQubitBasisDecomposer,
-    two_qubit_cnot_decompose,
     Quaternion,
-    decompose_clifford,
+    TwoQubitBasisDecomposer,
     XXDecomposer,
+    decompose_clifford,
+    two_qubit_cnot_decompose,
 )
-
-from .analysis import hellinger_distance, hellinger_fidelity, Z2Symmetries

qiskit/quantum_info/operators/__init__.py

@@ -14,12 +14,7 @@
 
 from .channel import PTM, Chi, Choi, Kraus, Stinespring, SuperOp
 from .dihedral import CNOTDihedral
-from .measures import (
-    average_gate_fidelity,
-    diamond_norm,
-    gate_error,
-    process_fidelity,
-)
+from .measures import average_gate_fidelity, diamond_norm, gate_error, process_fidelity
 from .operator import Operator
 from .scalar_op import ScalarOp
 from .symplectic import (
@@ -31,3 +26,4 @@
     StabilizerTable,
     pauli_basis,
 )
+from .utils import anti_commutator, commutator, double_commutator

qiskit/quantum_info/operators/utils/__init__.py

@@ -0,0 +1,19 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""
+Quantum information utility functions for operators.
+"""
+
+from .anti_commutator import anti_commutator
+from .commutator import commutator
+from .double_commutator import double_commutator

qiskit/quantum_info/operators/utils/anti_commutator.py

@@ -0,0 +1,35 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""Anti commutator function."""
+
+from typing import TypeVar
+
+from qiskit.quantum_info.operators.linear_op import LinearOp
+
+OperatorTypeT = TypeVar("OperatorTypeT", bound=LinearOp)
+
+
+def anti_commutator(a: OperatorTypeT, b: OperatorTypeT) -> OperatorTypeT:
+    r"""Compute anti-commutator of a and b.
+
+    .. math::
+
+        ab + ba.
+
+    Args:
+        a: Operator a.
+        b: Operator b.
+    Returns:
+        The anti-commutator
+    """
+    return a @ b + b @ a

qiskit/quantum_info/operators/utils/commutator.py

@@ -0,0 +1,35 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""Commutator function."""
+
+from typing import TypeVar
+
+from qiskit.quantum_info.operators.linear_op import LinearOp
+
+OperatorTypeT = TypeVar("OperatorTypeT", bound=LinearOp)
+
+
+def commutator(a: OperatorTypeT, b: OperatorTypeT) -> OperatorTypeT:
+    r"""Compute commutator of a and b.
+
+    .. math::
+
+        ab - ba.
+
+    Args:
+        a: Operator a.
+        b: Operator b.
+    Returns:
+        The commutator
+    """
+    return a @ b - b @ a

qiskit/quantum_info/operators/utils/double_commutator.py

@@ -0,0 +1,75 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""Double commutator function."""
+
+from typing import TypeVar
+
+from qiskit.quantum_info.operators.linear_op import LinearOp
+
+OperatorTypeT = TypeVar("OperatorTypeT", bound=LinearOp)
+
+
+def double_commutator(
+    a: OperatorTypeT, b: OperatorTypeT, c: OperatorTypeT, *, commutator: bool = True
+) -> OperatorTypeT:
+    r"""Compute symmetric double commutator of a, b and c.
+
+    See also Equation (13.6.18) in [1].
+
+    If `commutator` is `True`, it returns
+
+    .. math::
+
+         [[A, B], C]/2 + [A, [B, C]]/2
+         = (2ABC + 2CBA - BAC - CAB - ACB - BCA)/2.
+
+    If `commutator` is `False`, it returns
+
+    .. math::
+         \lbrace[A, B], C\rbrace/2 + \lbrace A, [B, C]\rbrace/2
+         = (2ABC - 2CBA - BAC + CAB - ACB + BCA)/2.
+
+    Args:
+        a: Operator a.
+        b: Operator b.
+        c: Operator c.
+        commutator: If ``True`` compute the double commutator,
+            if ``False`` the double anti-commutator.
+
+    Returns:
+        The double commutator
+
+    References:
+
+        [1]: R. McWeeny.
+            Methods of Molecular Quantum Mechanics.
+            2nd Edition, Academic Press, 1992.
+            ISBN 0-12-486552-6.
+    """
+    sign_num = -1 if commutator else 1
+
+    ab = a @ b
+    ba = b @ a
+    ac = a @ c
+    ca = c @ a
+
+    abc = ab @ c
+    cba = c @ ba
+    bac = ba @ c
+    cab = c @ ab
+    acb = ac @ b
+    bca = b @ ca
+
+    res = abc - sign_num * cba + 0.5 * (-bac + sign_num * cab - acb + sign_num * bca)
+
+    return res

releasenotes/notes/add-commutator-96ef07433e8ca4e7.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Add utility functions :func:`~qiskit.quantum_info.commutator`,
+    :func:`~qiskit.quantum_info.anti_commutator`, and
+    :func:`~qiskit.quantum_info.double_commutator` to compute commutators
+    for any objects implementing a :class:`.LinearOp`.

test/python/quantum_info/operators/test_utils.py

@@ -0,0 +1,58 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""Tests utility functions for operator classes."""
+
+import unittest
+
+from ddt import data, ddt, unpack
+
+from qiskit.quantum_info import SparsePauliOp, anti_commutator, commutator, double_commutator
+from qiskit.test import QiskitTestCase
+
+I = SparsePauliOp("I")
+X = SparsePauliOp("X")
+Y = SparsePauliOp("Y")
+Z = SparsePauliOp("Z")
+zero = SparsePauliOp("I", 0)
+
+
+@ddt
+class TestOperatorUtils(QiskitTestCase):
+    """Test utility functions for operator classes."""
+
+    @unpack
+    @data((Z, I, zero), (Z, X, 2j * Y))
+    def test_commutator(self, a, b, com):
+        """Test commutator function on SparsePauliOp."""
+        self.assertTrue(commutator(a, b).equiv(com))
+
+    @unpack
+    @data((Z, X, zero), (Z, I, 2 * Z))
+    def test_anti_commutator(self, a, b, com):
+        """Test anti_commutator function on SparsePauliOp."""
+        self.assertTrue(anti_commutator(a, b).equiv(com))
+
+    @unpack
+    @data(
+        (X, Y, Z, True, zero),
+        (X, Y, X, True, -4 * Y),
+        (X, Y, Z, False, 4j * I),
+        (X, Y, X, False, zero),
+    )
+    def test_double_commutator(self, a, b, c, com, expected):
+        """Test double_commutator function on SparsePauliOp."""
+        self.assertTrue(double_commutator(a, b, c, commutator=com).equiv(expected))
+
+
+if __name__ == "__main__":
+    unittest.main()