Operation abstract base class

qiskit/circuit/__init__.py

@@ -234,6 +234,7 @@
 from .controlledgate import ControlledGate
 from .instruction import Instruction
 from .instructionset import InstructionSet
+from .operation import Operation
 from .barrier import Barrier
 from .delay import Delay
 from .measure import Measure

qiskit/circuit/barrier.py

@@ -14,9 +14,10 @@
 
 from qiskit.exceptions import QiskitError
 from .instruction import Instruction
+from .operation import Operation
 
 
-class Barrier(Instruction):
+class Barrier(Instruction, Operation):
     """Barrier instruction."""
 
     _directive = True

qiskit/circuit/gate.py

@@ -20,9 +20,10 @@
 from qiskit.circuit.parameterexpression import ParameterExpression
 from qiskit.circuit.exceptions import CircuitError
 from .instruction import Instruction
+from .operation import Operation
 
 
-class Gate(Instruction):
+class Gate(Instruction, Operation):
     """Unitary gate."""
 
     def __init__(

qiskit/circuit/instruction.py

@@ -76,9 +76,9 @@ def __init__(self, name, num_qubits, num_clbits, params, duration=None, unit="dt
             raise CircuitError(
                 "bad instruction dimensions: %d qubits, %d clbits." % num_qubits, num_clbits
             )
-        self.name = name
-        self.num_qubits = num_qubits
-        self.num_clbits = num_clbits
+        self._name = name
+        self._num_qubits = num_qubits
+        self._num_clbits = num_clbits
 
         self._params = []  # a list of gate params stored
         # Custom instruction label
@@ -546,3 +546,33 @@ def condition_bits(self) -> List[Clbit]:
             return [self.condition[0]]
         else:  # ClassicalRegister
             return list(self.condition[0])
+
+    @property
+    def name(self):
+        """Return the name."""
+        return self._name
+
+    @name.setter
+    def name(self, name):
+        """Set the name."""
+        self._name = name
+
+    @property
+    def num_qubits(self):
+        """Return the number of qubits."""
+        return self._num_qubits
+
+    @num_qubits.setter
+    def num_qubits(self, num_qubits):
+        """Set num_qubits."""
+        self._num_qubits = num_qubits
+
+    @property
+    def num_clbits(self):
+        """Return the number of clbits."""
+        return self._num_clbits
+
+    @num_clbits.setter
+    def num_clbits(self, num_clbits):
+        """Set num_clbits."""
+        self._num_clbits = num_clbits

qiskit/circuit/measure.py

@@ -17,10 +17,11 @@
 import warnings
 
 from qiskit.circuit.instruction import Instruction
+from qiskit.circuit.operation import Operation
 from qiskit.circuit.exceptions import CircuitError
 
 
-class Measure(Instruction):
+class Measure(Instruction, Operation):
     """Quantum measurement in the computational basis."""
 
     def __init__(self):

qiskit/circuit/operation.py

@@ -0,0 +1,61 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2017.
+#
+# 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 Operation Mixin."""
+
+from abc import ABC, abstractmethod
+
+
+class Operation(ABC):
+    """Quantum Operation Interface Class.
+    For objects that can be added to a :class:`~qiskit.circuit.QuantumCircuit`.
+    These objects include :class:`~qiskit.circuit.Gate`, :class:`~qiskit.circuit.Reset`,
+    :class:`~qiskit.circuit.Barrier`, :class:`~qiskit.circuit.Measure`,
+    and operators such as :class:`~qiskit.quantum_info.Clifford`.
+    The main purpose is to add an :class:`~qiskit.circuit.Operation` to a
+    :class:`~qiskit.circuit.QuantumCircuit` without synthesizing it before the transpilation.
+
+    Example:
+
+        Add a Clifford and a Toffoli gate to a QuantumCircuit.
+
+        .. jupyter-execute::
+
+            from qiskit import QuantumCircuit
+            from qiskit.quantum_info import Clifford, random_clifford
+
+            qc = QuantumCircuit(3)
+            cliff = random_clifford(2)
+            qc.append(cliff, [0, 1])
+            qc.ccx(0, 1, 2)
+            qc.draw()
+    """
+
+    __slots__ = ()
+
+    @property
+    @abstractmethod
+    def name(self):
+        """Unique string identifier for operation type."""
+        raise NotImplementedError
+
+    @property
+    @abstractmethod
+    def num_qubits(self):
+        """Number of qubits."""
+        raise NotImplementedError
+
+    @property
+    @abstractmethod
+    def num_clbits(self):
+        """Number of classical bits."""
+        raise NotImplementedError

qiskit/circuit/reset.py

@@ -17,9 +17,10 @@
 import warnings
 
 from qiskit.circuit.instruction import Instruction
+from qiskit.circuit.operation import Operation
 
 
-class Reset(Instruction):
+class Reset(Instruction, Operation):
     """Qubit reset."""
 
     def __init__(self):

qiskit/extensions/quantum_initializer/initializer.py

@@ -20,7 +20,7 @@
 from qiskit.exceptions import QiskitError
 from qiskit.circuit import QuantumCircuit
 from qiskit.circuit import QuantumRegister
-from qiskit.circuit import Instruction
+from qiskit.circuit import Instruction, Operation
 from qiskit.circuit.exceptions import CircuitError
 from qiskit.circuit.library.standard_gates.x import CXGate, XGate
 from qiskit.circuit.library.standard_gates.h import HGate
@@ -32,7 +32,7 @@
 _EPS = 1e-10  # global variable used to chop very small numbers to zero
 
 
-class Initialize(Instruction):
+class Initialize(Instruction, Operation):
     """Complex amplitude initialization.
 
     Class that implements the (complex amplitude) initialization of some

qiskit/extensions/quantum_initializer/isometry.py

@@ -25,6 +25,7 @@
 
 from qiskit.circuit.exceptions import CircuitError
 from qiskit.circuit.instruction import Instruction
+from qiskit.circuit.operation import Operation
 from qiskit.circuit.quantumcircuit import QuantumCircuit
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.exceptions import QiskitError
@@ -35,7 +36,7 @@
 _EPS = 1e-10  # global variable used to chop very small numbers to zero
 
 
-class Isometry(Instruction):
+class Isometry(Instruction, Operation):
     """
     Decomposition of arbitrary isometries from m to n qubits. In particular, this allows to
     decompose unitaries (m=n) and to do state preparation (m=0).

qiskit/quantum_info/operators/dihedral/dihedral.py

@@ -23,12 +23,12 @@
 from qiskit.quantum_info.operators.scalar_op import ScalarOp
 from qiskit.quantum_info.synthesis.cnotdihedral_decompose import decompose_cnotdihedral
 from qiskit.quantum_info.operators.mixins import generate_apidocs, AdjointMixin
-from qiskit.circuit import QuantumCircuit, Instruction
+from qiskit.circuit import QuantumCircuit, Instruction, Operation
 from .dihedral_circuits import _append_circuit
 from .polynomial import SpecialPolynomial
 
 
-class CNOTDihedral(BaseOperator, AdjointMixin):
+class CNOTDihedral(BaseOperator, AdjointMixin, Operation):
     """An N-qubit operator from the CNOT-Dihedral group.
 
      The CNOT-Dihedral group is generated by the quantum gates,
@@ -160,6 +160,16 @@ def __init__(self, data=None, num_qubits=None, validate=True):
         if validate and not self._is_valid():
             raise QiskitError("Invalid CNOTDihedral element.")
 
+    @property
+    def name(self):
+        """Unique string identifier for operation type."""
+        return "cnotdihedral"
+
+    @property
+    def num_clbits(self):
+        """Number of classical bits."""
+        return 0
+
     def _z2matmul(self, left, right):
         """Compute product of two n x n z2 matrices."""
         prod = np.mod(np.dot(left, right), 2)

qiskit/quantum_info/operators/symplectic/clifford.py

@@ -16,7 +16,7 @@
 import numpy as np
 
 from qiskit.exceptions import QiskitError
-from qiskit.circuit import QuantumCircuit, Instruction
+from qiskit.circuit import QuantumCircuit, Instruction, Operation
 from qiskit.circuit.library.standard_gates import IGate, XGate, YGate, ZGate, HGate, SGate
 from qiskit.quantum_info.operators.base_operator import BaseOperator
 from qiskit.quantum_info.operators.operator import Operator
@@ -27,7 +27,7 @@
 from .clifford_circuits import _append_circuit
 
 
-class Clifford(BaseOperator, AdjointMixin):
+class Clifford(BaseOperator, AdjointMixin, Operation):
     """An N-qubit unitary operator from the Clifford group.
 
     **Representation**
@@ -137,6 +137,21 @@ def __init__(self, data, validate=True):
         # Initialize BaseOperator
         super().__init__(num_qubits=self._table.num_qubits)
 
+    @property
+    def name(self):
+        """Unique string identifier for operation type."""
+        return "clifford"
+
+    @property
+    def num_qubits(self):
+        """Number of qubits."""
+        return self._table.num_qubits
+
+    @property
+    def num_clbits(self):
+        """Number of classical bits."""
+        return 0
+
     def __repr__(self):
         return f"Clifford({repr(self.table)})"
 

qiskit/quantum_info/operators/symplectic/pauli.py

@@ -20,7 +20,7 @@
 
 import numpy as np
 
-from qiskit.circuit import Instruction, QuantumCircuit
+from qiskit.circuit import Instruction, QuantumCircuit, Operation
 from qiskit.circuit.barrier import Barrier
 from qiskit.circuit.library.generalized_gates import PauliGate
 from qiskit.circuit.library.standard_gates import IGate, XGate, YGate, ZGate
@@ -31,7 +31,7 @@
 from qiskit.utils.deprecation import deprecate_function
 
 
-class Pauli(BasePauli):
+class Pauli(BasePauli, Operation):
     r"""N-qubit Pauli operator.
 
     This class represents an operator :math:`P` from the full :math:`n`-qubit
@@ -202,6 +202,16 @@ def __init__(self, data=None, x=None, *, z=None, label=None):
             raise QiskitError("Input is not a single Pauli")
         super().__init__(base_z, base_x, base_phase)
 
+    @property
+    def name(self):
+        """Unique string identifier for operation type."""
+        return "pauli"
+
+    @property
+    def num_clbits(self):
+        """Number of classical bits."""
+        return 0
+
     def __repr__(self):
         """Display representation."""
         return f"Pauli('{self.__str__()}')"

releasenotes/notes/Operation-abstract-base-class-99f46180c7ddaa6b.yaml

@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    A new :class:`~qiskit.circuit.Operation` base class for objects
+    that can be added to a :class:`~qiskit.circuit.QuantumCircuit`.
+    These objects include :class:`~qiskit.circuit.Gate`, :class:`~qiskit.circuit.Reset`,
+    :class:`~qiskit.circuit.Barrier`, :class:`~qiskit.circuit.Measure`,
+    and operators such as :class:`~qiskit.quantum_info.Clifford`.
+    The main purpose is to add an :class:`~qiskit.circuit.Operation` to a
+    :class:`~qiskit.circuit.QuantumCircuit` without synthesizing it before the transpilation.