Use singletons for standard library unparameterized, non-controlled gates

qiskit/circuit/__init__.py

@@ -279,6 +279,7 @@
    InstructionSet
    Operation
    EquivalenceLibrary
+   SingletonGate
 
 Control Flow Operations
 -----------------------
@@ -366,6 +367,7 @@
 
 # pylint: disable=cyclic-import
 from .controlledgate import ControlledGate
+from .singleton_gate import SingletonGate
 from .instruction import Instruction
 from .instructionset import InstructionSet
 from .operation import Operation

qiskit/circuit/add_control.py

@@ -55,7 +55,9 @@ def add_control(
         # attempt decomposition
         operation._define()
     cgate = control(operation, num_ctrl_qubits=num_ctrl_qubits, label=label, ctrl_state=ctrl_state)
-    cgate.base_gate.label = operation.label
+    if operation.label is not None:
+        cgate.base_gate = cgate.base_gate.to_mutable()
+        cgate.base_gate.label = operation.label
     return cgate
 
 

qiskit/circuit/gate.py

@@ -24,7 +24,15 @@
 class Gate(Instruction):
     """Unitary gate."""
 
-    def __init__(self, name: str, num_qubits: int, params: list, label: str | None = None) -> None:
+    def __init__(
+        self,
+        name: str,
+        num_qubits: int,
+        params: list,
+        label: str | None = None,
+        duration=None,
+        unit="dt",
+    ) -> None:
         """Create a new gate.
 
         Args:
@@ -34,7 +42,7 @@ def __init__(self, name: str, num_qubits: int, params: list, label: str | None =
             label: An optional label for the gate.
         """
         self.definition = None
-        super().__init__(name, num_qubits, 0, params, label=label)
+        super().__init__(name, num_qubits, 0, params, label=label, duration=duration, unit=unit)
 
     # Set higher priority than Numpy array and matrix classes
     __array_priority__ = 20

qiskit/circuit/instruction.py

@@ -94,16 +94,43 @@ def __init__(self, name, num_qubits, num_clbits, params, duration=None, unit="dt
             self._label = label
         # tuple (ClassicalRegister, int), tuple (Clbit, bool) or tuple (Clbit, int)
         # when the instruction has a conditional ("if")
-        self.condition = None
+        self._condition = None
         # list of instructions (and their contexts) that this instruction is composed of
         # empty definition means opaque or fundamental instruction
         self._definition = None
-
         self._duration = duration
         self._unit = unit
 
         self.params = params  # must be at last (other properties may be required for validation)
 
+    @property
+    def mutable(self) -> bool:
+        """Is this instance is a mutable unique instance or not.
+
+        If this attribute is ``False`` the gate instance is a shared singleton
+        and is not mutable.
+        """
+        return True
+
+    def to_mutable(self):
+        """Return a mutable copy of this gate.
+
+        This method will return a new mutable copy of this gate instance.
+        If a singleton instance is being used this will be a new unique
+        instance that can be mutated. If the instance is already mutable it
+        will be a deepcopy of that instance.
+        """
+        return self.copy()
+
+    @property
+    def condition(self):
+        """The classical condition on the instruction."""
+        return self._condition
+
+    @condition.setter
+    def condition(self, condition):
+        self._condition = condition
+
     def __eq__(self, other):
         """Two instructions are the same if they have the same name,
         same dimensions, and same params.
@@ -409,7 +436,7 @@ def c_if(self, classical, val):
             # Casting the conditional value as Boolean when
             # the classical condition is on a classical bit.
             val = bool(val)
-        self.condition = (classical, val)
+        self._condition = (classical, val)
         return self
 
     def copy(self, name=None):

qiskit/circuit/instructionset.py

@@ -132,7 +132,7 @@ def c_if(self, classical: Clbit | ClassicalRegister | int, val: int) -> "Instruc
         if self._requester is not None:
             classical = self._requester(classical)
         for instruction in self._instructions:
-            instruction.operation.c_if(classical, val)
+            instruction.operation = instruction.operation.c_if(classical, val)
         return self
 
     # Legacy support for properties.  Added in Terra 0.21 to support the internal switch in

qiskit/circuit/library/standard_gates/dcx.py

@@ -12,13 +12,13 @@
 
 """Double-CNOT gate."""
 
-from qiskit.circuit.gate import Gate
+from qiskit.circuit.singleton_gate import SingletonGate
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array
 
 
 @with_gate_array([[1, 0, 0, 0], [0, 0, 0, 1], [0, 1, 0, 0], [0, 0, 1, 0]])
-class DCXGate(Gate):
+class DCXGate(SingletonGate):
     r"""Double-CNOT gate.
 
     A 2-qubit Clifford gate consisting of two back-to-back
@@ -48,9 +48,14 @@ class DCXGate(Gate):
             \end{pmatrix}
     """
 
-    def __init__(self):
+    def __init__(self, label=None, duration=None, unit=None, _condition=None):
         """Create new DCX gate."""
-        super().__init__("dcx", 2, [])
+        if unit is None:
+            unit = "dt"
+
+        super().__init__(
+            "dcx", 2, [], label=label, _condition=_condition, duration=duration, unit=unit
+        )
 
     def _define(self):
         """

qiskit/circuit/library/standard_gates/ecr.py

@@ -15,16 +15,16 @@
 import numpy as np
 
 from qiskit.circuit._utils import with_gate_array
-from qiskit.circuit.gate import Gate
 from qiskit.circuit.quantumregister import QuantumRegister
+from qiskit.circuit.singleton_gate import SingletonGate
 from .rzx import RZXGate
 from .x import XGate
 
 
 @with_gate_array(
     sqrt(0.5) * np.array([[0, 1, 0, 1.0j], [1, 0, -1.0j, 0], [0, 1.0j, 0, 1], [-1.0j, 0, 1, 0]])
 )
-class ECRGate(Gate):
+class ECRGate(SingletonGate):
     r"""An echoed cross-resonance gate.
 
     This gate is maximally entangling and is equivalent to a CNOT up to
@@ -84,9 +84,13 @@ class ECRGate(Gate):
                 \end{pmatrix}
     """
 
-    def __init__(self):
+    def __init__(self, label=None, _condition=None, duration=None, unit=None):
         """Create new ECR gate."""
-        super().__init__("ecr", 2, [])
+        if unit is None:
+            unit = "dt"
+        super().__init__(
+            "ecr", 2, [], label=label, _condition=_condition, duration=duration, unit=unit
+        )
 
     def _define(self):
         """

qiskit/circuit/library/standard_gates/h.py

@@ -15,7 +15,7 @@
 from typing import Optional, Union
 import numpy
 from qiskit.circuit.controlledgate import ControlledGate
-from qiskit.circuit.gate import Gate
+from qiskit.circuit.singleton_gate import SingletonGate
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
 from .t import TGate, TdgGate
@@ -25,7 +25,7 @@
 
 
 @with_gate_array(_H_ARRAY)
-class HGate(Gate):
+class HGate(SingletonGate):
     r"""Single-qubit Hadamard gate.
 
     This gate is a \pi rotation about the X+Z axis, and has the effect of
@@ -54,9 +54,13 @@ class HGate(Gate):
             \end{pmatrix}
     """
 
-    def __init__(self, label: Optional[str] = None):
+    def __init__(self, label: Optional[str] = None, duration=None, unit=None, _condition=None):
         """Create new H gate."""
-        super().__init__("h", 1, [], label=label)
+        if unit is None:
+            unit = "dt"
+        super().__init__(
+            "h", 1, [], label=label, _condition=_condition, duration=duration, unit=unit
+        )
 
     def _define(self):
         """
@@ -94,8 +98,7 @@ def control(
             ControlledGate: controlled version of this gate.
         """
         if num_ctrl_qubits == 1:
-            gate = CHGate(label=label, ctrl_state=ctrl_state)
-            gate.base_gate.label = self.label
+            gate = CHGate(label=label, ctrl_state=ctrl_state, _base_label=self.label)
             return gate
         return super().control(num_ctrl_qubits=num_ctrl_qubits, label=label, ctrl_state=ctrl_state)
 
@@ -162,10 +165,21 @@ class CHGate(ControlledGate):
                 \end{pmatrix}
     """
 
-    def __init__(self, label: Optional[str] = None, ctrl_state: Optional[Union[int, str]] = None):
+    def __init__(
+        self,
+        label: Optional[str] = None,
+        ctrl_state: Optional[Union[int, str]] = None,
+        _base_label=None,
+    ):
         """Create new CH gate."""
         super().__init__(
-            "ch", 2, [], num_ctrl_qubits=1, label=label, ctrl_state=ctrl_state, base_gate=HGate()
+            "ch",
+            2,
+            [],
+            num_ctrl_qubits=1,
+            label=label,
+            ctrl_state=ctrl_state,
+            base_gate=HGate(label=_base_label),
         )
 
     def _define(self):

qiskit/circuit/library/standard_gates/i.py

@@ -13,12 +13,12 @@
 """Identity gate."""
 
 from typing import Optional
-from qiskit.circuit.gate import Gate
+from qiskit.circuit.singleton_gate import SingletonGate
 from qiskit.circuit._utils import with_gate_array
 
 
 @with_gate_array([[1, 0], [0, 1]])
-class IGate(Gate):
+class IGate(SingletonGate):
     r"""Identity gate.
 
     Identity gate corresponds to a single-qubit gate wait cycle,
@@ -45,9 +45,13 @@ class IGate(Gate):
              └───┘
     """
 
-    def __init__(self, label: Optional[str] = None):
+    def __init__(self, label: Optional[str] = None, duration=None, unit=None, _condition=None):
         """Create new Identity gate."""
-        super().__init__("id", 1, [], label=label)
+        if unit is None:
+            unit = "dt"
+        super().__init__(
+            "id", 1, [], label=label, _condition=_condition, duration=duration, unit=unit
+        )
 
     def inverse(self):
         """Invert this gate."""

qiskit/circuit/library/standard_gates/iswap.py

@@ -16,15 +16,15 @@
 
 import numpy as np
 
-from qiskit.circuit.gate import Gate
+from qiskit.circuit.singleton_gate import SingletonGate
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array
 
 from .xx_plus_yy import XXPlusYYGate
 
 
 @with_gate_array([[1, 0, 0, 0], [0, 0, 1j, 0], [0, 1j, 0, 0], [0, 0, 0, 1]])
-class iSwapGate(Gate):
+class iSwapGate(SingletonGate):
     r"""iSWAP gate.
 
     A 2-qubit XX+YY interaction.
@@ -85,9 +85,13 @@ class iSwapGate(Gate):
             \end{pmatrix}
     """
 
-    def __init__(self, label: Optional[str] = None):
+    def __init__(self, label: Optional[str] = None, duration=None, unit=None, _condition=None):
         """Create new iSwap gate."""
-        super().__init__("iswap", 2, [], label=label)
+        if unit is None:
+            unit = "dt"
+        super().__init__(
+            "iswap", 2, [], label=label, _condition=_condition, duration=duration, unit=unit
+        )
 
     def _define(self):
         """

qiskit/circuit/library/standard_gates/s.py

@@ -18,7 +18,7 @@
 import numpy
 
 from qiskit.circuit.controlledgate import ControlledGate
-from qiskit.circuit.gate import Gate
+from qiskit.circuit.singleton_gate import SingletonGate
 from qiskit.circuit.library.standard_gates.p import CPhaseGate, PhaseGate
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit._utils import with_gate_array, with_controlled_gate_array
@@ -29,7 +29,7 @@
 
 
 @with_gate_array(_S_ARRAY)
-class SGate(Gate):
+class SGate(SingletonGate):
     r"""Single qubit S gate (Z**0.5).
 
     It induces a :math:`\pi/2` phase, and is sometimes called the P gate (phase).
@@ -59,9 +59,13 @@ class SGate(Gate):
     Equivalent to a :math:`\pi/2` radian rotation about the Z axis.
     """
 
-    def __init__(self, label: Optional[str] = None):
+    def __init__(self, label: Optional[str] = None, duration=None, unit=None, _condition=None):
         """Create new S gate."""
-        super().__init__("s", 1, [], label=label)
+        if unit is None:
+            unit = "dt"
+        super().__init__(
+            "s", 1, [], label=label, _condition=_condition, duration=duration, unit=unit
+        )
 
     def _define(self):
         """
@@ -90,7 +94,7 @@ def power(self, exponent: float):
 
 
 @with_gate_array(_SDG_ARRAY)
-class SdgGate(Gate):
+class SdgGate(SingletonGate):
     r"""Single qubit S-adjoint gate (~Z**0.5).
 
     It induces a :math:`-\pi/2` phase.
@@ -120,9 +124,13 @@ class SdgGate(Gate):
     Equivalent to a :math:`-\pi/2` radian rotation about the Z axis.
     """
 
-    def __init__(self, label: Optional[str] = None):
+    def __init__(self, label: Optional[str] = None, duration=None, unit=None, _condition=None):
         """Create new Sdg gate."""
-        super().__init__("sdg", 1, [], label=label)
+        if unit is None:
+            unit = "dt"
+        super().__init__(
+            "sdg", 1, [], label=label, _condition=_condition, duration=duration, unit=unit
+        )
 
     def _define(self):
         """