Add min_qubits kwarg to UnitarySynthesis pass

qiskit/extensions/quantum_initializer/squ.py

@@ -59,7 +59,7 @@ def __init__(self, unitary_matrix, mode="ZYZ", up_to_diagonal=False, u=None):
         self._diag = None
 
         # Create new gate
-        super().__init__("unitary", 1, [unitary_matrix])
+        super().__init__("squ", 1, [unitary_matrix])
 
     def inverse(self):
         """Return the inverse.

qiskit/transpiler/passes/synthesis/unitary_synthesis.py

@@ -91,6 +91,7 @@ def __init__(
         natural_direction: Union[bool, None] = None,
         synth_gates: Union[List[str], None] = None,
         method: str = "default",
+        min_qubits: int = None,
     ):
         """Synthesize unitaries over some basis gates.
 
@@ -131,11 +132,14 @@ def __init__(
                 ['unitary']. If None and `pulse_optimzie` == True,
                 default to ['unitary', 'swap']
             method (str): The unitary synthesis method plugin to use.
-
+            min_qubits: The minimum number of qubits in the unitary to synthesize. If this is set
+                and the unitary is less than the specified number of qubits it will not be
+                synthesized.
         """
         super().__init__()
         self._basis_gates = basis_gates
         self._approximation_degree = approximation_degree
+        self._min_qubits = min_qubits
         self.method = method
         self.plugins = plugin.UnitarySynthesisPluginManager()
         self._coupling_map = coupling_map
@@ -194,6 +198,8 @@ def run(self, dag: DAGCircuit) -> DAGCircuit:
             plugin_method._approximation_degree = self._approximation_degree
 
         for node in dag.named_nodes(*self._synth_gates):
+            if self._min_qubits is not None and len(node.qargs) < self._min_qubits:
+                continue
             if plugin_method.supports_coupling_map:
                 kwargs["coupling_map"] = (
                     self._coupling_map,

qiskit/transpiler/preset_passmanagers/level0.py

@@ -113,12 +113,14 @@ def _choose_layout_condition(property_set):
 
     # 3. Decompose so only 1-qubit and 2-qubit gates remain
     _unroll3q = [
+        # Use unitary synthesis for basis aware decomposition of UnitaryGates
         UnitarySynthesis(
             basis_gates,
             approximation_degree=approximation_degree,
             coupling_map=coupling_map,
             backend_props=backend_properties,
             method=unitary_synthesis_method,
+            min_qubits=3,
         ),
         Unroll3qOrMore(),
     ]
@@ -155,9 +157,27 @@ def _swap_condition(property_set):
     elif translation_method == "translator":
         from qiskit.circuit.equivalence_library import SessionEquivalenceLibrary as sel
 
-        _unroll = [UnrollCustomDefinitions(sel, basis_gates), BasisTranslator(sel, basis_gates)]
+        _unroll = [
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                backend_props=backend_properties,
+                method=unitary_synthesis_method,
+            ),
+            UnrollCustomDefinitions(sel, basis_gates),
+            BasisTranslator(sel, basis_gates),
+        ]
     elif translation_method == "synthesis":
         _unroll = [
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                backend_props=backend_properties,
+                method=unitary_synthesis_method,
+                min_qubits=3,
+            ),
             Unroll3qOrMore(),
             Collect2qBlocks(),
             ConsolidateBlocks(basis_gates=basis_gates),

qiskit/transpiler/preset_passmanagers/level1.py

@@ -133,12 +133,14 @@ def _not_perfect_yet(property_set):
 
     # 4. Decompose so only 1-qubit and 2-qubit gates remain
     _unroll3q = [
+        # Use unitary synthesis for basis aware decomposition of UnitaryGates
         UnitarySynthesis(
             basis_gates,
             approximation_degree=approximation_degree,
             coupling_map=coupling_map,
             method=unitary_synthesis_method,
             backend_props=backend_properties,
+            min_qubits=3,
         ),
         Unroll3qOrMore(),
     ]
@@ -175,9 +177,31 @@ def _swap_condition(property_set):
     elif translation_method == "translator":
         from qiskit.circuit.equivalence_library import SessionEquivalenceLibrary as sel
 
-        _unroll = [UnrollCustomDefinitions(sel, basis_gates), BasisTranslator(sel, basis_gates)]
+        _unroll = [
+            # Use unitary synthesis for basis aware decomposition of UnitaryGates before
+            # custom unrolling
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                method=unitary_synthesis_method,
+                backend_props=backend_properties,
+            ),
+            UnrollCustomDefinitions(sel, basis_gates),
+            BasisTranslator(sel, basis_gates),
+        ]
     elif translation_method == "synthesis":
         _unroll = [
+            # Use unitary synthesis for basis aware decomposition of UnitaryGates before
+            # collection
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                method=unitary_synthesis_method,
+                backend_props=backend_properties,
+                min_qubits=3,
+            ),
             Unroll3qOrMore(),
             Collect2qBlocks(),
             ConsolidateBlocks(basis_gates=basis_gates),

qiskit/transpiler/preset_passmanagers/level2.py

@@ -167,12 +167,14 @@ def _csp_not_found_match(property_set):
 
     # 3. Unroll to 1q or 2q gates
     _unroll3q = [
+        # Use unitary synthesis for basis aware decomposition of UnitaryGates
         UnitarySynthesis(
             basis_gates,
             approximation_degree=approximation_degree,
             coupling_map=coupling_map,
             backend_props=backend_properties,
             method=unitary_synthesis_method,
+            min_qubits=3,
         ),
         Unroll3qOrMore(),
     ]
@@ -209,9 +211,31 @@ def _swap_condition(property_set):
     elif translation_method == "translator":
         from qiskit.circuit.equivalence_library import SessionEquivalenceLibrary as sel
 
-        _unroll = [UnrollCustomDefinitions(sel, basis_gates), BasisTranslator(sel, basis_gates)]
+        _unroll = [
+            # Use unitary synthesis for basis aware decomposition of UnitaryGates before
+            # custom unrolling
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                backend_props=backend_properties,
+                method=unitary_synthesis_method,
+            ),
+            UnrollCustomDefinitions(sel, basis_gates),
+            BasisTranslator(sel, basis_gates),
+        ]
     elif translation_method == "synthesis":
         _unroll = [
+            # Use unitary synthesis for basis aware decomposition of UnitaryGates before
+            # collection
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                backend_props=backend_properties,
+                method=unitary_synthesis_method,
+                min_qubits=3,
+            ),
             Unroll3qOrMore(),
             Collect2qBlocks(),
             ConsolidateBlocks(basis_gates=basis_gates),

qiskit/transpiler/preset_passmanagers/level3.py

@@ -108,12 +108,14 @@ def level_3_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
 
     # 1. Unroll to 1q or 2q gates
     _unroll3q = [
+        # Use unitary synthesis for basis aware decomposition of UnitaryGates
         UnitarySynthesis(
             basis_gates,
             approximation_degree=approximation_degree,
             coupling_map=coupling_map,
             backend_props=backend_properties,
             method=unitary_synthesis_method,
+            min_qubits=3,
         ),
         Unroll3qOrMore(),
     ]
@@ -212,9 +214,27 @@ def _swap_condition(property_set):
     elif translation_method == "translator":
         from qiskit.circuit.equivalence_library import SessionEquivalenceLibrary as sel
 
-        _unroll = [UnrollCustomDefinitions(sel, basis_gates), BasisTranslator(sel, basis_gates)]
+        _unroll = [
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                backend_props=backend_properties,
+                method=unitary_synthesis_method,
+            ),
+            UnrollCustomDefinitions(sel, basis_gates),
+            BasisTranslator(sel, basis_gates),
+        ]
     elif translation_method == "synthesis":
         _unroll = [
+            UnitarySynthesis(
+                basis_gates,
+                approximation_degree=approximation_degree,
+                coupling_map=coupling_map,
+                backend_props=backend_properties,
+                method=unitary_synthesis_method,
+                min_qubits=3,
+            ),
             Unroll3qOrMore(),
             Collect2qBlocks(),
             ConsolidateBlocks(basis_gates=basis_gates),

releasenotes/notes/squ-gate-name-785b7896300a92ef.yaml

@@ -0,0 +1,18 @@
+---
+features:
+  - |
+    The :class:`~qiskit.transpiler.passes.UnitarySynthesis` transpiler pass in
+    :mod:`qiskit.transpiler.passes` has a new kwarg in the constructor,
+    ``min_qubits``. When specified this can be set to an ``int`` value which
+    is the minimum size :class:`~qiskit.extensions.UnitaryGate` object to
+    run the unitary synthesis on. If a :class:`~qiskit.extensions.UnitaryGate`
+    in a :class:`~qiskit.circuit.QuantumCircuit` uses fewer qubits it will
+    be skipped by that instance of the pass.
+upgrade:
+  - |
+    The :attr:`~qiskit.extensions.SingleQubitUnitary.name` attribute of the
+    :class:`~qiskit.extensions.SingleQubitUnitary` gate class has been changed
+    from ``unitary`` to ``squ``. This was necessary to avoid a conflict with
+    the :class:`~qiskit.extensions.UnitaryGate` class's name which was also
+    ``unitary`` since the 2 gates are not the same and don't have the same
+    implementation (and can't be used interchangeably).