Add option to set a minimum size to unitarysynthesis pass

This commit adds a new option to the UnitarySynthesis pass constructor,
min_qubits, which is used to specify a minimimum size unitary to
synthesize. If the unitary is smaller than that it will be skipped. This
is then used by the UnitarySynthesis instance in the unroll3q phase so
we don't decompose 1 or 2q unitaries before routing.

qiskit/transpiler/passes/synthesis/unitary_synthesis.py

@@ -57,7 +57,9 @@ def _choose_euler_basis(basis_gates):
 class UnitarySynthesis(TransformationPass):
     """Synthesize gates according to their basis gates."""
 
-    def __init__(self, basis_gates: List[str], approximation_degree: float = 1):
+    def __init__(
+        self, basis_gates: List[str], approximation_degree: float = 1, min_qubits: int = None
+    ):
         """
         Synthesize unitaries over some basis gates.
 
@@ -68,10 +70,14 @@ def __init__(self, basis_gates: List[str], approximation_degree: float = 1):
         Args:
             basis_gates: List of gate names to target.
             approximation_degree: closeness of approximation (0: lowest, 1: highest).
+            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
 
     def run(self, dag: DAGCircuit) -> DAGCircuit:
         """Run the UnitarySynthesis pass on `dag`.
@@ -92,6 +98,8 @@ def run(self, dag: DAGCircuit) -> DAGCircuit:
             decomposer2q = TwoQubitBasisDecomposer(kak_gate, euler_basis=euler_basis)
 
         for node in dag.named_nodes("unitary"):
+            if self._min_qubits is not None and len(node.qargs) < self._min_qubits:
+                continue
 
             synth_dag = None
             if len(node.qargs) == 1:

qiskit/transpiler/preset_passmanagers/level0.py

@@ -107,7 +107,7 @@ 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),
+        UnitarySynthesis(basis_gates, approximation_degree=approximation_degree, min_qubits=3),
         Unroll3qOrMore(),
     ]
 

qiskit/transpiler/preset_passmanagers/level1.py

@@ -127,7 +127,7 @@ 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),
+        UnitarySynthesis(basis_gates, approximation_degree=approximation_degree, min_qubits=3),
         Unroll3qOrMore(),
     ]
 

qiskit/transpiler/preset_passmanagers/level2.py

@@ -161,7 +161,7 @@ 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),
+        UnitarySynthesis(basis_gates, approximation_degree=approximation_degree, min_qubits=3),
         Unroll3qOrMore(),
     ]
 

qiskit/transpiler/preset_passmanagers/level3.py

@@ -102,7 +102,7 @@ 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),
+        UnitarySynthesis(basis_gates, approximation_degree=approximation_degree, min_qubits=3),
         Unroll3qOrMore(),
     ]
 

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

@@ -1,4 +1,13 @@
 ---
+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