fix issue/12311 with fractional gate in basis (Qiskit#12511)

* linting

* linting...relax check for 3-qubit circuit

* update test docstrings

* black update

* bind RZZ to pi/2 if Rzz(theta) in basis

* Apply suggestions from code review

---------

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

qiskit/transpiler/passes/synthesis/unitary_synthesis.py

@@ -37,6 +37,7 @@
     CZGate,
     RXXGate,
     RZXGate,
+    RZZGate,
     ECRGate,
     RXGate,
     SXGate,
@@ -781,6 +782,8 @@ def _replace_parameterized_gate(op):
                 op = RXXGate(pi / 2)
             elif isinstance(op, RZXGate) and isinstance(op.params[0], Parameter):
                 op = RZXGate(pi / 4)
+            elif isinstance(op, RZZGate) and isinstance(op.params[0], Parameter):
+                op = RZZGate(pi / 2)
             return op
 
         try:

test/python/transpiler/test_basis_translator.py

@@ -16,6 +16,7 @@
 import os
 
 from numpy import pi
+import scipy
 
 from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
 from qiskit import transpile
@@ -33,13 +34,17 @@
     XGate,
     SXGate,
     CXGate,
+    RXGate,
+    RZZGate,
 )
 from qiskit.converters import circuit_to_dag, dag_to_circuit, circuit_to_instruction
 from qiskit.exceptions import QiskitError
+from qiskit.providers.fake_provider import GenericBackendV2
 from qiskit.quantum_info import Operator
 from qiskit.transpiler.target import Target, InstructionProperties
 from qiskit.transpiler.exceptions import TranspilerError
 from qiskit.transpiler.passes.basis import BasisTranslator, UnrollCustomDefinitions
+from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
 from qiskit.circuit.library.standard_gates.equivalence_library import (
     StandardEquivalenceLibrary as std_eqlib,
 )
@@ -1223,3 +1228,48 @@ def __init__(self):
 
         out = BasisTranslator(eq_lib, {"my_h", "my_cx"}, target)(qc)
         self.assertEqual(out, expected)
+
+    def test_fractional_gate_in_basis_from_string(self):
+        """Test transpiling with RZZ in basis with only basis_gates option."""
+        num_qubits = 2
+        seed = 9169
+        basis_gates = ["rz", "rx", "rzz"]
+        qc = QuantumCircuit(num_qubits)
+        mat = scipy.stats.unitary_group.rvs(2**num_qubits, random_state=seed)
+        qc.unitary(mat, range(num_qubits))
+        pm = generate_preset_pass_manager(
+            optimization_level=1, basis_gates=basis_gates, seed_transpiler=134
+        )
+        cqc = pm.run(qc)
+        self.assertEqual(Operator(qc), Operator(cqc))
+
+    def test_fractional_gate_in_basis_from_backendv2(self):
+        """Test transpiling with RZZ in basis of backendv2."""
+        num_qubits = 2
+        seed = 9169
+        basis_gates = ["rz", "rx", "rzz"]
+        qc = QuantumCircuit(num_qubits)
+        mat = scipy.stats.unitary_group.rvs(2**num_qubits, random_state=seed)
+        qc.unitary(mat, range(num_qubits))
+        backend = GenericBackendV2(num_qubits, basis_gates=basis_gates)
+        target = backend.target
+        pm = generate_preset_pass_manager(optimization_level=1, target=target, seed_transpiler=134)
+        cqc = pm.run(qc)
+        self.assertEqual(Operator(qc), Operator.from_circuit(cqc))
+
+    def test_fractional_gate_in_basis_from_custom_target(self):
+        """Test transpiling with RZZ in basis of custom target."""
+        num_qubits = 2
+        seed = 9169
+        qc = QuantumCircuit(num_qubits)
+        mat = scipy.stats.unitary_group.rvs(2**num_qubits, random_state=seed)
+        qc.unitary(mat, range(num_qubits))
+        target = Target()
+        target.add_instruction(RZGate(self.theta), {(i,): None for i in range(qc.num_qubits)})
+        target.add_instruction(RXGate(self.phi), {(i,): None for i in range(qc.num_qubits)})
+        target.add_instruction(
+            RZZGate(self.lam), {(i, i + 1): None for i in range(qc.num_qubits - 1)}
+        )
+        pm = generate_preset_pass_manager(optimization_level=1, target=target, seed_transpiler=134)
+        cqc = pm.run(qc)
+        self.assertEqual(Operator(qc), Operator.from_circuit(cqc))