Fix QPY deserialization of UCRX, UCRY, and UCRZ gates (Qiskit#7933)

This commit fixes the deserialization of the UCRX, UCRY, and UCRZ gates.
These gates aren't constructed in the standard way as other gate objects
where the parameters are passed directly as args to the class
constructor. Instead they behave the same as Initialize and the params
are passed as a list to the constructor to make a new gate object. This
commit fixes the handling in deserialization to treat these gates the
same as Initialize so that it can be properly deserialized by
qpy.load().

Fixes Qiskit#7847

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/qpy/binary_io/circuits.py

@@ -240,7 +240,7 @@ def _read_instruction(file_obj, circuit, registers, custom_instructions, version
     if gate_name in {"IfElseOp", "WhileLoopOp"}:
         gate = gate_class(condition_tuple, *params)
     else:
-        if gate_name == "Initialize":
+        if gate_name in {"Initialize", "UCRXGate", "UCRYGate", "UCRZGate"}:
             gate = gate_class(params)
         else:
             if gate_name == "Barrier":

releasenotes/notes/ucr-gates-qpy-b8f6fb1e34fae258.yaml

@@ -0,0 +1,18 @@
+---
+fixes:
+  - |
+    Fixed an issue with QPY deserialization via the :func:`.qpy.load` function
+    of the :class:`~.UCRXGate`, :class:`~.UCRYGate`, and :class:`~.UCRZGate`.
+    Previously, a QPY file that contained any of these gates would error
+    when trying to load the file.
+    Fixed `#7847 <https://github.com/Qiskit/qiskit-terra/issues/7847>`__
+issues:
+  - |
+    QPY deserialization with the :func:`.qpy.load` function of a directly
+    instantiated :class:`~.UCPauliRotGate` object in a circuit will fail
+    because the rotation axis argument to the class isn't stored in a standard
+    place. To workaround this you can instead use the subclasses:
+    :class:`~.UCRXGate`, :class:`~.UCRYGate`, or :class:`~.UCRZGate` (based on
+    whether you're using a rotation axis of ``"X"``, ``"Y"``, or ``"Z"``
+    respectively) which embeds the rotation axis in the class constructor and
+    will work correctly in QPY.

test/python/circuit/test_circuit_load_from_qpy.py

@@ -941,3 +941,16 @@ def test_empty_tuple_param(self):
         qpy_file.seek(0)
         new_circuit = load(qpy_file)[0]
         self.assertEqual(qc, new_circuit)
+
+    def test_ucr_gates(self):
+        """Test qpy with UCRX, UCRY, and UCRZ gates."""
+        qc = QuantumCircuit(3)
+        qc.ucrz([0, 0, 0, -np.pi], [0, 1], 2)
+        qc.ucry([0, 0, 0, -np.pi], [0, 2], 1)
+        qc.ucrx([0, 0, 0, -np.pi], [2, 1], 0)
+        qc.measure_all()
+        qpy_file = io.BytesIO()
+        dump(qc, qpy_file)
+        qpy_file.seek(0)
+        new_circuit = load(qpy_file)[0]
+        self.assertEqual(qc.decompose().decompose(), new_circuit.decompose().decompose())