Allow correct retrieval of circuit with initialize instruction from qpy file (backport #11206)

qiskit/qpy/binary_io/circuits.py

@@ -319,9 +319,17 @@ def _read_instruction(
         if condition:
             gate = gate.c_if(*condition)
     else:
-        if gate_name in {
-            "Initialize",
-            "StatePreparation",
+        if gate_name in {"Initialize", "StatePreparation"}:
+            if isinstance(params[0], str):
+                # the params are the labels of the initial state
+                gate = gate_class("".join(label for label in params))
+            elif instruction.num_parameters == 1:
+                # the params is the integer indicating which qubits to initialize
+                gate = gate_class(int(params[0].real), instruction.num_qargs)
+            else:
+                # the params represent a list of complex amplitudes
+                gate = gate_class(params)
+        elif gate_name in {
             "UCRXGate",
             "UCRYGate",
             "UCRZGate",

releasenotes/notes/fix-qpy-statepreparation-retrieval-1feea5eb74db7f1e.yaml

@@ -0,0 +1,5 @@
+fixes:
+  - |
+    Fixed QPY deserialization of the :class:`.StatePreparation` and :class:`.Initialize` circuit
+    instructions with string and integer parameters (as opposed to an explicit statevector, which
+    was already working).  Fixed `#11158 <https://github.com/Qiskit/qiskit/issues/11158>`__.

test/python/circuit/test_circuit_load_from_qpy.py

@@ -1685,6 +1685,18 @@ def test_qpy_deprecation(self):
             # pylint: disable=no-name-in-module, unused-import, redefined-outer-name, reimported
             from qiskit.circuit.qpy_serialization import dump, load
 
+    @ddt.data(0, "01", [1, 0, 0, 0])
+    def test_valid_circuit_with_initialize_instruction(self, param):
+        """Tests that circuit that has initialize instruction can be saved and correctly retrieved"""
+        qc = QuantumCircuit(2)
+        qc.initialize(param, qc.qubits)
+        with io.BytesIO() as fptr:
+            dump(qc, fptr)
+            fptr.seek(0)
+            new_circuit = load(fptr)[0]
+        self.assertEqual(qc, new_circuit)
+        self.assertDeprecatedBitProperties(qc, new_circuit)
+
 
 class TestSymengineLoadFromQPY(QiskitTestCase):
     """Test use of symengine in qpy set of methods."""