Support save_statevector for QuantumCircuit from QASM3 string

qiskit_aer/library/default_qubits.py

@@ -13,7 +13,7 @@
 Helper function
 """
 
-from qiskit.circuit import QuantumCircuit, QuantumRegister
+from qiskit.circuit import QuantumRegister
 from qiskit.extensions.exceptions import ExtensionError
 
 
@@ -38,18 +38,8 @@ def default_qubits(circuit, qubits=None):
     if isinstance(qubits, QuantumRegister):
         qubits = qubits[:]
     if not qubits:
-        tuples = []
-        if isinstance(circuit, QuantumCircuit):
-            for register in circuit.qregs:
-                tuples.append(register)
-        if not tuples:
+        qubits = list(circuit.qubits)
+        if len(qubits) == 0:
             raise ExtensionError("no qubits for snapshot")
-        qubits = []
-        for tuple_element in tuples:
-            if isinstance(tuple_element, QuantumRegister):
-                for j in range(tuple_element.size):
-                    qubits.append(tuple_element[j])
-            else:
-                qubits.append(tuple_element)
 
     return qubits

releasenotes/notes/save_statevector_for_qasm3_circ-642ade99af3ff0d2.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    :meth:``QuantumCircuit.save_statevector()`` does not work if the circuit
+    is generated from OpenQASM3 text because its quantum registers have duplicated
+    qubit instances. With this commit, :meth:``QuantumCircuit.save_statevector()``
+    uses :data:``QuantumCircuit.qubits`` to get qubits to be saved.

requirements-dev.txt

@@ -16,3 +16,4 @@ seaborn>=0.9.0
 qiskit_sphinx_theme~=1.12.0
 sphinx-design>=0.2.0
 nbsphinx
+qiskit_qasm3_import

test/terra/backends/aer_simulator/test_save_statevector.py

@@ -13,10 +13,12 @@
 Integration Tests for SaveStatevector instruction
 """
 
+import sys
 from ddt import ddt
 import qiskit.quantum_info as qi
 from qiskit import QuantumCircuit, transpile
 from test.terra.backends.simulator_test_case import SimulatorTestCase, supported_methods
+from qiskit.qasm3 import dumps, loads
 
 
 @ddt
@@ -201,3 +203,44 @@ def test_save_statevector_cache_blocking(self, method, device):
         self.assertIn(label, simdata)
         value = simdata[label]
         self.assertEqual(value, target)
+
+    @supported_methods(
+        [
+            "automatic",
+            "statevector",
+            "matrix_product_state",
+            "extended_stabilizer",
+            "tensor_network",
+        ]
+    )
+    def test_save_statevector_for_qasm3_circuit(self, method, device):
+        """Test save statevector instruction"""
+        # qiskit_qasm3_import, which is used in qiskit.qasm3 does not support 3.7
+        if sys.version_info < (3, 8):
+            return
+
+        backend = self.backend(method=method, device=device)
+
+        # Stabilizer test circuit
+        circ = QuantumCircuit(3)
+        circ.h(0)
+        circ.sdg(0)
+        circ.cx(0, 1)
+        circ.cx(0, 2)
+
+        # Target statevector
+        target = qi.Statevector(circ)
+
+        circ = loads(dumps(circ))
+
+        # Add save to circuit
+        label = "state"
+        circ.save_statevector(label=label)
+
+        # Run
+        result = backend.run(transpile(circ, backend, optimization_level=0), shots=1).result()
+        self.assertTrue(result.success)
+        simdata = result.data(0)
+        self.assertIn(label, simdata)
+        value = simdata[label]
+        self.assertEqual(value, target)