FIx 0q operation handling in Statevector (backport #10031)

qiskit/quantum_info/operators/op_shape.py

@@ -123,6 +123,9 @@ def num_qargs(self):
     @property
     def shape(self):
         """Return a tuple of the matrix shape"""
+        if self._num_qargs_l == self._num_qargs_r == 0:
+            # Scalar shape is op-like
+            return (1, 1)
         if not self._num_qargs_r:
             # Vector shape
             return (self._dim_l,)

releasenotes/notes/fix-0q-operator-statevector-79199c65c24637c4.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Construction of a :class:`~.quantum_info.Statevector` from a :class:`.QuantumCircuit` containing
+    zero-qubit operations will no longer raise an error.  These operations impart a global phase on
+    the resulting statevector.

test/python/quantum_info/states/test_statevector.py

@@ -24,7 +24,7 @@
 from qiskit import QiskitError
 from qiskit import QuantumRegister, QuantumCircuit
 from qiskit import transpile
-from qiskit.circuit.library import HGate, QFT
+from qiskit.circuit.library import HGate, QFT, GlobalPhaseGate
 from qiskit.providers.basicaer import QasmSimulatorPy
 
 from qiskit.quantum_info.random import random_unitary, random_statevector, random_pauli
@@ -186,6 +186,13 @@ def test_from_circuit(self):
         psi = Statevector.from_instruction(circuit)
         self.assertEqual(psi, target)
 
+        # Test 0q instruction
+        target = Statevector([1j, 0])
+        circuit = QuantumCircuit(1)
+        circuit.append(GlobalPhaseGate(np.pi / 2), [], [])
+        psi = Statevector.from_instruction(circuit)
+        self.assertEqual(psi, target)
+
     def test_from_instruction(self):
         """Test initialization from an instruction."""
         target = np.dot(HGate().to_matrix(), [1, 0])