Throw execption for multi measurement (#2593)

mitiq/observable/pauli.py

@@ -279,11 +279,26 @@ def _measure_in(
 
         basis_rotations = set()
         support = set()
+        qubits_with_measurements = set[cirq.Qid]()
+
+        # Find any existing measurement gates in the circuit
+        for _, op, _ in circuit.findall_operations_with_gate_type(
+            cirq.MeasurementGate
+        ):
+            qubits_with_measurements.update(op.qubits)
+
         for pauli in paulis.elements:
             basis_rotations.update(pauli._basis_rotations())
             support.update(pauli._qubits_to_measure())
         measured = circuit + basis_rotations + cirq.measure(*sorted(support))
 
+        if support & qubits_with_measurements:
+            raise ValueError(
+                f"More than one measurement found for qubits: "
+                f"{support & qubits_with_measurements}. Only a single "
+                f"measurement is allowed per qubit."
+            )
+
         # Transform circuit back to original qubits.
         reverse_qubit_map = dict(zip(qubit_map.values(), qubit_map.keys()))
         return measured.transform_qubits(lambda q: reverse_qubit_map[q])

mitiq/observable/tests/test_pauli.py

@@ -137,6 +137,17 @@ def test_pauli_measure_in_bad_qubits_error():
         pauli.measure_in(circuit)
 
 
+def test_pauli_measure_in_multi__measurement_per_qubit():
+    n = 4
+    pauli = PauliString(spec="Z" * n)
+    circuit = cirq.Circuit(cirq.H.on_each(cirq.LineQubit.range(n)))
+
+    # add a measurement to qubit 0 and 1
+    circuit = circuit + cirq.measure(cirq.LineQubit(0), cirq.LineQubit(1))
+    with pytest.raises(ValueError, match="More than one measurement"):
+        pauli.measure_in(circuit)
+
+
 def test_can_be_measured_with_single_qubit():
     pauli = PauliString(spec="Z")