Raise value error if pauli string passed to cirq.measure_single_pauli…

…string does not have a coefficient of +1 (quantumlib#5623)

Fixes quantumlib#5615

cirq/ops/measure_util.py

@@ -45,12 +45,16 @@ def measure_single_paulistring(
         An operation measuring the pauli observable.
 
     Raises:
-        ValueError: if the observable is not an instance of PauliString.
+        ValueError: if the observable is not an instance of PauliString or if the coefficient
+            is not +1.
     """
     if not isinstance(pauli_observable, pauli_string.PauliString):
         raise ValueError(
             f'Pauli observable {pauli_observable} should be an instance of cirq.PauliString.'
         )
+    if pauli_observable.coefficient != 1:
+        raise ValueError(f"Pauli observable {pauli_observable} must have a coefficient of +1.")
+
     if key is None:
         key = _default_measurement_key(pauli_observable)
     return PauliMeasurementGate(pauli_observable.values(), key).on(*pauli_observable.keys())

cirq/ops/measure_util_test.py

@@ -103,6 +103,10 @@ def test_measure_single_paulistring():
     with pytest.raises(ValueError, match='should be an instance of cirq.PauliString'):
         _ = cirq.measure_single_paulistring(q)
 
+    # Coefficient != +1
+    with pytest.raises(ValueError, match='must have a coefficient'):
+        _ = cirq.measure_single_paulistring(-ps)
+
 
 def test_measure_paulistring_terms():
     # Correct application