def Get_Histogram_key(qubitOperator):
    """
     Function to obtain histogram key string for Cirq Simulator.

    e.g.
            PauliWord = QubitOperator('X0 Z2 Y3', 0.5j)

            returning: histogram_string = '0,2,3'


    Args:
        qubitOperator (openfermion.ops._qubit_operator.QubitOperator): QubitOperator

    Returns:
        histogram_string (str): Returns string corresponding to histogram key (required for Cirq simulator)

    """

    qubit_No, PauliStr = zip(*list(*qubitOperator.terms.keys()))
    histogram_string = ','.join([str(i) for i in qubit_No])
    return histogram_string