def char_decay(A, alpha, m):
    """
    From Helsen et al. A new class of efficient RB protocols.

    Theory in Helsen et al. arXiv:1806.02048
    Eq. 8 of Xue et al. ArXiv 1811.04002v1 (experimental implementation)

    Parameters
    ----------
    A (float):
        Scaling factor of the decay
    alpha (float):
        depolarizing parameter to be estimated
    m (array)
        number of cliffords

    returns:
       A * α**m
    """
    return A * alpha ** m