def METHOD_NAME(self, exp_op: OperatorBase) -> Union[list, float]:
    r"""
    Compute the variance of the expectation estimator. Because Aer takes this expectation
    with matrix multiplication, the estimation is exact and the variance is always 0,
    but we need to return those values in a way which matches the Operator's structure.
    Args:
        exp_op: The full expectation value Operator after sampling.
    Returns:
         The variances or lists thereof (if exp_op contains ListOps) of the expectation value
         estimation, equal to 0.
    """
    # Need to do this to mimic Op structure
    def sum_variance(operator):
        if isinstance(operator, ComposedOp):
            return 0.0
        elif isinstance(operator, ListOp):
            return operator.combo_fn([sum_variance(op) for op in operator.oplist])
        raise TypeError(f"Variance cannot be computed for {operator.__class__.__name__}.")
    return sum_variance(exp_op)