How can I continue the following term, in order to get a efficient way to compute it. $$|\sum_{l}z_{l}|^{2}=\sum_{l}z_{l}\overline{\sum_{l}z_{l}}=\sum_{l}|z_{l}|^{2}+\sum_{l \neq k}z_{l}\overline{z_{k}}$$ The second term is squared in terms of time of calculation.
Thanks