We introduce a necessary and sufficient criterion for the non-Markovianity of
Gaussian quantum dynamical maps based on the violation of divisibility. The
criterion is derived by defining a general vectorial representation of the
covariance matrix which is then exploited to determine the condition for the
complete positivity of partial maps associated to arbitrary time intervals.
Such construction does not rely on the Choi-Jamiolkowski representation and
does not require optimization over states.Comment: 5 pages, 1 figure. Published versio