Quantum non-Markovianity elusive to interventions.

The non-Markovian nature of open quantum dynamics lies in the structure of the multitime correlations, which are accessible by means of interventions. Here, by examining multitime correlations, we show that it is possible to engineer non-Markovian systems with only long-term memory but seemingly no short-term memory, so that their non-Markovianity is completely non-detectable by any interventions up to an arbitrarily large time. Our results raise the question about the assessibility of non-Markovianity: in principle, non-Markovian effects that are perfectly elusive to interventions may emerge at much later times.