QUROPE Quantum Information Processing and Communication in Europe

We provide a class of bound entangled states that have a positive distillable secure key rate. The smallest state of this kind is 44. Our class is a generalization of the class presented in Horodecki et al (2008 IEEE Trans. Inf. Theory 54 2621–5). It is much wider, containing, in particular, states from the boundary of PPT entangled states (all of the states in the previous class were of this kind) and also states inside the set of PPT entangled states, even approaching the separable states. This generalization comes at a price: for the wider class, a positive key rate requires, in general, apart from the one-way Devetak–Winter protocol (used in the previous case) also the recurrence preprocessing and thus is effectively a two-way protocol. We also analyze the amount of noise that can be admixtured to the states of our class without losing the key distillability property which may be crucial for experimental realization. The wider class contains key-distillable states with higher entropy (up to 3.524, as opposed to 2.564 for the previous class).