Transmitting data reliably over noisy communication channels is one of the most important applications of information theory, and is well understood for channels modelled by classical physics. However, when quantum effects are involved, we do not know how to compute channel capacities. This is because the formula for the quantum capacity involves maximizing the coherent information over an unbounded number of channel uses. In fact, entanglement across channel uses can even increase the coherent information from zero to non-zero. In Nature Communications 6, 6739, the authors study the number of channel uses necessary to detect positive coherent information. In all previous known examples, two channel uses already sufficed. It might be that only a finite number of channel uses is always sufficient. It is shown that this is not the case: for any number of uses, there are channels for which the coherent information is zero, but which nonetheless have capacity. | POSITION OPENINGSNew positions in the group: PhD, Postdoc and Senior Postdoc. Topics: mathematical problems arising in quantum many body systems, mainly in the context of Tensor Network States. More info and contact details: see Job Offers. Press article at Tribuna Complutense20/22-05-2015Presentations at TQC2015: Sampling quantum nonlocal correlations with high probability and Area law for fixed points of rapidly mixing dissipative quantum systems.10-03-2015David Pérez has been awarded an ERC Consolidator grant, for the project Spectral gaps in interacting quantum systems.01-09-2014Carlos González defended his PhD dissertation. Congratulations to the new doctor! |