A bstract Motivated by recent proposals for information recovery from black holes via non-isometric maps and post-selection in an effective description, we set up and investigate a teleportation scenario in a 2d CFT involving a local operator quench and projection on a portion of space onto a Cardy state with the theory in the vacuum state. Using conformal invariance the system can be mapped to CFT with boundary (BCFT). Rényi entropies for spatial intervals in the projected state can then be computed as a function of the location of the quench, either using the replica method, or using twist fields, the latter employing universal results for correlators at large c . We find qualitatively distinct behaviours in the two systems. Our replica computations reveal a surprising universal n dependence of Rényi entropies which implies that teleportation does occur but is not optimal as would be expected because the projector is not especially tuned. We also find that the curious n dependence of the Rényi entropies means that the limit to the von Neumann entropy is not straightforward.
The additivity of minimum output entropies is a central problem in quantum information theory. Nonadditivity is known for every Rényi order
p>1, at the von Neumann point
p=1, and for sufficiently small positive
p, while much of the interval
0<p<1 has remained open. In this work, we prove that for every
p>0 there exist finite-dimensional quantum channels whose minimum output
p-Rényi entropies violate additivity. Our proof combines two constructions: random projection-induced channels yield nonadditivity for
p>3/4, and antisymmetric postprocessing extends the violation to all positive Rényi orders. Our estimates also improve the output-dimension bound obtained by Belinschi, Collins, and Nechita for additivity violation of minimum output von Neumann entropy.