# Stuttgart 2012 – wissenschaftliches Programm

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# Q: Fachverband Quantenoptik und Photonik

## Q 69: Quanteninformation: Kommunikation

### Q 69.6: Vortrag

### Freitag, 16. März 2012, 15:15–15:30, V38.04

**Gaussian super-activation of the quantum channel capacity requires nonlinear interaction** — •Daniel Lercher^{1}, Géza Giedke^{1,2}, and Michael Marc Wolf^{1} — ^{1}Department of Mathematics, Technische Universität München, 85748 Garching, Germany — ^{2}Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany

The quantum capacity of bosonic Gaussian quantum channels was recently shown to be non-additive in a particularly striking way: a pair of such optical-fibre type channels can individually have zero quantum capacity but super-activate each other such that the combined channel has strictly positive capacity. This has been shown in [Nature Photon. 5, 6(2011)] where it was conjectured that non-linear squeezing is necessary for this phenomenon. We prove a stronger version of this conjecture by showing that super-activation in this scheme is not possible if the system-environment interaction can be obtained by means of linear optics, even if the environment is in an arbitrary, possibly squeezed, state.

Technically, we prove that a Gaussian channel whose interaction matrix admits a diagonal symplectic singular value decomposition is entanglement breaking if its Choi matrix is PPT. Consequences on the preparation of bound entangled Gaussian states are outlined.