---
abstract: 'Quantum discord is the minimal bipartite resource which is needed for a secure quantum key distribution, being a cryptographic primitive equivalent to non-orthogonality. Its role becomes crucial in device-dependent quantum cryptography, where the presence of preparation and detection noise (inaccessible to all parties) may be so strong to prevent the distribution and distillation of entanglement. The necessity of entanglement is re-affirmed in the stronger scenario of device-independent quantum cryptography, where all sources of noise are ascribed to the eavesdropper.'
author:
- Stefano Pirandola
title: Quantum discord as a resource for quantum cryptography
---

*Introduction*.– One of the hot topics in the quantum information theory is the quest for the most appropriate measure and quantification of quantum correlations. For pure quantum states, this quantification is provided by quantum entanglement [@Nielsen] which is the physical resource at the basis of the most powerful protocols of quantum communication and computation [@Tele; @Ekert; @Shor]. However, we have recently understood that the characterization of quantum correlations is much more subtle in the general case of mixed quantum states [Qdiscord,Qdiscord2]{}.

There are in fact mixed states which, despite being separable, have correlations so strong to be irreproducible by any classical probability distribution. These residual 