We introduce a scenario of discrimination between bipartite boxes and apply it to boxes with two binary inputs and two binary outputs (2 × 2). We develop the analogy between the theory of Bell non-locality and the theory of entanglement by considering the class of completely locality preserving (CLP) operations. A CLP operation satisfies two conditions: 1) transforms boxes with local hidden variable model (LHVM) into boxes with LHVM (i.e., is locality preserving) and 2) when tensored with an identity operation, forms a new operation, which is also locality preserving. We derive linear program, which gives an upper bound on the probability of success of discrimination between different isotropic boxes using this class of operations. In particular, we provide an upper bound on the probability of success of discrimination between isotropic boxes with the same mixing parameter. As a counterpart of entanglement monotone, we use the non-locality cost. Discrimination is restricted by the fact that non-locality cost does not increase under considered class of operations and geometry of 2 × 2 boxes. We provide an example of CLP operations, which are called comparing operations (COP). The latter operations consist of direct measurement of the shared box by both the parties, followed by a predefined strategy in order to establish the guess, which may depend on the obtained outcomes. We then show that with the help of the COP operations, one can distinguish perfectly any two extremal boxes in 2 × 2 case and any local extremal box from any other extremal box in case of two inputs and two outputs of arbitrary cardinalities.

The problem of device-independent randomness amplification against no-signaling adversaries has so far been studied under the assumption that the weak source of randomness is uncorrelated with the (quantum) devices used in the amplification procedure. In this paper, we relax this assumption, and reconsider the original protocol of Colbeck and Renner using a Santha-Vazirani (SV) source. To do so, we introduce an SV-like condition for devices, namely that any string of SV source bits remains weakly random conditioned upon any other bit string from the same SV source and the outputs obtained when this further string is input into the devices. Assuming this condition, we show that a quantum device using a singlet state to violate the chained Bell inequalities leads to full randomness in the asymptotic scenario of a large number of settings, for a restricted set of SV sources (with$0 \leq \varepsilon < (2^{(1/12)} - 1)/(2(2^{(1/12)} + 1)) \approx 0.0144$). We also study a device-independent protocol that allows for correlations between the sequence of boxes used in the protocol and the SV source bits used to choose the particular box from whose output the randomness is obtained. Assuming the SV-like condition for devices, we show that the honest parties can achieve amplification of the weak source, for the parameter range$0 \leq \varepsilon <0.0132$, against a class of attacks given as a mixture of product box sequences, made of extremal no-signaling boxes, with additional symmetry conditions. Composable security proof against this class of attacks is provided.

In this paper, we develop a formalism for distilling a classical key from a quantum state in a systematic way, expanding on our previous work on a secure key from bound entanglement (Horodecki, 2005). More detailed proofs, discussion, and examples are provided of the main results. Namely, we demonstrate that all quantum cryptographic protocols can be recast in a way which looks like entanglement theory, with the only change being that instead of distilling Einstein–Podolsky–Rosen (EPR) pairs, the parties distill private states. The form of these general private states are given, and we show that there are a number of useful ways of expressing them. Some of the private states can be approximated by certain states, which are bound entangled. Thus, distillable entanglement is not a requirement for a private key. We find that such bound entangled states are useful for a cryptographic primitive we call a controlled private quantum channel (PQC). We also find a general class of states, which have negative partial transpose (are NPT), but which appear to be bound entangled. The relative entropy distance is shown to be an upper bound on the rate of a key. This allows us to compute theexactvalue of a distillable key for a certain class of private states.

New measures of multipartite entanglement are constructed based on two definitions of multipartite information and different methods of optimizing over extensions of the states. One is a generalization of the squashed entanglement where one takes the mutual information of parties conditioned on the state's extension and takes the infimum over such extensions. Additivity of the multipartite squashed entanglement is proved for both versions of the multipartite information which turn out to be related. The second one is based on taking classical extensions. This scheme is generalized, which enables to construct measures of entanglement based on the mixed convex roof of a quantity, which in contrast to the standard convex roof method involves optimization over all decompositions of a density matrix rather than just the decompositions into pure states. As one of the possible applications of these results we prove that any multipartite monotone is an upper bound on the amount of multipartite distillable key. The findings are finally related to analogous results in classical key agreement.

In this paper, we prove unconditional security for a quantum key distribution (QKD) protocol based on distilling pbits (twisted ebits) from an arbitrary untrusted state that is claimed to contain distillable key. Our main result is that we can verify security using only public communication-via parameter estimation of the given untrusted state. The technique applies even to bound-entangled states, thus extending QKD to the regime where the available quantum channel has zero quantum capacity. We also show how to convert our purification-based QKD schemes to prepare/measure schemes.

In this paper, we provide a class of bound entangled states that have positive distillable secure key rate. The smallest state of this kind is 4 otimes 4, which shows that peculiar security contained in bound entangled states does not need high-dimensional systems. We show that for these states a positive key rate can be obtained byone-wayDevetak-Winter (DW) protocol. Subsequently, the volume of bound entangled key-distillable states formotimesnHilbert space withm,n> 4 is shown to be nonzero. We provide a scheme of verification of cryptographic quality of experimentally prepared state in terms of local observables. Proposed set of seven collective settings is proven to be optimal in number of settings.