Valerio Scarani

Device-independent quantum key distribution (DIQKD) is the art of using untrusted devices to distribute secret keys in an unsecure network. It thus represents the ultimate form of cryptography, offering not only information-theoretic security against channel attacks, but also against attacks exploiting implementation loopholes~\cite{lydersen2010hacking}. At its heart, DIQKD utilises nonlocal correlations---detected and certified by a Bell inequality---to establish secret correlations between the users. In recent years, much progress has been made towards realising the first DIQKD experiments, but current proposals are just out of reach of today’s loophole-free Bell experiments. Here, in this work, we close the gap between the theory and practice of DIQKD with a simple variant of the original protocol based on the celebrated Clauser-Horne-Shimony-Holt (CHSH) Bell inequality. In using two randomly chosen key generating bases instead of one, we show that the noise tolerance of DIQKD can be significantly improved. In particular, the extended feasibility region now covers some of the most recent loophole-free CHSH experiments, hence indicating that the first realisation of DIQKD already lies within the range of these experiments.

When separated measurements on entangled quantum systems are performed, the theory predicts correlations that cannot be explained by any classical mechanism. All optical demonstrations of such violations have involved discrete degrees of freedom and are plagued by the detection-efficiency loophole. We present a simple method for generating large violations for feasible states using both photon counting and homodyne detections. Our scheme may lead to the first violation of Bell inequalities using continuous-variable measurements and pave the way for a loophole-free Bell test.

Violation of a Bell inequality can be used to generate certified random numbers. Given the high rate at which pairs of entangled photons can be produced, they constitute promising candidates for high rate randomness generation. However, this requires closing the detection loophole. Here we present our progresses toward an experimental demonstration of randomness generation with photons, certified by the violation of a Bell inequality with a closed detection loophole.

We give an explicit construction for classes of quantum states of qubits, whose complexity grows super-polynomially in the number of qubits, and discuss the properties of these states. The complexity measure considered is the tree size of a quantum state which is in principle computable and is closely related to the size of a multi-linear formula. Several conjectures and partially known relations to entanglement and quantum computational power will be discussed. Based on arXiv:1303.4843.