Our strategy within CRYPTASC is twofold. On the one hand, we intend to pursue the development of Quantum Key Distribution (QKD) along directions that our teams have followed over the last years, in particular the use of higher-dimensional or even continuous alphabets. In this direction, we expect to obtain results which may be commercialised within a few years. On the other hand, we plan to explore, in a longer-term perspective, the possibility of applying quantum technology to other cryptographic primitives, such as authentication, digital signature, fair exchange protocols, etc.

CRYPTASC can be viewed as an “umbrella”, which also incorporates the general themes of projects :

1. Prospective research on a high-rate continuous-variable quantum key distribution and possibilities of its implementation in the Brussels Capital Region in the framework of the program Prospective Research in Brussels (PRFB call 2006) aimed at the fundamental studies on this particular class of quantum cryptographic systems as well as the investigation of the needs and possible markets for it in the Brussels-Capital Region.
2. « SeQuR » in the framework of the program Spin off in Brussels (SOIB call 2007) aimed at the creation of a spin-off in the Brussels-capital region specialized in quantum technologies-enabled security, in particular quantum random number generators.

The cut between CRYPTASC and project 1 is “vertical”, in the sense that a particular technique (continuous-variable systems) is developed in more details in the “satellite” project 1, up to the analysis of valorization, while the general theory related to the quantum key distribution remains in CRYPTASC. In contrast, the cut between CRYPTASC and project 2 is “horizontal”, in the sense that the aspects which are the closest to valorization, especially the quantum random number generator, are developed in the “satellite” project 2, while more conceptual proof-of-principle aspects are treated in CRYPTASC.

In this “hardware-oriented” work package most of experimental work on Quantum Key Distribution (and beyond) will be concentrated

We start from a theoretical task is devoted to device independent protocols of Quantum Key Distribution based on non-locality. An important extension of this work is the realization that if one uses non-local correlations to realize quantum cryptography, he can use mistrusted equipment, even bought from eavesdropper, and nevertheless the cryptographic scheme will be secure.

Then we go to experimental realizations of

• an important application of cryptography, coin tossing which was recently demonstrated experimentally by one of us; we intend to improve our previous exprerimental results with the help of new, recently proposed scheme; a continuous variable version of this cryptographic primitive will be developed theoretically in WP4;
• a SIC POVM based QKD protocol using qutrits, quantum states of dimension three, which was shown to be competitive relatively to two-dimensional qubit schemes; this work will be based on the preliminary theoretical study in WP 2.

As a complement to the spin off project 2, SeQuR, we will study the possibility of realizing prototypes for high bit rate random number generators based on the randomness inherent in the quantum measurement process.

The main goal of this “software-oriented” work package is the fundamental theoretical study of quantum secret key distribution with discrete variables (or discrete alphabets). Virtually all the quantum key distribution systems developed to date are based on binary alphabets (quantum bits). It has been shown however, that increasing the size of the alphabet may come with a security advantage.

We will study in detail the qutrit generalization of the qubit Singapore protocol, in which the users of the cryptographic channel would establish a key based on the correlations exhibited by a two qutrit entangled signal that they measure by performing qutrit SIC POVMs measurements. The realization is planned in WP1.

In a continuation to previous work of several partners of the present project, we will make an attempt at improving the post-processing stage of such discrete-variable systems, based on a quest for clever ways to encode the signal aiming at the development of higher-dimensional versions of this protocol, which are expected to result in enhanced effective bit rates and present several advantages from the point of view of robustness and practicability. We expect a possibility for the security verification of the developed scheme by the meta-logical program developed in WP3.

Complementary to the project 1 supported by the program Prospective Research for Brussels we will investigate the limit of SIC POVM-based protocols when the dimension d becomes high (say 50). In this regime, the discrete variable is analogous to a discretized continuous variable so that high dimensional d-level systems constitute a bridge between discrete and continuous variables.

The objective of this work package is to provide high-level security proofs for quantum key distribution as well as software for verifying the security of the quantum key distributing hardware implemented through optical components. The latter will essentially be a meta-logical program that will allow different customers to estimate the security of their hardware in their concrete environment.

In order to achieve our objective we propose to further develop a simulator developed and specifically port it to a parallel environment (Cell processor) to improve efficiency.

Along with the first two work packages, this third work package is anticipated to yield applicable results within a medium-term horizon, in any case before the end of the project. We expect the developed meta-logical program will be able to check the security of other cryptographic primitives developed in WP4.

Our fourth objective, which is longer-term oriented, is to extend the techniques used in the area of secure quantum secret keys distribution to other cryptographic primitives and other fundamental security protocols including:

• quantum stream cipher scheme
• message authentication code, identification
• digital signature (non-repudiation)
• fair exchange protocols
• delayed authorization for key exchange
• coin tossing, string flipping, string commitment.

Research on these topics will essentially be theoretical, but if necessary we may consider the possibility of a practical realization in the laboratory in order to validate our theoretical ideas.