QKD based on symmetric entangled Bernstein-Vazirani

This paper introduces a novel entanglement-based QKD protocol, that makes use of a modified symmetric version of the Bernstein-Vazirani algorithm, in order to achieve a secure and efficient key distribution. Two variants of the protocol, one fully symmetric and one semi-symmetric, are presented. In both cases, the spatially separated Alice and Bob share multiple EPR pairs, one qubit of the pair each. The fully symmetric version allows both parties to input a secret key from the irrespective location and, finally, acquire in the end a totally new and original key, an idea which was inspired by the Diffie-Hellman key exchange protocol. In the semi-symmetric version, Alice sends her chosen secret key to Bob (or vice versa). Furthermore, their performance against an eavesdropper's attack is analyzed. Finally, in order to illustrate the operation of the protocols in practice, two small scale but detailed examples are given.Comment: 16 pages, 8 figure

Quantum Tapsilou -- a quantum game inspired from the traditional Greek coin tossing game tapsilou

This paper introduces a new quantum game called Quantum Tapsilou that is inspired from the classical traditional Greek coin tossing game tapsilou. The new quantum game, despite its increased complexity and scope, retains the crucial characteristic of the traditional game, namely that of fairness. In the classical game, both players have $\frac { 1 } { 4 }$ probability to win. In its quantum version, both players have equal chances to win too, but now the probability to win varies considerably, depending on previous choices. The two most important novelties of Quantum Tapsilou can be attributed to its implementation of entanglement via the use of rotation gates instead of Hadamard gates, which generates Bell-like states with unequal probability amplitudes, and the integral use of groups. In Quantum Tapsilou both players agree on a specific cyclic rotation group of order $n$, for some sufficiently large $n$, which is the group upon which the game will be based, in the sense both players will pick rotations from this group to realize their actions using the corresponding $R_{ y }$ rotation gates. This fact is in accordance with a previous result in the literature showing that symmetric quantum games, where both players draw their moves from the same group, are fair

A 2 & 3 Player Scheme for Quantum Direct Communication

This paper introduces two information-theoretically secure protocols that achieve quantum secure direct communication between Alice and Bob in the first case, and among Alice, Bod and Charlie in the second case. Both protocols use the same novel method to embed the secret information in the entangled composite system of the players. The way of encoding the information is the main novelty of this paper and the distinguishing feature compared to previous works in the field. The advantage of this method is that it is easily extensible and can be generalized to a setting involving three, or even more, players, as demonstrated with the second protocol. This trait can be beneficial when two spatially separated players posses only part of the secret information that must be combined and transmitted to Alice in order for her to reveal the complete secret. Using the three player protocol, this task can be achieved in one go, without the need to apply a typical QSDC protocol twice, where Alice first receives Bob's information and afterwards Charlie's information. Another characteristic of both protocols is their simplicity and uniformity. The two player protocol relies on EPR pairs, and the three player protocol on GHZ triples, which can be easily prepared with our current technology. In the same vein, the local quantum circuits are similar or identical, and are easily constructible as they employ only Hadamard and CNOT gates

An entanglement-based protocol for simultaneous reciprocal information exchange between 2 players

Let us consider a situation where two information brokers, whose currency is, of course, information, need to reciprocally exchange information. The two brokers, being somewhat distrustful, would like a third, mutually trusted, entity to be involved in the exchange process so as to guarantee the successful completion of the transaction, and also verify that it indeed took place. Can this be done in such a way that both brokers receive their information simultaneously and securely, and without the trusted intermediary ending up knowing the exchanged information? This work presents and rigorously analyzes a new quantum entanglement-based protocol that provides a solution to the above problem. The proposed protocol is aptly named entanglement-based reciprocal simultaneous information exchange protocol. Its security is ultimately based on the assumption of the existence of a third trusted party. Although, the reciprocal information flow is between our two information brokers, the third entity plays a crucial role in mediating this process, being a guarantor and a verifier. The phenomenon of quantum entanglement is the cornerstone of this protocol, as it makes possible its implementation even when all entities are spatially separated, and ensuring that, upon completion, the trusted third party remains oblivious of the actual information that was exchanged

A Quantum Detectable Byzantine Agreement Protocol using only EPR pairs

In this paper, we introduce a new quantum protocol for Detectable Byzantine Agreement. What distinguishes the proposed protocol among similar quantum protocols, is the fact that it uses only EPR pairs, and, in particular, $\Psi^{ + }$ pairs. There are many sophisticated quantum protocols that guarantee Detectable Byzantine Agreement, but they do not easily lend themselves to practical implementations, due to present-day technological limitations. For a large number $n$ of players, GHZ $n$-tuples, or other more exotic entangled states, are not easy to produce, a fact which might complicate the scalability of such protocols. In contrast, Bell states are, undoubtedly, the easiest to generate among maximally entangled states. This will, hopefully, facilitate the scalability of the proposed protocol, as only EPR pairs are required, irrespective of the number $n$ of players. Finally, we mention that, even for arbitrary many players $n$, our protocol always completes in a constant number of rounds, namely $4$.Comment: Corrected typos, expanded proofs and added reference

On relating CTL to Datalog

CTL is the dominant temporal specification language in practice mainly due to the fact that it admits model checking in linear time. Logic programming and the database query language Datalog are often used as an implementation platform for logic languages. In this paper we present the exact relation between CTL and Datalog and moreover we build on this relation and known efficient algorithms for CTL to obtain efficient algorithms for fragments of stratified Datalog. The contributions of this paper are: a) We embed CTL into STD which is a proper fragment of stratified Datalog. Moreover we show that STD expresses exactly CTL -- we prove that by embedding STD into CTL. Both embeddings are linear. b) CTL can also be embedded to fragments of Datalog without negation. We define a fragment of Datalog with the successor build-in predicate that we call TDS and we embed CTL into TDS in linear time. We build on the above relations to answer open problems of stratified Datalog. We prove that query evaluation is linear and that containment and satisfiability problems are both decidable. The results presented in this paper are the first for fragments of stratified Datalog that are more general than those containing only unary EDBs.Comment: 34 pages, 1 figure (file .eps

Finite Automata Capturing Winning Sequences for All Possible Variants of the PQ Penny Flip Game

The meticulous study of finite automata has produced many important and useful results. Automata are simple yet efficient finite state machines that can be utilized in a plethora of situations. It comes, therefore, as no surprise that they have been used in classic game theory in order to model players and their actions. Game theory has recently been influenced by ideas from the field of quantum computation. As a result, quantum versions of classic games have already been introduced and studied. The P Q penny flip game is a famous quantum game introduced by Meyer in 1999. In this paper, we investigate all possible finite games that can be played between the two players Q and Picard of the original P Q game. For this purpose, we establish a rigorous connection between finite automata and the P Q game along with all its possible variations. Starting from the automaton that corresponds to the original game, we construct more elaborate automata for certain extensions of the game, before finally presenting a semiautomaton that captures the intrinsic behavior of all possible variants of the P Q game. What this means is that, from the semiautomaton in question, by setting appropriate initial and accepting states, one can construct deterministic automata able to capture every possible finite game that can be played between the two players Q and Picard. Moreover, we introduce the new concepts of a winning automaton and complete automaton for either player