225 research outputs found
Weak randomness completely trounces the security of QKD
In usual security proofs of quantum protocols the adversary (Eve) is expected
to have full control over any quantum communication between any communicating
parties (Alice and Bob). Eve is also expected to have full access to an
authenticated classical channel between Alice and Bob. Unconditional security
against any attack by Eve can be proved even in the realistic setting of device
and channel imperfection. In this Letter we show that the security of QKD
protocols is ruined if one allows Eve to possess a very limited access to the
random sources used by Alice. Such knowledge should always be expected in
realistic experimental conditions via different side channels
Entanglement swapping between multi-qudit systems
We generalize the entanglement swapping scheme originally proposed for two pairs of qubits to an arbitrary number of systems composed from an arbitrary number of qudits. Each of the system is supposed to be prepared in a maximally entangled state of qudits, while different systems are not correlated at all. We show that when a set particles (from each of the systems particles are measured) are subjected to a generalized Bell-type measurement, the resulting set of particles will collapse into a maximally entangled state
Purification and correlated measurements of bipartite mixed states
We prove that all purifications of a non-factorable state (i.e., the state
which cannot be expressed in a form ) are
entangled. We also show that for any bipartite state there exists a pair of
measurements which are correlated on this state if and only if the state is
non-factorable.Comment: 4 revtex pages, to appear in Phys. Rev.
Trajectories in the Context of the Quantum Newton's Law
In this paper, we apply the one dimensional quantum law of motion, that we
recently formulated in the context of the trajectory representation of quantum
mechanics, to the constant potential, the linear potential and the harmonic
oscillator. In the classically allowed regions, we show that to each classical
trajectory there is a family of quantum trajectories which all pass through
some points constituting nodes and belonging to the classical trajectory. We
also discuss the generalization to any potential and give a new definition for
de Broglie's wavelength in such a way as to link it with the length separating
adjacent nodes. In particular, we show how quantum trajectories have as a limit
when the classical ones. In the classically forbidden regions,
the nodal structure of the trajectories is lost and the particle velocity
rapidly diverges.Comment: 17 pages, LateX, 6 eps figures, minor modifications, Title changed,
to appear in Physica Script
Etude de la toxicité des feuilles du pommier de Sodome du Sénégal et du tourteau de graines de thé chez le tilapia du Nil en aménagement piscicole
Les toxicitĂ©s des feuilles du pommier de Sodome du SĂ©nĂ©gal (Calotropis procera) et du tourteau de graines de thĂ© (Camellia sp) chez le tilapia du Nil (Oreochromis niloticus) en pisciculture ont Ă©tĂ© Ă©tudiĂ©es. La rĂ©alisation des rĂ©actions de caractĂ©risation chimique a rĂ©vĂ©lĂ© la forte prĂ©sence des coumarines et dâanthocyanes mais seulement des traces de saponosides dans les feuilles de C. procera. Cependant elle nâa pas rĂ©vĂ©lĂ© la prĂ©sence dâautres groupes particuliĂšrement ichtyotoxiques. Les rĂ©sultats ont notamment montrĂ© quâavec les macĂ©rĂ©s aqueux de feuilles fraĂźches du pommier de Sodome du SĂ©nĂ©gal, il faut une concentration de 10000 mg/l pour que 50% des poissons meurent en 4 heures 10 mn et 100% meurent en 5 heures 59 mn. Avec le tourteau de graines de thĂ©, 50% de morts des poissons sont obtenus en 2 heures 37 mn et 100% de morts en 3 heures 23 mn avec la concentration de 25 mg/l. Ceci pourrait expliquer la relative faible ichtyotoxicitĂ© des macĂ©rĂ©s aqueux de feuilles du pommier de Sodome du SĂ©nĂ©gal comparĂ©e Ă celle du tourteau de thĂ©.Mots clĂ©s : Feuilles de Calotropis procera, ichtyotoxicitĂ©, tourteau, graines Camellia sp, tilapia du Nile dâĂ©levage
Optimality of private quantum channels
We addressed the question of optimality of private quantum channels. We have
shown that the Shannon entropy of the classical key necessary to securely
transfer the quantum information is lower bounded by the entropy exchange of
the private quantum channel and von Neumann entropy of the ciphertext
state . Based on these bounds we have shown that decomposition
of private quantum channels into orthogonal unitaries (if exists) is optimizing
the entropy. For non-ancillary single qubit PQC we have derived the optimal
entropy for arbitrary set of plaintexts. In particular, we have shown that
except when the (closure of the) set of plaintexts contains all states, one bit
key is sufficient. We characterized and analyzed all the possible single qubit
private quantum channels for arbitrary set of plaintexts. For the set of
plaintexts consisting of all qubit states we have characterized all possible
approximate private quantum channels and we have derived the relation between
the security parameter and the corresponding minimal entropy.Comment: no commen
The Relativistic Quantum Motions
Using the relativistic quantum stationary Hamilton-Jacobi equation within the
framework of the equivalence postulate, and grounding oneself on both
relativistic and quantum Lagrangians, we construct a Lagrangian of a
relativistic quantum system in one dimension and derive a third order equation
of motion representing a first integral of the relativistic quantum Newton's
law. Then, we plot the relativistic quantum trajectories of a particle moving
under the constant and the linear potentials. We establish the existence of
nodes and link them to the de Broglie's wavelength.Comment: Latex, 18 pages, 3 eps figure
Fair and optimistic quantum contract signing
We present a fair and optimistic quantum contract signing protocol between
two clients that requires no communication with the third trusted party during
the exchange phase. We discuss its fairness and show that it is possible to
design such a protocol for which the probability of a dishonest client to cheat
becomes negligible, and scales as N^{-1/2}, where N is the number of messages
exchanged between the clients. Our protocol is not based on the exchange of
signed messages: its fairness is based on the laws of quantum mechanics. Thus,
it is abuse-free, and the clients do not have to generate new keys for each
message during the Exchange phase. We discuss a real-life scenario when the
measurement errors and qubit state corruption due to noisy channels occur and
argue that for real, good enough measurement apparatus and transmission
channels, our protocol would still be fair. Our protocol could be implemented
by today's technology, as it requires in essence the same type of apparatus as
the one needed for BB84 cryptographic protocol. Finally, we briefly discuss two
alternative versions of the protocol, one that uses only two states (based on
B92 protocol) and the other that uses entangled pairs, and show that it is
possible to generalize our protocol to an arbitrary number of clients.Comment: 11 pages, 2 figure
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