On the Scattering of Gravitons on Two Parallel D-Branes

I discuss the scattering of a graviton (or a dilaton, or an anti-symmetric tensor) on two parallel static Dp-branes. The graviton belongs to a type II string in 10D.Comment: 1+9 pages, TeX with macros include

Scattering Amplitudes and the CPT theorem in String Theory

We discuss the role of the CPT transformation in first quantized string theories, both on the world-sheet and in the space-time. We explicitly show that the space-time CPT theorem holds for all first-quantized (perturbative) string theories in a Minkowski background of even dimension $D>2$.Comment: LaTeX2e, 8 pages, talk given at the Conference "Gauge Theories, Applied Supersymmetry and Quantum Gravity", London, July 199

A Note on Softly Broken MQCD

We consider generic MQCD configurations with matter described by semi-infinte D4-branes and softly broken supersymmetry. We show that the matter sector does not introduce supersymmetry breaking parameters so that the most relevant supersymmetry breaking operator at low energies is the gaugino mass term. By studying the run-away properties of these models in the decoupling limit of the adjoint matter, we argue that these softly broken MQCD configurations fail to capture the infrared physics of QCD at scales below the gaugino mass scale.Comment: 10+1 pages, TeX. V2: small comments and acknowledgements added, conclusions unchange

Quantum cryptography with 3-state systems

We consider quantum cryptographic schemes where the carriers of information are 3-state particles. One protocol uses four mutually unbiased bases and appears to provide better security than obtainable with 2-state carriers. Another possible method allows quantum states to belong to more than one basis. The security is not better, but many curious features arise.Comment: 11 pages Revte

Softly Broken MQCD and the Theta Angle

We consider a family of non-supersymmetric MQCD five-brane configurations introduced by Witten, and discuss the dependence of the curves on the microscopic Theta angle and its relation with CP. We find evidence for a non-trivial spectral-flow of the curves (vacua) and for the level-crossing of adjacent curves at a particular value of the Theta angle, with spontaneous breaking of CP symmetry, providing an MQCD analog of the phase transitions in Theta proposed by 't Hooft

Quantum key distribution for d-level systems with generalized Bell states

Using the generalized Bell states and controlled not gates, we introduce an enatanglement-based quantum key distribution (QKD) of d-level states (qudits). In case of eavesdropping, Eve's information gain is zero and a quantum error rate of (d-1)/d is introduced in Bob's received qudits, so that for large d, comparison of only a tiny fraction of received qudits with the sent ones can detect the presence of Eve.Comment: 8 pages, 3 figures, REVTEX, references added, extensive revision, to appear in Phys. Rev.

The effectiveness of quantum operations for eavesdropping on sealed messages

A quantum protocol is described which enables a user to send sealed messages and that allows for the detection of active eavesdroppers. We examine a class of eavesdropping strategies, those that make use of quantum operations, and we determine the information gain versus disturbance caused by these strategies. We demonstrate this tradeoff with an example and we compare this protocol to quantum key distribution, quantum direct communication, and quantum seal protocols.Comment: 10 pages, 2 figures. Third Feynman Festival, 25 -- 29 August 2006, University of Maryland, College Park, Maryland, U.S.

A Note on Interactions of (Non-Commutative) Instantons Via AdS/CFT

We consider the interaction between instantons and anti-instantons in four-dimensional N=4 super-Yang-Mills theory at large N and large 't Hooft coupling as described by D-instantons via AdS/CFT duality. We give an estimate of the strength of the interaction in various regimes. We discuss also the case of Non-Commutative super Yang-Mills theory where the interaction between instantons and anti-instantons can be used as a way to probe the locality properties of the theory in the supergravity picture, without explicit reference to the definition of local operators.Comment: harvmac, 13 p

Quantum Key Distribution between N partners: optimal eavesdropping and Bell's inequalities

Quantum secret-sharing protocols involving N partners (NQSS) are key distribution protocols in which Alice encodes her key into $N-1$ qubits, in such a way that all the other partners must cooperate in order to retrieve the key. On these protocols, several eavesdropping scenarios are possible: some partners may want to reconstruct the key without the help of the other ones, and consequently collaborate with an Eve that eavesdrops on the other partners' channels. For each of these scenarios, we give the optimal individual attack that the Eve can perform. In case of such an optimal attack, the authorized partners have a higher information on the key than the unauthorized ones if and only if they can violate a Bell's inequality.Comment: 14 pages, 1 figur