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Saugata Basu, University College, London

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Simone Severini, Institute for Quantum Computation, University of Waterloo

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Guruprasad Kar, Indian Statistical Insitute, Kolkata

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Jozef Gruska, Masaryk University, Brno, Czech Republik.

Title: FROM CLASSICAL CRYPTOGRAPHY to QUANTUM PHYSICS through QUANTUM CRYPTOGRAPHY

ABSTRACT

Quantum cryptography, as an area of science and technology, should be seen both as an attempt to develop a new, and more adequate, theory of broadly understood cryptography, and new cryptographic tools and technologies and also as a new way to get a deeper insight into the physical world, into its basic concepts, models, laws and limitations. Quantum cryptography, in a broad sense, should be seen as an area of science that brings also new paradigms, goals, value systems, concepts, methods and tools to exploit (quantum) physical world. Development of security providing technology has been the original goal of quantum cryptography, but its implications and contributions for the study of the physical and especially quantum world are far reaching. The goal of the talk is first to take a broader view of classical and quantum cryptography and then to discuss impacts of quantum cryptography to quantum physics (understanding). The talk will present main developments in the process Classical Cryptography ---- Quantum Cryptography - Quantum Physics

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Andreas Winter, University of Bristol

Title: "Information-theoretic uncertainty relations - conjectures and examples"

ABSTRACT

In this talk we will look at formulations of the quantum theoretic uncertainty principle in terms of information quantities, in particular entropies of mutually incompatible observables. There is considerable previous work, concentrating on the case of two observables, in particular the case of "conjugate" variables. One of the strongest and most versatile results there is an inequality due to Maassen and Uffink for maximal measurements in arbitrary finite dimension. More recent, and partly motivated by quantum information theory (the phenomenon of "information locking" and quantum cryptography), is the interest in similar trade-off relations for three or more observables. I will survey what is known about these, drawing in recent joint work with P Hayden, D Leung and P Shor (quant-ph/0307104), ongoing work of A Ambainis on mutually unbiased bases, as well as joint work with Stephanie Wehner on the case of arbitrary numbers of two-valued observables. The main purpose is to highlight the open questions and to show where the limitations on possible answers lie.

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V. Subramaniyam, IIT Kanpur

Title: Spin decohenrence in quantum dots.

ABSTRACT

The decoherence of spin states of electrons interacting with nuclear spins in quantum dots has been studied. In the case of single-qubit states, an effective magnetic field (a feature of unitary evolution) and an effective temperature (a non-unitary feature) describe the effective dynamics of the qubit. The decoherence time scale is calculated as a function of the nuclear spin distribution and the nuclear polarizations in various spin channels. The averaged auto correlation function of the qubit spin, averaged over all possible initial states of the qubit, depends on the nuclear spin distribution, but is insensitive to the nuclear polarizations. In the case of two-qubit states, the time evolution is more complicated. There are two physically different situations, viz. either both qubits see the same nuclear spin environment or each qubit interact with a different nuclear bath, depending on the overlap of the spatial wave functions of the two electrons. The decoherence time scale is determined as a function of the bath-spin distribution and the polarizations of the initial two-qubit state. States with large decoherence times are identified by performing a minimization over all the two-qubit pure states.

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D Goswami, IIT Kanpur

Title: Probing coherence aspects of adiabatic quantum computation

ABSTACT

Typical problems in experimental implementation of quantum computing exist in the complexity of the experimental setup and in scaling the number of qubits. Various implementation approaches are being pursued to find ways to circumvent such problems. Optical approaches could be attractive but for the rapid decoherence time scales involved. We show that quantum interference between multiple excitation pathways can be used to cancel the couplings to the unwanted, nonradiative channels resulting in robustly controlling decoherence through adiabatic coherent control approaches. We propose a useful quantification of the two-level character in a multilevel system by considering the evolution of the coherent character in the quantum system as represented by the off-diagonal density matrix elements, which switches from real to imaginary as the excitation process changes from being resonant to completely adiabatic. Such counterintuitive results can be explained in terms of continuous population exchange in comparison to no population exchange under the adiabatic condition.

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F. Guinea, ICMM, Madrid

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F. Sols, Computensa, Madrid

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R. Srikanth, Poornaprajna Institute of Scientific Research, Bangalore

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