QCQC-2007 will be held at Centre for Theoretical Studies, IIT Kharagpur during December 11-13, 2007
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Confirmed Invited Speakers

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Saugato Basu, University College, London

Title : Quantum Communication and Entanglement Distribution Through Spin Chains and Allied Systems

Abstract :

I will start with a brief introduction to the use of a spin chains for quantum communications. Following this, I will describe some methods to perfect these communications using certain reasonbable strategies when one uses chains in their ferromagnetic ground state. I will then briefly point out the use of antiferromagnetic spin chains for the same purpose. Next, I will discuss the use of a bosons hopping freely in a one dimensional lattice to generate entanglement between its ends. I will also describe the generation of entanglement from the ground state of a chain of qudits coupled by purely exchange interactions. I will also describe how spin chains act as interesting forms of quantum channels with memory in which the noise operators of the channel on a subsequent use depends on the state transmitted during an earlier use.

 

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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

Abstract :

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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Tobias Stauber, ICMM, Madrid

Title : Entanglement at the boundary of spin chains and of dissipative systems

Abstract :

We analyze the entanglement properties of spins (qubits) attached to the boundary of spin chains near quantum critical points, or to dissipative environments near a boundary critical point, such as Kondo-like systems or the dissipative two level system. In the first case, we show that the properties of the entanglement are significantly different from those for bulk spins. The influence of the proximity to a transition is less marked at the boundary. In the second case, our results indicate that the entanglementchanges abruptly at the point where coherent quantum oscillations cease to exist. The phase transition modifies significantly less the entanglement. We argue that this might be a general property of the entanglement of dissipative systems.

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F. Sols, Computensa, Madrid

Title : Emission of entangled electron pairs from superconductors

Abstract :

The emission of entangled electron pairs from a superconductor into a normal metal is investigated. The equivalence between hole Andreev reflection and two-electron emission is established, each picture corresponding to a different choice of normal metal vacuum. We analyze both the case of a tunnelling interface of arbitrary size and that of two point-like apertures. We show that the distance dependence of the non-locally entangled current is quite sensitive to the choice of tunnelling Hamiltonian. Finally, we investigate a resonant tunnelling structure that emits pair electrons in diverging directions, which solves the problem of spatial beam separation. The relation with current experimental expertise is discussed. E. Prada and F. Sols, Eur. Phys. J. B 40, 379 (2004); New J. Phys. 7, 231 (2005).

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R. Srikanth, Poornaprajna Institute of Scientific Research, Bangalore

Title :From quantum measurement to black hole evaporation: foundational problems in a computation theoretic and metamathematical light

Abstract :

A question of fundamental interest is that of the impact on physics of basic issues in computation theory, such as the \mbox{P vs NP} problem, and in metamathematics, such as G\"odel's theorem. Often it is not even clear where to look for their "footprints", or what questions to ask. Here we will try to argue that two well-known puzzles in quantum theory, the quantum measurement problem and the black hole information paradox, may well harbor difficulties which computation theoretic or metamathematical insights can shed light on. As one key observation, we note the close correspondence between the efficiency and power of abstract algorithms on the one hand, and physical computers on the other, and consider how this may resolve the quantum measurement problem. As a corrolary, we will be led to the view that the universe is not only computable, but also a polynomial place. In another problem, we will argue that the black hole information localization problem is a manifestation of a G\"odel-like inconsistent self-reference in the semiclassical theory of black hole evolution, and consider possible resolutions of the paradox based on this understanding. Finally, we speculate on how metamathematical considerations may constrain the ultimate degrees of freedom, which are possibly strings or something even more fundamental, and on what physical reality may be at the deepest level of structure.

 

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Bikas K. Chakrabarti, SINP, Kolkata

Title : Long Range Transverse Ising Antiferromagnets

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Arnab Das , SINP, Kolkata

Title : Quantum Annealing and Analog Quantum Computation

Abstract :

Here we review the recent developments in quantum annealing, which is a novel technique of finding the ground state of complex glassy systems with rugged energy landscape using quantum fluctuations. This can be utilize to construct a very general framework for approximate solution of hard optimization problems in a manner similar to that of the celebrated classical simulated annealing algorithm, where thermal fluctuations are employed. Quantum annealing paves a way towards realization of analog quantum computer, which is a promising and powerful complement to the gate-based digital quantum computation.

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Giuseppe E. Santoro , SISSA, Italy

Title : Computation and Optimization using Quantum Mechanics: Quantum annealing dynamics in a random Ising chain

Abstract :

I will start by briefly reviewing some recent work in the field of quantum annealing, alias adiabatic quantum computation. The idea of quantum annealing is doing optimization by a quantum adiabatic evolution which tracks the ground state of a suitable time-dependent Hamiltonian, where quantum fluctuation are slowly switched off. I will illustrate some applications of quantum annealing strategies, starting from textbook toy-models. I will then move to presenting a recent work of ours [1], where we perform an adiabatic real-time Schroedinger quantum dynamics study of a random Ising chain across its quantum critical point. The model investigated is an Ising chain in a transverse field with disorder present both in the exchange coupling and in the transverse field. The transverse field term is proportional to a function g (t) which is linearly reduced to zero in time with a rate $\tau^{-1}$, $\Gamma(t)=-t/\tau$, starting at $t=-\infty$ from the quantum disordered phase ($\Gamma=\infty$) and ending at $t=0$ in the classical ferromagnetic phase ($\Gamma=0$). We then present extensive numerical simulations for the residual energy $E_{\rm res}$ and density of defects $\rho_k$ at the end of the annealing, as a function of the annealing inverse rate $\tau$. Both the average $E_{\rm res}(\tau)$ and $\rho_k(\tau)$ are found to behave logarithmically for large $\tau$, but with different exponents, $[E_{\rm res}(\tau)/L]_{\rm av}\sim 1/\ln^{\zeta}(\tau)$ with $\zeta\approx 3.4$, and $[\rho_k(\tau)]_{\rm av}\sim 1/\ln^{2}(\tau)$. We propose a mechanism for $1/\ln^2{\tau}$-behavior of $[\rho_k]_{\rm av}$ based on the Landau-Zener tunneling theory and on a Fisher's type real-space renormalization group analysis of the relevant gaps at the critical point. The model proposed shows therefore a paradigmatic example of how an adiabatic quantum computation can become very slow when disorder is at play, even in absence of any source of frustration.

[1] T. Caneva, R. Fazio, and G.E. Santoro, arXiv:0706.1832 and Phys. Rev. B (to be published).

Work done in collaboration with Tommaso Caneva (SISSA, Via Beirut 2-4, I-34014 Trieste, Italy) and Rosario Fazio (SISSA, Via Beirut 2-4, I-34014 Trieste, Italy and NEST-CNR-INFM & Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy)

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© All rights reserved, Ujal Halder (ujal@cts.iitkgp.ernet.in), CTS, IIT Kharagpur