Confirmed Invited Speakers

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Saugato Bose, University College, London

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

Abstract :

The spin state of electrons injected into quantum dots suffers decoherence due to a hyperfine interaction with the nuclear spins. The qubit and nuclear spins get entangled thhrough the time evolution. Thus, the quantum unitary dynamics of the state of the qubits (electron spin states) and the nuclear spins leads to a gaussian decoherence for the qubit. The decoherence times depend on the interaction strength, and the polarization strengths of the nuclear spin bath in various spin channels.

In the case of single-qubit states (a single electron injected into the quantum dot), 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 averaged auto correlation function of the qubit, 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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Simone Severini, Institute for Quantum Computation, University of Waterloo

Title : A widefield picture of finite mathematics in quantum information and nearby regions

Abstract : There is lots to study about configurations of a finite number of items, when dealing with Hilbert space of small dimension and discrete quantum mechanical dynamics. In other words, lots of finite mathematics. I will take an obviously incomplete widefield picture of some recent uses of finite mathematics in quantum information and nearby regions.

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Guruprasad Kar, Indian Statistical Insitute, Kolkata

Title : Causality, Bell’s inequality and quantum mechanics

Abstract :

In a general probabilistic theory, violation of Bell’s inequality (even by its algebraic maximum) may not imply violation of causality. But there is a limit (Tsirelson’s bound) on the quantum violation of Bell’s inequality. This limit is not just a consequence of Hilbert space formalism of quantum mechanics, but it has physical consequence. Violation of this limit by quantum mechanics would imply instantaneous signaling. This is shown by using the joint measurement formalism of quantum mechanics.

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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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Sibasish Ghosh, Institute of Mathematical Sciences, Chennai, India

Title : Classical simulation of two spin-s singlet correlations involving spin measurements

Abstract :

In this work, we have given a classical (i.e., which uses classical resources) protocol to exactly simulate quantum correlations implied by the two spin-s singlet state corresponding to all integer as well as half-integer spin values. The class of measurements considered here consists of only those each of which corresponds to a spin - observable. The required amount of classical communication (the classical resource used here) is found to be $\lceling {\rm log}_2 (s + 1) \rceling$, in the worst case scenario, where $\lceling x \rceling$ is the least positive integer greater than or equal to x. We have also described here about some variations of the above-mentioned protocol to study the optimality issue.

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Andreas Winter, University of Bristol

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 spin state of electrons injected into quantum dots suffers decoherence due to a hyperfine interaction with the nuclear spins. The qubit and nuclear spins get entangled thhrough the time evolution. Thus, the quantum unitary dynamics of the state of the qubits (electron spin states) and the nuclear spins leads to a gaussian decoherence for the qubit. The decoherence times depend on the interaction strength, and the polarization strengths of the nuclear spin bath in various spin channels.

In the case of single-qubit states (a single electron injected into the quantum dot), 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 averaged auto correlation function of the qubit, 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, Universidad Complutense de 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

Abstract :

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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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Sumathi Rao , HRI, Allahabad

Title :`Spintronics with NSN junction of one-dimensional quantum wires'

Abstract :

We demonstrate possible scenarios for production of pure spin current and large tunnelling magnetoresistance ratios from elastic co-tunnelling and crossed Andreev reflection across a superconducting junction comprising of normal metal-superconductor-normal metal, where, the normal metal is a one-dimensional interacting quantum wire. We show that there are fixed points in the theory which correspond to the case of pure spin current. We analyze the influence of electron-electron interaction and see how it stabilizes or de-stabilizes the production of pure spin current. These fixed points can be of direct experimental relevance for spintronics application of normal metal-superconductor-normal metal junctions of one-dimensional quantum wires. We generalise the study to include transport properties of a superconducting junction connected to many (N>2) quantum wires.

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Naresh Chandra , IIT Kharagpur

Title : GENERATION AND CHARACTERIZATION OF MATTER-MATTER AND MATTER-LIGHT QUANTUM CORRELATION IN THE FORM OF ENTANGLEMENT

Abstract :

Quantum correlation is known to manifest itself in different ways in different phenomena taking place in nature. The way it is manifested in quantum information (QI) [1] has come to be known as entanglement (E). The most profound difference [1] between the classical information and QI is that the later, unlike the former, is encoded in non-local correlation between the different parts of a physical system. This non-local correlation is represented [1] by E in QI. Two or more particles, with such non- local correlation between them, are known to be in an entangled (ie, non-separable) state [1]. Recent discoveries have shown that E is a resource [2], essential for QI science, and is also capable [3] of affecting macroscopic properties of systems in their solid phase. Experimental as well as theoretical studies for generation and/or characterization of E is one of the challenging open problems [4] in the field of QI.

One needs, at least, two particles to produce an entangled quantum state. Parametric down conversion [5] in quantum optics has so far been the most successful and widely used methods for producing a pair of entangled photons. Many applications of E, however, require non-separable states of two or more particles of matter, or a material particle and a photon. For, such E will be needed to test Bell’s inequalities and hitherto developed quantum communication protocols [1].

There are more than one processes (eg, photoionization, double photoionization, inner-shell pho- toionization followed by non-radiative/radiative decay of excited photoion, etc) in atomic and molecular physics which can produce two material particles (eg, one ion and one electron, two electrons), or one electron and a photon. Unlike in several other cases, the E between two particles, produced in such processes, is not imposed from outside, but naturally arises from the internal structure of the target controlled by the strong electrostatic Coulomb forces and weak spin-orbit interaction (SOI). Consequen-tially, E generated during such processes is least prone to the harmful effects like those of decoherence and dissipation.

E between the spins of an ion and an electron [6], or of two electrons [7], can be generated with [6(a), 7(a, b)] or without [6(b), 7(a, c)] SOI taken into account; but, E between spin of an electron and polarization of a photon is not possible [8] in the absence of SOI. The electrostatic E between (ie, a non-separable, spin-entangled state, generated without SOI, of) an ion and one electron, or two electrons, can readily be characterized [6(b), 7(a, c)] experimentally merely by measuring energies of its particles by an ion/electron spectrometer. But, the presence of SOI has very significant effects on ion-electron [6(a)], electron-electron [7(a, b)], and electron-photon [8] E making it anisotropic, tunable, and kinematics as well as dynamics dependent.

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T.P.Pareek , HRI, Allahabad

Title : Spintronics and quantum computation

Abstract : We discusses the solid state based spintronics and quantum computation. Prinmary focus will be on generating spin polarization from unpolarized source which can be used to intialize qbits and enhance the purity of outgoing beam in absence of magnetic filed. We develope a spin density matrix scattering theory. Using this we study different aspect of spintronics and quantum computation in solid state system.

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Sudhansu Mandal , IACS, Kolkata

Title : Spin transport in superconductors

Abstract : Superconductors with different form of heterostructures may serve as low temperature spintronic device. In this talk, I will discuss spin transport in different kinds of superconductor heterostructures. I shall also discuss how spin polarized Josephson current may be controlled in different systems.

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