Welcome to PAMU, ISI

Forthcoming Seminars

28.02.2018,
at 3.00 pm

Seminar room, PAMU
(Kolmogorov Bhavan)

Dark Photon Searches and Bounds: Resonant Production in Beam Dump for 8Be anomaly

Speaker: Anish Ghoshal, Universit`a di Roma Tre and INFN, Italy

Abstract: Search for dark photon and axion-like particles will be described and present bounds on them will be shown. Such a candidate if responsible for 8Be anomaly in the ATOMKI experiment can be produced in PADME experiment in LNF-INFN. Sensitivity and parameter space concerned to this will be discussed.

Past Seminar List

15.02.2018, at 3.00 pm Seminar room, PAMU (Kolmogorov Bhavan)	Exotic pairs of stellar undeads: binary radio pulsars with compact companions Speaker: Manjari Bagchi, IMSc Chennai Abstract: Binary radio pulsars with white-dwarfs or other neutron stars or black holes as companions serve as excellent laboratories to test various aspects of basic physics. In this talk, I will explain how orbital and stellar properties (classical and general relativistic) of pulsars, can be probed by timing stable pulsars for long enough time. I will emphasize the prospects of putting more stringent constraints on alternative theories of gravity with the help of pulsar - black hole binaries (to be discovered). I will also discuss the prospect of constraining dense matter equation of state by timing neutron star-neutron star binaries. Finally, I will give a brief update on Indian efforts to detect nano-Hz gravitational waves by timing an ensemble of stable pulsars.
07.12.2017, at 4.15 pm Seminar room, PAMU (Kolmogorov Bhavan)	Granger Causality of Diffusion Processes Speaker: Prof. Dr. Jan Freund, Complex Systems Group, ICBM, University of Oldenburg, Germany Abstract: The reconstruction of causal interactions between process components from empirical data, viz. multivariate time series, is not possible from plain cross-correlation analyses but can be done in the context of Granger causality. Operational definitions are based on the assumption that multivariate time series can be viewed as realizations of a class of linear stochastic processes, usually vector autoregressive processes of order p (VAR[p]). For nonlinear dynamics this assumption may be far from justified and transfer entropy (predictive information flow) was devised to treat also strongly nonlinear dynamics – at the expense of diminished statistical power. Here I will present an alternative approach that is based on a local linearization of a nonlinear diffusion processes and that renders a Granger causality map which quantifies local prediction improvement on an attractor. This generalization may prove useful for a characterization of brain dynamics or for the analysis of multivariate data recorded at wind farms.
07.12.2017, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Critical Transitions Due to Shocks Speaker: Prof. Dr. Ulrike Feudel, Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University Oldenburg, Germany Abstract: Natural or technical systems possess often several possible stable states of operation. Linear stability theory is the appropriate tool to study the stability properties of such states with respect to small perturbations. However, in nature perturbations are not necessarily small but are finite in size. We discuss two different methods how to investigate the stability with respect to large perturbations such as single shocks. Both methods aim to determine the distance to the boundary of the basin of attraction or the edge of chaos, respectively. The first method determines the minimal destabilizing perturbation for large dynamical systems such as networks. Besides the size of this perturbations the method allows also to obtain the direction of this perturbation. We illustrate this method using pollinator networks in ecology and energy networks and identify relations between the topology of a network and its stability properties. The second method measures return times to a stable state at the edge of chaos. This is demonstrated for the transition from laminar to turbulent motion in a shear flow.
07.12.2017, at 3.00 pm Seminar room, PAMU (Kolmogorov Bhavan)	The Search For the Smallest Chimera Speaker: Prof. Tomasz Kapitaniak, Division of Dynamics, Lodz University of Technology, Lodz, Poland Abstract: We demonstrate that chimera behavior can be observed in small networks consisting of three identical oscillators, with mutual all-to-all coupling. Three different types of chimeras, characterized by the coexistence of two coherent oscillators and one incoherent oscillator (i.e., rotating with another frequency) have been identified, where the oscillators show periodic (two types) and chaotic (one type) behaviors. Typical bifurcations at the transitions from full synchronization to chimera states and between different types of chimeras have been described. Parameter regions for the chimera states are obtained in the form of Arnold tongues, issued from a singular parameter point. Our analysis suggests that chimera states can be observed in small networks relevant to various real-world systems.
25.10.2017, at 12.00 noon Seminar room, PAMU (Kolmogorov Bhavan)	Universal Laws of Thermodynamics Speaker: Manabendra Nath Bera, ICFO, Barcelona, Spain Abstract: Thermodynamics is one of the most successful physical theories ever formulated. Though it was initially developed to deal with steam engines and, in particular, the problem of conversion of heat into mechanical work, it has prevailed even after the scientific revolutions of relativity and quantum mechanics. Despite its wide range of applicability, it is known that the laws of thermodynamics break down when systems are correlated with their environments. In the presence of correlations, anomalous heat flows from cold to hot baths become possible, as well as memory erasure accompanied by work extraction instead of heat dissipation. Here, we generalize thermodynamics to physical scenarios which allow presence of correlations, including those where quantum entanglements are present. We exploit the connection between information and physics, and introduce a consistent redefinition of heat dissipation by systematically accounting for the information flow from system to bath in terms of the conditional entropy. As a consequence, the formula for the Helmholtz free energy is accordingly modified. Such a remedy not only fixes the apparent violations of Landauer's erasure principle and the second law due to anomalous heat flows, but it also leads to a reformulation of the laws of thermodynamics that are universally respected. In this information-theoretic approach, correlations between system and environment store work potential. Thus, in this view, the apparent anomalous heat flows are the refrigeration processes driven by such potentials.
19.09.2017, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Estimating hidden signals violating isotropy underlying CMB maps Speaker: Pavan Kumar Aluri, KIAS, South Korea Abstract: The standard model of cosmology is based on the cosmological principal which states that the universe is homogeneous and isotropic on very large scales. However cosmic microwave background which is a prediction of Big Bang theory is found to have signals of isotropy violation on large angular scales. One of the important aspects of CMB data analysis is to remove the effect of local microwave emission from our own galaxy in order to obtain pristine cosmic CMB signal. In this talk I will introduce a method to estimate hidden signals violating isotropy underlying CMB sky directly from a partial sky (masked CMB sky with some regions of the CMB sky set to zero to avoid bias due to residual contamination). The effect of masking on the estimation process is studied in detail, and an estimator is proposed to directly use the partial sky data accounting for the mask. We then applied it to PLANCK 2015 CMB maps as part of India/IUCAA Planck team. This method is also applicable to recover weak lensing field.
19.09.2017, at 11.30 am Seminar room, PAMU (Kolmogorov Bhavan)	State independent contextuality advances one-way communication Speaker: Debashis Saha, ITPA, University of Gdansk, Poland Abstract: Kochen-Specker theorem (more generally state independent contextuality (SIC)) is one of the most fundamental research areas in quantum physics. We are celebrating the 50th year of this discovery, and the role of quantum contextual correlations has been attracting an increasing attention in quantum information over the years, resulting in a rich variety of investigations. However, all the proposed applications of single system SIC lack in two important aspects. One is the universality of the quantum advantage for each SIC proof and more importantly, the advantages over classical system are in restricted scenarios instead of impossibility of a task itself. This talk presents a new perspective in this line of study by connecting SIC with the advantage of quantum channel over classical in one-way communication. We study a family of one-way communication problem based on orthogonal graphs of SIC sets of vectors. First, we reveal that if the dimension of the communicated system is bounded, sending quantum systems, which correspond to the vectors of each SIC set, is advantageous over classical communication. Then, we propose a general framework of oblivious communication and show that under certain oblivious condition for the same communication task, the quantum strategy corresponds to each Kochen-Specker set outperforms classical communication of arbitrary large dimensional system. The quantum protocol allows the parties to accomplish the communication task perfectly. In the case of classical communication, we present a general method to obtain the best possible strategy. Our results provide a fully operational significance to single system SIC and open up the possibility of quantum information processing based on that.
25.08.2017, at 3.00 pm Seminar room, PAMU (Kolmogorov Bhavan)	On thermalization of two-level quantum systems Speaker: Sibasish Ghosh, IMSc, Chennai Abstract: It has always been a difficult issue in Statistical Mechanics to provide a generic interaction Hamiltonian among the microscopic constituents of a macroscopic system which would give rise to equilibration of the system. One tries to evade this problem by incorporating the so-called $H-theorem$, according to which, the (macroscopic) system arrives at equilibrium when its entropy becomes maximum over all the accessible micro states. This approach has become quite useful for thermodynamic calculations using the (thermodynamic) equilibrium states of the system. Nevertheless, the original problem has still not been resolved. In the context of resolving this problem it is important to check the validity of thermodynamic concepts -- known to be valid for macroscopic systems -- in the microscopic world. Quantum thermodynamics is an effort in that direction. As a toy model towards this effort, we look here at the process of thermalization of a two-level quantum system under the action of a Markovian master equation corresponding to memory-less action of a heat bath, kept at certain temperature. A two-qubit interaction Hamiltonian (H_{th}, say) is then designed -- with a single-qubit mixed state as the initial state of the bath -- which gives rise to thermalization of the system qubit in the infinite time limit. We then look at the question of equilibration by taking the simplest case of a two-qubit system A + B, under some interaction Hamiltonian H_{int} (which is of the form of H_{th}) with the individual qubits being under the action of separate heat baths of temperatures T_A, and T_B. Different equilibrium phases of the two-qubit system are shown to appear -- both the qubits or one of them get cooled down. In the passing, we will also comment on thermalization of a qubit using non-Markovian evolution.
07.07.2017, at 3.00 pm Seminar room, PAMU (Kolmogorov Bhavan)	Periodically driven dynamics using two-rate protocols Speaker: Satyaki Kar,Theoretical Physics Department, Indian Association for the Cultivation of Science, Kolkata Abstract: We study the non-equilibrium dynamics in closed quantum systems periodically driven via time dependent parameters with two fiequencies ω₁ and ω₂ = rω. Tuning of the ratio r there can unleash plenty of dynamical phenomena to occur. Our study includes integrable models like Ising and XY models in d=1 and Kitaev model in d=1 and 2 and can also be extended to Dirac fermions in graphene. We witness the wave-function overlap or dynamic freezing to occur within some small/ intermediate frequency regimes in the (ω,r) plane (with r≠0) when the ground state is evolved through single cycle of driving. However, evolved states soon become steady with long driving and the freezing scenario gets rarer. We extend the formalism of adiabatic-impulse ipproximation for many cycles driving within our two-rate protocol and show the near-exact comparisons at small frequencies. An extension of the rotating wave approximation is also developed to gather an analytical framework of the dynamics at high frequencies. Finally we compute the entanglement entropy in the stroboscopically evolved states within the gapped phases of the system and observe how it gets tuned with the ratio r in our protocol. The minimally entangled states are found to fall within the regime of dynamical freezing. In general, the results indicate that the entanglement entropy in our driven short-ranged integrable systems follow genuine non-area law of scaling and show a convergence (with a r dependent pace) towards volume scaling behavior as the driving is continued for long time.
08.06.2017, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Fermionic dark matter and supernova SN1987A cooling Speaker: Prasanta Kumar Das, BITS Goa Abstract: Light dark matter (1 - 30 MeV) particles which can be pair produced in electron-positron annihilation e^-e⁺ → xx inside the supernova SN1987A core take away the energy released in the supernova SN1987A explosion. Using the Raffelt's criteria on the energy loss rate and using the optical depth criteria on the free streaming of the dark matter fermion, we find that the lower bound on the scale Λ of the dark matter effective theory to be Λ ∼ 1.0E+08 TeV for mx = 30 MeV. We extend our study in q-deformed statistics scenario and study the impact of it on the scale Λ.
20.04.2017, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Highlights from IceCube Neutrino Observatory Speaker: Dr. Debanjan Bose, Sung Kyun Kwan University, South Korea Abstract: IceCube is a cubic-kilometer neutrino observatory buried deep in the ice sheet at the geographic South Pole. A total of 5160 Digital Optical Modules (DOMs) are deployed on 86 strings forming a three dimensional detector array. Over the past decade, South Pole has emerged as a leading site for neutrino astronomy, particle astrophysics and neutrino physics. IceCube's discovery of a diffuse flux of astrophysical neutrinos started a new era of neutrino astronomy. In this talk I will describe IceCube neutrino detector and will discuss some of IceCube's results. Also, an overview will be given of plans to upgrade IceCube in the future.
06.01.2017, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Dense Axion Stars Speaker: Abhishek Mohapatra, The Ohio State University, USA Abstract: Axions can be described by a relativistic field theory with a real scalar field whose self-interaction potential is a periodic function of the field. Low-energy axions, such as those produced in the early universe, can be described more simply by a nonrelativistic effective field theory with a complex scalar field. I discuss how to determine the coefficients in the expansion of the effective potential in powers of the complex scalar field. I also discuss a possible way to systematically improve the effective potential that resums terms of all orders in the complex scalar field. A simple application of this effective potential is to axion stars. If the dark matter particles are axions, gravity can cause them to coalesce into axion stars, which are stable gravitationally bound systems of axions. The previously known solutions are dilute axion stars,whose mass cannot exceed a critical mass of about 10 to the power (-14) Solar mass. I discuss a possible new branch of dense axion stars whose mass can range from about 10 to the power (-20) Solar mass to about 1 Solar mass. If a dilute axion star with the critical mass accretes additional axions and collapses, it could produce a bosenova, leaving a dense axion star as the remnant.
05.01.2017, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Resolution of a Singularity Problem in Gravity Speaker: Dr. Anupam Mazumdar, Lancaster University, UK & Kapteyn Institute for Astronomy, The Netherlands Abstract: I will show how to resolve cosmological and blackhole singularity problems in the Einstein's theory of gravity at a classical level. In the infrared the theory mimics all the predictions of Einstein's General Relativity. I will then construct a potential quantum theory of gravity - which becomes asymptotically free in the ultraviolet.
02.01.2017, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Magnons in a honeycomb ferromagnet Speaker: Saikat Banerjee, Nordic Institute of Theoretical Physics, Nordita Abstract: The original discovery of the Dirac electron dispersion in graphene led naturally to the question of Dirac cone stability with respect to interactions, and the Coulomb interaction between electrons was shown to induce a logarithmic renormalization of the Dirac dispersion. With the rapid expansion of the list of compounds and quasiparticle bands with linear band touching, the concept of bosonic Dirac materials has emerged. At the single particle level, these materials closely resemble the fermionic counterparts. However, how the changed particle statistics affects the stability of Dirac cones has yet to be determined. Here we study the effect of interactions focusing on the honeycomb ferromagnet - where the quasiparticles are magnetic spin waves (magnons) with the same dispersion as the electrons for graphene. We demonstrate that magnon-magnon interactions lead to a significant renormalization of the bare band structure. The charge neutrality and Dirac spectrum of magnons result in finite lifetime effects with significant momentum dependence near the nodes and a temperature dependent shift of the magnon bands. We also address the question of the edge and surface states for a finite system. We applied these results to chromium tri-halides CrX3 (X = F, Cl, Br and I), the class of ferromagnets where the magnetic Cr atoms are arranged in weakly coupled honeycomb layers. Our theory qualitatively accounts for hitherto unexplained anomalies in neutron scattering data from 40 years ago for CrBr3. We expand the theory of ferromagnets beyond the standard Dyson theory and point to new exciting physics of Bose systems on non-Bravais lattices (e.g. honeycomb).
29.12.2016, at 2.00 pm Seminar room, PAMU (Kolmogorov Bhavan)	Vertical Structure of Disk Galaxies & their Dark Matter Halos Speaker: Dr Arunima Banerjee, IUCAA Abstract: According to the modern theory of galaxy formation, galaxies form because of cooling of baryons and star formation at the centres of gigantic halos of dark matter. The cold neutral hydrogen (HI) layer of the galactic disk serves as an effective tracer of the underlying gravitational potential of the dark matter halo in nearby, edge-on spiral galaxies. In the first part of the talk, I will discuss how the density profile of the dark matter halo can be constrained by using the observed HI rotation curve and the HI vertical thickness, as applied to the superthin low surface brightness galaxy UGC 7321, the Andromeda (M31) and our the Galaxy. In the second part, I will show how the superthin nature of the disk of the stars in the low surface brightness galaxy UGC7321 can be traced back to the presence of a dense and compact dark matter halo in this galaxy. Our findings may have important implications for the cosmology and structure formation history of the universe.
29.12.2016, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Noncommutative spaces with generalised uncertainty principle and their implications in quantum information theory Speaker: Dr Sanjib Dey, Centre de Recherches Mathematiques, Universite de Montreal, Canada Abstract: The existence of space-time noncommutativity is very promising in various fields of modern science. Several different types of noncommutative structure have been suggested depending on their significance in different contexts; such as, quantum gravity, quantum cosmology, string theory as well as quantum mechanics. We discuss about the construction of one such noncommutative framework originating from a mathematical background. The physical reality of such models are ensured by exploiting some standard techniques of PT-symmetric non-Hermitian Hamiltonian systems, which is yet another interesting subject in recent days. Nevertheless, we study some interesting applications of this particular noncommutative structure in different areas of mathematics and physics, particularly, in quantum optics and quantum information theory.
04.05.2016, at 3.30 pm Seminar room, PAMU (Kolmogorov Bhavan)	Existence and Uniqueness of solution of ODE; an overview Speaker: Swarup Poria, Department of Applied Mathematics, University of Calcutta Abstract: The study of existence and uniqueness of solutions became important due to the lack of a general formula for solving nonlinear ordinary differential equations (ODE). Compact form of existence and uniqueness theory appears nearly 200 years after the development of the theory of differential equation. In this talk, we shall discuss briefly about existence and uniqueness of solutions of a first order ODE. Special emphasis will be given on the Lipschitz continuous functions.
22.03.2016, at 3.00 pm Seminar room, PAMU (Kolmogorov Bhavan)	A necessary condition for local distinguishability of two-qudit maximally entangled states completely characterises that of the generalized Bell states for $d = 4$ Speaker: Prof. Sibasish Ghosh, IMSC, Chennai Abstract: The (im)possibility of local distinguishability of orthogonal multipartite quantum states still re- mains an intriguing question. Beyond ${C\!\!\!I}^3 \otimes {C\!\!\!I}^3$, the problem remains unsolved even for maximally entangled states (MES). We develop a very simple necessary condition for the perfect local distin- guishability of any set of MES in ${C\!\!\!I}^d \otimes {C\!\!\!I}^d$. This condition places constraints after the first round of measurement of the LOCC protocol, and, by doing so, reduces the complexity of the distin- guishability problem, particularly for any set of $d$ no. of MES. This necessary condition solves the longstanding problem of the existence of $d$ locally indistinguishable orthogonal states taken from the set of generalized Bell states in ${C\!\!\!I}^d \otimes {C\!\!\!I}^d$ by showing that for d = 4, there indeed exist four locally indistinguishable generalized Bell states. The full classification of sets of four generalized Bell states in ${C\!\!\!I}^4 \otimes {C\!\!\!I}^4$ has been made, and it is shown that any four states from such a set are either locally indistinguishable or distinguishable by one-way LOCC using only projective measurements.
06.01.2016, at 3.00 pm Platinum Jubilee Auditorium	The Neutrino Story Speaker: Sreerup Raychaudhuri, Tata Institute of Fundamental Research Abstract: Neutrinos are the most mysterious particles known and many aspects of neutrinos still remain a mystery. This talk will survey, in simple,nontechnical language, the physics of these elusive particles, starting from their discovery in nuclear beta decay processes to the discovery of neutrino flavours and, most importantly, flavour oscillations which won the 2015 Nobel Prize. The role of neutrinos in astrophysics and cosmology will also be touched upon At the end, we will briefly discuss the upcoming Indian Neutrino Observatory and its physics.
21.12.2015, at 3.00 pm PAMU Seminar Room (Kolmogorov Bhavan)	Dynamical State Reduction Models: Pedagogic Introduction and Advantages Speaker: Sujoy K. Modak, JSPS Fellow, KEK High Energy Accelerator Research Organization, Japan Abstract: Dynamical State Reduction Models are modified version of quantum mechanics which were historically formulated to overcome certain problems associated with the Copenhagen interpretation of quantum mechanics. I shall discuss one promising version of them, known as ``Continuous Spontaneous Localization’’ (CSL) theory. Over the years they have evolved to give predictions that are verifiable/falsifiable. In recent times these theories were also used to explain generation of seeds of cosmic structures during inflation which is otherwise not understood. I shall briefly review these aspects and then mostly concentrate on my own work where a resolution to black hole information paradox is proposed using a tailored version of CSL theory.
08.10.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Action Principle for General Relativity and its Relation to Null Surfaces Speaker: Sumanta Chakraborty, IUCAA Abstract: Constructing a well-posed variational principle and characterizing the appropriate degrees of freedom that need to be fixed at the boundary are non-trivial issues in general relativity. I will discuss a few toy examples in classical mechanics and field theory before going into general relativity. For spacelike and timelike boundaries I will show that the action principle for general relativity is well posed, only when a suitable counter-term [the Gibbons-Hawking-York (GHY) counter-term] is added to the action principle. Also I will show that the degrees of freedom to be fixed on the boundary are contained in the induced 3-metric. These results, however, do not directly generalize to null boundaries on which the 3-metric becomes degenerate. In this talk I will address the following questions: (i) What is the counter-term that may be added on a null boundary to make the variational principle well-posed? (ii) How do we characterize the degrees of freedom which need to be fixed at the null boundary?
18.09.2015, at 2.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Flat space holography: field theory and gravitational aspects Speaker: Dr. Rudranil Basu, IISER Pune Abstract: The holographic principle, as inspired by string theory is an important concept that has helped in better understanding of various unsolved physical problems which range from strongly interacting quantum field theories to quantum gravity. Much of developments in this field however has been in the context of negatively curved space-time, ie in the guise of AdS/CFT duality. One side of the duality deals with a space-time with gravitational interactions whereas the other side describes a quantum field theory living at the boundary of that space-time. We, on the other hand, are studying how the principle of holography can work for physically relevant flat (as opposed to negatively curved) space-time scenarios. In this talk I will describe advances made both in the gravity side as well as the field theory side. Curiously enough, the field theory, although conformal in nature, is one observed by very fast observers.
15.09.2015, at 2.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Search for Supersymmetry: Some avenues less explored Speaker: Dr. Arindam Chatterjee, IISER Pune Abstract: After the discovery of Higgs at LHC, search for Beyond Standard Model (BSM) Physics is in full swing. Supersymmetric theories provide a well motivated extension of the Standard Model. Apart from addressing the hierarchy problem, R-parity conserving supersymmetric scenarios also assure the stability of the Lightest Supersymmetric particle (LSP). The LSP, if neutral, can be a good Dark Matter (DM) candidate. Therefore, the nature of the LSP, in such scenarios, can be probed or constrained using both Dark Matter (DM) and collider searches. At LHC, the decay of strongly interacting sparticles, which can have large production cross-sections, can lead to different signals for different LSPs. We consider two different scenarios in this context: one featuring a left-sneutrino as the LSP and the other with a neutralino as the LSP. We show that in both cases distinct collider signals from the stop sector may be observed in the (near) future. The viability of these LSPs as Dark Matter candidates are also explored. In the second part of the talk, we will discuss the possibility of accommodating an inflaton candidate within the minimal supersymmetric model in the context of ``inflection-point inflation". Further, we will comment on generic features and constraints on these scenarios in the light of recent data from PLANCK and BICEP.
19.08.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Quantum Rational Secret Sharing Speaker: Dr. Goutam Paul, Cryptology and Security Research Unit, ISI Abstract: A rational secret sharing scheme is a game in which each party responsible for reconstructing a secret tries to maximize his or her utility by obtaining the secret alone. While there exist schemes for classical rational secret sharing, quantum secret sharing schemes, derived either from quantum teleportation or from quantum error correcting code, do not succeed when we assume rational participants. This is because all the existing quantum secret sharing schemes consider that the secret is reconstructed by a party chosen by the dealer. We, for the first time, propose a quantum secret sharing scheme which is resistant to rational parties. The proposed scheme achieves fairness, correctness and strict Nash equilibrium.
08.07.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Collective dynamics of non-linear oscillators Speaker: Dr. Punit Parmananda, Physics Department, IIT Bombay Abstract: The dynamics of the Mercury Beating Heart (MBH) system is analyzed experimentally. Subsequently, two such oscillators were coupled and the resultant synchronization phenomena studied. Finally, the collective dynamics for an ensemble of MBH oscillators were investigated, and Kuramoto transitions were verfied experimentally. The dynamics of the Mercury Beating Heart (MBH) system is analyzed experimentally. Subsequently, two such oscillators were coupled and the resultant synchronization phenomena studied. Finally, the collective dynamics for an ensemble of MBH oscillators were investigated, and Kuramoto transitions were verfied experimentally.
01.06.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Associated Higgs production with a Z boson in the Non-Commutative Standard Model Speaker: Dr. Prasanta Kumar Das, BITS-Pilani Abstract: We Study the associated Higgs production with Z boson at future linear colliders in the framework of the minimal non-commutative standard model. Using the Seiberg-Witten map, we calculate the production cross-section considering all orders of the non-commutative parameter Θμν. We consider the effect of earth’s rotation on the orientation of θμν with respect to the laboratory frame and thus on the total cross-section, it’s azimuthal distribution etc for the machine energy ranging from 0.5 TeV to TeV corresponding to the non-commutative scale Λ ≥ 0.5 TeV.
13.05.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Universal detection of bi-partite entanglement in measurement-device-independent way Speaker: Prof. Sibasish Ghosh, IMSc Abstract: Whenever a source produces a bi-partite state, how to verify experimentally whether the state is entangled? One generally performs measurement -- to be realized locally on the individual subsystems -- of some observable (called as entanglement witness operator (EWO)) on the state for this verification. Unfortunately, the detectors themselves may not be reliable to the extent that even a separable state may be revealed as an entangled one. Is it at all possible to witness entanglement in a measurement-device-independent (MDI) way? The answer turned out to be in the affirmative through a recent work by Branciard et al. [Phys. Rev. Lett., vol. 110, pp. 060405 (2013)], provided a form of an EWO, witnessing the entanglement in the `given' state is known beforehand. It is important to note here that any such EWO depends generally, on the given state with an exception in the case of two-qubit states. In fact, it was shown by Augusiak et al. [Phys. Rev. A, vol. 77, pp. 030301 (2008)] that entanglement in an arbitrary two-qubit state can be witnessed by a state-independent EWO provided four copies of such a state are available. Using this latter result, we give a scheme here to detect entanglement in any two-qubit state in a state-independent but MDI way. We also provide here a scheme to distinguish between PPT and NPT states in state-independent but MDI way -- valid for any dimension. Finally, we conjecture that entanglement in any bi-partite state can always be witnessed in a state-independent but MDI way, whatever be the dimension of the system.
11.05.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	PT symmetric optical lattices : some intriguing features Speaker: Dr. Anjana Sinha, Jadavpur University Abstract: With the experimental verification of PT symmetric concepts in optics, PT symmetric optical lattices have opened up a very interesting area of study in the last few years. These systems have complex refractive index of the form n(x) = n0+nR(x)+inI(x) with nR,I less than n0 . The intriguing behaviour of the scattering coefficients — reflection \|T\| and transmission \|R\| — is quite different from what we observe in conventional quantum me- chanics. The scattering amplitudes do not necessarily add up to unity. Rather, they obey a modified relationship. Furthermore, their behaviour depends on the direction of the incident wave, and also on the parameters in the optical potential. In case of spontaneous breakdown of PT symmetry at some value of the potential parameter, one observes the fascinating phenomena of Spectral Singularity, where reflection and transmission coeffi- cients tend to diverge. The talk will discuss some interesting features related to scattering in PT symmetric optical devices.
08.05.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Constraining the Nature of Dark Energy from Cosmological Observations Speaker: Dr. Ujjaini Alam Abstract: Dark energy is one of the most tantalizing mysteries in current cosmological research. A host of observations confirm that about two-thirds of the energy content of the universe comprises of this negative-pressure ''dark energy'' component that causes the expansion of the universe to accelerate. In my talk, I will explore two distinct facets of dark energy research. Many different theoretical models have been suggested for dark energy. One aspect of my research is to study these theoretical models, such as early dark energy and modified gravity models, and constrain them in light of current observations. A second approach to the dark energy problem is to study the various observations available to us with different statistical tools, and optimize these methods for obtaining maximum information on the dark energy parameters. Using these two complementary approaches in conjunction, we expect to shed light on the nature of dark energy.
27.04.2015, at 3.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Exploring Novel Quantum Phenomena in Photonic Settings: From Fundamentals to Technological Applications Speaker: Dr. Somnath Ghosh, Calcutta University Abstract: Corroborating the analogy between non-Hermitian quantum system and counterpart optical geometries with suitable amount of simultaneous gain and loss, I plan to discuss an innovative scheme for asymmetric mode conversion in a coupled optical system under certain condition in the strong coupling regime (beyond the PT symmetry limit) exploiting singularities (in eigen values and eigen vectors) associated with avoided crossings (in the regime where adiabatic evolution fails) as an efficient tool. Novel propagation dynamics of light wave through this special optical structure is evident, which is being explored in the context of optical isolation for integrated/ on-chip photonics applications. From fundamental physics point of view and applications in photonics like realizing compact random lasing in one dimensional disordered system (longitudinal direction mapped onto time), I plan to focus on my latest findings to demonstrate that the simultaneous presence of spatial and refractive index disorder favors Anderson localization of light. This study has revealed several underlying interesting features of light confinement to a localized state in such a medium of finite length and showed that beyond the point of localization, light indeed propagates without any diffractive spread in the transverse direction in a disordered lattice, a feature that mimics waveguide-like propagation. The hallmark stochastic nature of the phenomena has been encountered in both simulations and experiments using ultrafast laser inscription (ULI) technique.
24.04.2015, at 12.30 pm PAMU Seminar Room (Kolmogorov Bhavan)	Generating Finite Dimensional Integrable Nonlinear Dynamical Systems and their Quantization Speaker: Prof. M. Lakshmanan, FNA,FTWAS, Centre for Nonlinear Dynamics, Bharathidasan University, Tiruchirapalli Abstract: We present an overview of some recent progress made in identifying and generating finite dimensional integrable nonlinear dynamical systems, exhibiting oscillatory and other solution properties, including quantum aspects. Particularly we will concentrate on Lienard type nonlinear oscillators and their generalizations and their coupled versions. Specific systems include Mathews-Lakshmanan oscillators, modified Emden equations, isochronous oscillators and generalizations. Nonstandard Lagrangian and Hamiltonian formulations of some of these systems will be briefly touched upon. Nonlocal transformations and linearization aspects will be briefly discussed.
21.11.2014, at 3.00 pm NAB 2 (Kolmogorov Bhavan)	Ancilla assisted suppression of decoherence Speaker: Sibasish Ghosh, IMSc Chennai Abstract: Most of the quantum mechanical systems are open in nature -- they often interact with their surroundings quite fast, leading to decoherence. This issue of decoherence is a major obstacle in physical realisations of different quantum information processing schemes. It not only destroys quantum coherence in individual subsystems, but as a result of that, it also (in general), destroys entanglement among the subsystems. Different mechanisms have been developed -- mostly specific to systems concerned -- to control this decoherence effect, most of which aim at weakening the system--environment interaction in a controlled manner. In the present work, we instead, aim at inserting an ancillary system so that the environment acts jointly on the system and the ancilla. Considering Markovian type environmental effect, we find that one can suppress both decoherence as well as decay of entanglement by this method. Moreover, we find that larger the size of the ancilla, better is the suppression mechanism.
11.11.2014, at 3.00 pm NAB 2 (Kolmogorov Bhavan)	Numerical Simulation with relativistic equation of state of astrophysical plasma Speaker: Indranil Chattopadhyay, Aryabhatta Research Institute for observational sciences (ARIES) Abstract: We discuss computational fluid dynamics in general and how they are applied in various astrophysical scenario. We focus on how to incorporate the relativistic equation of state of multi-species fluid in relativistic hydrodynamic code. We show that in the trans-relativistic temperature regime, composition plays a very important role in the temporal evolution of relativistic hydrodynamics. We discuss the impact this would have on observations.
07.11.2014, at 3.00 pm NAB 2 (Kolmogorov Bhavan)	Spinor field and anisotropic cosmological models Speaker: Bijan Saha, D.Sc, Joint Institute for Nuclear Research Dubna, Moscow reg. Abstract: Within the scope of a Bianchi type-I anisotropic cosmological model we study the evolution of the Universe filled with nonlinear spinor field. It is found that due to some specific behavior of the spinor field it imposes some restriction either of spinor field components or on metric functions. It is found that for some suitable choice of parameters the spinor field can give rise to singularity free spacetime; accelerate the isotropization process; explain the late time accelerated mode of expansion and simulate perfect fluid, quintessence, Chaplygin gas and different type of Dark energy.
28.08.2014, at 3.30 pm NAB 1 (Kolmogorov Bhavan)	Dynamical Localization in quantum spin-fermion systems: Suppressing the Relaxation of quantum magnets by a coherent periodic drive Speaker:Dr. Analabha Roy , SINP. Abstract: Dynamical localization is one of the most startling manifestations of quantum interference, where the evolution of a simple system is frozen out under a suitably tuned coherent periodic drive. In this talk, I shall demonstrate that such freezing occurs even in the presence of extensive disorder in a many-body system. I consider a disordered quantum Ising chain where the transverse magnetization relaxes exponentially with time with a decay time-scale [image: \tau] due to random longitudinal interactions between the spins. Using Floquet theory and Renormalization Group techniques, I will show that this relaxation can be slowed down ([image: \tau] is enhanced) by orders of magnitude due to employing periodic drives at certain specific values of frequencies and amplitudes (the freezing condition) regardless of the initial state. Under the freezing condition, [image: \tau] diverges exponentially with the drive frequency [image: \omega]. The results can be easily extended to a larger family of disordered fermionic and bosonic systems. I will then discuss universality in the relaxation behavior, generalizations to open quantum systems, as well as using this phenomenon to prevent the propagation of errors in systems of quantum bits.
23.07.2014, at 15.30 PAMU Computer Lab (Kolmogorov Bhavan)	Minimal state-dependent proof of measurement contextuality for a qubit Speaker: Dr. Sibasish Ghosh, IMSc, Chennai . Abstract: We show that three unsharp binary qubit measurements are enough to violate a generalized noncontextuality inequality, the LSW inequality, in a state-dependent manner. For the case of trine spin axes we calculate the optimal quantum violation of this inequality. Besides, we show that unsharp qubit measurements do not allow a state-independent violation of this inequality. We thus provide a minimal state-dependent proof of measurement contextuality requiring one qubit and three unsharp measurements. Our result rules out generalized noncontextual models of these measurements which were previously conjectured to exist. More importantly, this class of generalized noncontextual models includes the traditional Kochen-Specker (KS) noncontextual models as a proper subset, so our result rules out a larger class of models than those ruled out by a violation of the corresponding KS-inequality in this scenario.
09.07.2014, at 15.00 PAMU Computer Lab (Kolmogorov Bhavan)	Resurrecting left-sneutrino Dark Matter in SUSY in the light of direct Dark Matter searches Speaker: Dr. Arindam Chatterjee , HRI Allahabad . Abstract: In the minimal supersymmetric standard model (MSSM) the lightest superpartner of the left-handed neutrinos is ruled out as a Dark Matter candidate because of its large elastic cross-section with the nucleus mediated via Z-boson. We resurrect it by extending the MSSM with two (heavy) triplets of opposite hyper-charge. The addition of these triplets not only generates small Majorana mass for the left-handed active neutrinos, but also makes sneutrino (LSP) a candidate for dark matter (of inelastic type). We then discuss the relevant parameter space which can give rise to the right amount of (thermal) relic abundance while satisfying the current direct detection constraints from Xenon100 and LUX.
25.06.2014, at 15.00 PAMU Seminar Room (Kolmogorov Bhavan)	Device-independent quantum key distribution based on Hardy’s paradox Speaker: Dr. Ramij Rahaman, University of Allahabad . Abstract: We present a secure device-independent quantum key distribution (QKD) scheme based on Hardy’s paradox. Hardy’s paradox uses four conditions impossible for classical systems, but satisfied by predictions for a unique quantum two-particle state. In comparison with protocols based on Bell inequalities, our scheme has several novel features: (a) The bits used for the secret key do not come from the results of the measurements on an entangled state but from the choices of settings which are harder for an eavesdropper to influence; (b) Instead of a single security parameter (a violation of some Bell inequality) a set of them is used to estimate the level of trust in the secrecy of the key. This further restricts the eavesdropper's options. We prove the security of our protocol for both ideal and noisy cases.
28.04.2014, at 15.30 NAB 1 (Kolmogorov Bhavan)	Applicability of First Law for Rindler Horizon Speaker: Dr. Srijit Bhattacharjee, Department of Physics, IISER Mohali. Abstract: The physical process version of the first law for black holes states that the passage of energy and angular momentum through the horizon results in change in area κ/8π ∆A = ∆E − Ω∆J, so long as the passage is quasistationary. The validity of the process first law crucially depends upon the control over the horizon shear and expansion such that they remain small. I will discuss how such control can be established in the case of a weakly self-gravitating object crossing Rindler horizon.
15.04.2014, at 15.30 NAB 1 (Kolmogorov Bhavan)	Turbulence in mobile-bed streams Speaker: Prof. Subhasish Dey, Indian Institute of Technology Kharagpur. Abstract: The study is devoted to quantify the near-bed turbulence parameters in mobile-bed flows with bed-load transport. A reduction in near-bed velocity fluctuations due to the decrease of flow velocity relative to particle velocity of the transporting particles results in an excessive near- bed damping in Reynolds shear stress (RSS) distributions. The bed particles are associated with the momentum provided from the flow to maintain their motion overcoming the bed resistance. It leads to a reduction in RSS magnitude over the entire flow depth. In the logarithmic law, the von Kármán coefficient decreases in presence of bed-load transport. The turbulent kinetic energy budget reveals that for the bed-load transport, the pressure energy diffusion rate near the bed changes sharply to a negative magnitude, implying a gain in turbulence production. According to the quadrant analysis, sweep events in mobile-bed flows are the principal mechanism of bed-load transport. The universal probability density functions for turbulence parameters given by Bose and Dey have been successfully applied in mobile-bed flows..
06.02.2014	A short overview on Higgs physics at the Large Hadron Collider. Speaker: Dr Siba Prasad Das, Institute of Physics , Bhubaneswar.
Sept. 17, 2013	Turbulence in Loose Boundary Streams Speaker: Sankar Sarkar, Sikkim Manipal Institute of Technology , INDIA.
Sep. 6, 2013	Exact results in two dimensional hydrodynamics with gauge and gravitational anomalies. Speaker: Prof. Rabin Banerjee, SNBNCBS Kolkata,, India. Abstract: We give the exact constitutive relations that express the stress tensor and the charged current in terms of the fluid variables in two dimensional hydrodynamics in the presence of gauge and gravitational anomalies. We distinguish between anomalies in chiral and nonchiral theories. The interplay between conformal invariance and diffeomorphism invariance is highlighted. The modifications in the ideal fluid constitutive relations in the presence of chirality are illustrated.
June 04, 2013	The non-linearity of the Cepheid Period-Luminosity relation. Speaker: Prof. Shashi Kanbur, SUNY Oswego, USA. Abstract: The Cepheid Period-Luminosity relation is of paramount importance in developing an extra-galactic distance scale that is independent of the Cosmic Microwave Background. Developing such an independent distance scale that is accurate to less than 3% is important in placing constraints on the dark energy equation of state. For the last 70 years, the assumption has been that the PL relation is linear. Here we review recent evidence supporting a nonlinear PL relation, the impact on estimates of Hubble's constant and on stellar pulsation and also comment on recent advances in the statistical description of the quantitative structure on variable star light curves.
May 9, 2013	Art of writing academic article and modern trends in publishing. Speaker: S. K. Venkateshan, TNQ Books and Journals, India. Abstract:
May 10, 2013	Turbulance in fluid flows and the breaking of implicit symmetries. Speaker: S. K. Venkateshan, TNQ Books and Journals, India. Abstract:
February 25, 2013	Study of Astrophysical Sources in Very High Energy Regime Using Ground Based Gamma-ray and Neutrino Telescopes. Speaker: Dr. Debanjan Bose, Vrije Universiteit Brussel, Belgium. Abstract: In this presentation I will talk about sources like pulsars, AGN and GRBs in very high energy regime. At these energies we study non-themal Universe. From observations we know that relativistic effects are taking place inside these objects. These sources emit high energy gamma-rays and are also expected to emit neutrinos. I will explain detection principle of gamma-rays and neutrinos with earth based detectors. I will also discuss some important observations for the above mentioned sources with ground based Cherenkov telescopes and IceCube neutrino observatory.
January 10, 2013	Dark Energy Model Building and Observational Signatures Speaker: Dr. Anjan Ananda Sen, Jamia Millia Islamia, New Delhi, India. Abstract: In this talk I shall review the current research on the late time acceleration of the Universe, which is one of the most challenging problems for cosmologists at present. I shall review different approaches to explain such a late time acceleration, in particular the scalar field models. I shall also review the current status of the observational aspects for the dark energy models.
November 30, 2012	Statistical physics of glasses Speaker: Prof. Deepak Dhar, Tata Institute of Fundamental Research, Mumbai. Abstract: In this talk, after a general introduction to the problem of describing the glassy state, I will describe our work on a simple model where we explicitly take into account the fact that ergodicity is broken, and one should calculate averages of physical quantities using restricted partition functions, that sum only over accessible states of the system.
November 23, 2012	Black Holes in Your Bathtub Speaker: Dr. Tapas K Das, Harish Chandra Research Institute, Allahabad. Abstract: I plan to talk about the analogue gravity phenomena, where one can create certain astrophysical (black hole) and cosmological (Friedmann Robertson Walker expanding Universe) spacetime in the laboratory, using classical (water in a bathtub or wash basin, for example) or quantum (Bose Einstein Condensates or liquid Helium, for example) fluids. In a series of papers, I have recently developed, for the first time in the literature, an analytical model which studies the analogue spacetime embedded within the relativistic accreting fluid onto astrophysical black holes. This is a unique example of the classical analogue model naturally found in the Universe where the analogue space time is studied in the strong gravity environment. Such a configuration is the only analogue system available so far which contains both the gravitational as well as the acoustic horizons.
November 15, 2012	Probing the Universe's first light: Statistical detection of reionization 21 cm signal Speaker: Dr. Kanan K. Datta, , Stockholm University, Sweden. Abstract: The cosmic reionization is one the most important missing pictures in the history of our Universe. It is believed that during this epoch the first sources of light (e.g first stars, quasars) formed in the Universe and they subsequently reionized the neutral hydrogen in the intergalactic medium. Radio interferometric observations of redshifted 21-cm radiation are considered to constitute the most promising tool to probe the reionization epoch. The first generation radio telescopes (e.g, GMRT, LOFAR, MWA) are trying to detect the signal statistically. I will discuss the statistical quantities that will be measured by such observations. The statistical detection of ionized regions around bright reionizing sources by means of a matched filter technique will also be discussed. At the end I will also discuss constraints on reionizing source parameters that can be achieved by instrument like LOFAR.
July 9, 2012	Parameter estimation and data analysis in Cosmology Speaker: Trina Chakraborty, Indian Statistical Institute, India Abstract: The observational evidence for the current accelerated expansion of the universe is presented. Emphasis has been laid on a number of scalar field dark energy models besides the conventional cosmological constant. To verify the signatures of dynamical dark energy models, diagnostic parameters will be discussed in details and how they are extracted out from observations. Further, methodology for estimation of various cosmological parameters from single dataset or combination of datasets will be presented.
July 9, 2012	Field theory and particle physics aspect of early universe Speaker: Sayantan Choudhury, Indian Statistical Institute, India Abstract: We propose two models of inflation in the framework of braneworld and standard cosmology. First starting from bulk supergravity we construct the inflaton potential on the brane and employ it to investigate for the consequences to inflationary paradigm. To this end, we derive the expressions for the important parameters in brane inflation, which are somewhat different from their counterparts in standard cosmology. We have studied extensively reheating phenomenology, which explains the thermal history of the universe and leptogenesis through the production of thermal gravitino. Next we propose another model of inflation originated from gauge invariant MSSM flat directions comprising of QQQL, QuQd, QuLe and uude. To this end, we further estimate the observable parameters for both the models and find them to fit well with recent observational data. Finally, we analyze one loop RG flow to determine the appropriate parameter space for inflation.
07-06-2012	Energy Efficiency in Architectural Designs in the Indus Valley Civilization: Lessons learnt for new designing Speaker: Satyajit Ghosh , University of Leeds, U.K. Abstract: The Indus Valley civilization flourished between 2750-1900 B.C. We present a case study for Lothal, an extension of the Indus Valley civilization currently located in the state of Gujarat in India. Their double storied buildings were clustered around geometrical grids comprising of three divisions, including a citadel, a middle town and a lower town. An on-site visit revealed that the buildings were suitably oriented for maximum solar gains. The standardised stone blocks used had thermal transmissivities ("U" values in today's architectural parlance) that retained indoor coolth substantially. The rectangular building forms ensured the presence of active and passive zones even then. As Mechanical engineers, we have used Autodesk Ecotect Analysis to calculate Daylight Factors, Solar Radiation and Water Usage. We have explored quantitatively the fascinating world of light and shadows, coolth and warmth, and obtained answers for the following questions: 1. Were the orientation and the clustering perfect in the city of Lothal? 2. Can these architectural designs prevail with some modifications in today's times? After all, modernists believe that vernacular architecture can also be sustainable. 3. How feasible is it to use engineered eco-fabrics in modern settlements (keeping in mind the versatility of traditional fabrics)? In this paper we shall present a prototype of a new proposed city called "The Sanctuary" housing 40,000 inhabitants (possibly the population in Lothal in 2500 B.C.). Our analysis shows how architectural structures adapt to driving rain, the ingress of winds and a glaring sun. Ecotect calculations elucidate contrasting patterns of energy use in residential areas of Lothal vis-a-vis the dwellings in The Sanctuary. To our knowledge, this is a first study exploring energy efficiency in Lothal.
21-05-2012	Floating orbits around rotating black holes and imprints of massive scalars Speaker: Sayan Chakrabarti, Instituto Superior Tecnico, Lisbon, Portugal. Abstract: In this talk I plan to discuss the coupling of massive scalar fields to matter in orbit around rotating black holes. It is generally expected that orbiting bodies will lose energy in gravitational waves, slowly inspiralling into the black hole. Instead, we show that the coupling of the field to matter leads to a surprising effect: because of superradiance, matter can hover into "floating orbits" for which the net gravitational energy loss at infinity is entirely provided by the black hole's rotational energy. Orbiting bodies remain floating until they extract sufficient angular momentum from the black hole, or until perturbations or nonlinear effects disrupt the orbit.
15-05-2012	*Zero discordness of initial system-environment correlation versus complete positivity of the dynamical map of the system* Speaker: Sibasish Ghosh, The Institute of Mathematical Sciences, India Abstract: For more than three decades, the scaling theory of localization asserted that a two dimensional (2D) system of quantum particles with even infinitesimal disorder cannot have a metallic state : it will always be localized. When recent experiments uncovered a metal-insulator transition (MIT) in two-dimensional electron gases in high-mobility semiconductor hetero-structures, this view was challenged. However, whether a true MIT exists as a quantum phase transition in these systems is still controversial. We have used extensive Quantum Monte Carlo (QMC) simulations to show that, in a related model (the Anderson-Hubbard model in two dimensions) a quantum phase transition exists between a metallic state and an Anderson-localized state. We will also discuss the possible nature of this metallic state, connect this transition with the superconductor-insulator transition (SIT) in 2D, and discuss surprising magnetic properties of a 2D fermionic system when both interaction.
10.05.2012	*Presence of Quantum diffusion in two dimensions : effect of inter-particle interactions on Anderson localization* Speaker: Prabuddha Chakraborty, ISI, Chennai Abstract Quantum systems are quite fragile -- they often interact with their respective environments, which eventually leads to non-unitary dynamics of the states of the systems. Although this dynamics is non-unitary, it can be realized as a unitary dynamics of the system and its environment together and thereby ignoring the environmental degrees of freedom. In this way of realization, the non-unitary dynamics is guaranteed to be a physical evolution (i.e., a completely positive map) allowed by the rules of Quantum Mechanics if the initial joint state is taken to be the tensor product of any state of the system and a fixed state of the environment. In recent years, there has been a growing interest in relaxing the initial correlation between the system and the environment to the extent where the non-unitary dynamical map would still remain completely positive (CP), and the zero discordness of the initial correlation has been proposed to be the characterising feature for the CP-ness of the dynamical map. We argue in this paper that for the CP-ness of the dynamical map, the aforesaid product structure of the initial correlation is inevitable.
27.04.2012	*Scattering of water waves by a thin elastic vertical plate* Speaker: Rumpa Chakraborty, ISI, Kolkata Abstract The problem of scattering of water waves by a thin elastic vertical plate either partially immersed or completely submerged in infinitely deep water or in finite depth water, is investigated here. Within the framework of linearised theory of water waves. The boundary condition on the elastic plate is derived from the Bernoulli-Euler equation of motion of the plate and is given as the normal velocity on the plate to be prescribed in terms of an integral involving the difference of potentials describing the motion in the two sides of the plate, multiplied by an approximate Green's function. The two ends of the barrier may be clamped or free, and the end conditions play a crucial role in the construction of the Green's function. The reflection and transmission coefficients are obtained in terms of integrals involving three unknown functions satisfying two Fredholm integral equations. The integral equations are solved by the Nystrom method and the numerical values of the reflection and transmission coefficients are depicted graphically in a number of figures.