Das, Saurya

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Browsing Das, Saurya by Author "Das, Saurya"

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    Adiabatic quantum computation and Deutsch's algorithm
    (American Physical Society, 2002) Das, Saurya; Kobes, Randy; Kunstatter, Gabor
    We show that by a suitable choice of a time-dependent Hamiltonian, Deutsch’s algorithm can be implemented by an adiabatic quantum computer. We extend our analysis to the Deutsch-Jozsa problem and estimate the required running time for both global and local adiabatic evolutions.
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    Aspects of Planckian scattering beyond the eikonal
    (Indian Academy of Sciences, 1998) Das, Saurya; Majumdar, Parthasarathi
    We discuss an approach to compute two-particle scattering amplitudes for spinless light particles colliding at Planckian centre-of-mass energies, with increasing momentum transfer away from the eikonal limit. The leading corrections to the eikonal amplitude, in our 'external metric' approach, are shown to be vanishingly small in the limit of the source particle mass going to zero. For massless charged particles, the electromagnetic and gravitational interactions decouple in the eikonal limit, hut mix non-trivially for the leading order corrections.
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    Black hole emission rates and the AdS/CFT correspondence
    (SISSA, 1999) Das, Saurya; Dasgupta, Arundhati
    We study the emission rates of scalar, spinor and vector particles from a 5 dimensional black hole for arbitrary partial waves. The solution is lifted to 6 dimensions, and the near horizon BTZ S3 geometry of the black hole solution is probed to determine the greybody factors. We show that the exact decay rates can be reproduced from a (1 + 1)-dimensional conformal eld theory which lies on the boundary of the near horizon geometry. The AdS/CFT correspondence is used to determine the dimension of the CFT operators corresponding to the bulk elds. These operators couple to plane waves incident on the CFT from in nity to produce emission in the bulk.
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    Black hole thermodynamics: entropy, information and beyond
    (2015-12-16) Das, Saurya
    We review some recent advances in black hole thermodynamics, including statistical mechanical origins of black hole entropy and its leading order corrections, from the viewpoints of various quantum gravity theories. We then examine the information loss problem and some possible approaches to its resolution. Finally, we study some proposed experiments which may be able to provide experimental signatures of black holes.
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    Can MOND type hypotheses be tested in a free fall laboratory environment?
    (2015-12-15) Das, Saurya; Patitsas, Steve
    The extremely small accelerations of objects required for the the onset of modified Newtonian dynamics, or MOND, makes testing the hypothesis in conventional terrestrial laboratories virtually impossible. This is due to the large background acceleration of Earth, which is transmitted to the acceleration of test objects within an apparatus. We show however, that it may be possible to test MOND-type hypotheses with experiments using a conventional apparatus capable of tracking very small accelerations of its components, but performed in locally inertial frames such as artifi- cial satellites and other freely falling laboratories. For example, experiments involving an optical interferometer or a torsion balance in these laboratories would show nonlinear dynamics, and dis- placement amplitudes larger than expected. These experiments may also be able to test potential violations of the strong equivalence principle by MOND and to distinguish between its two possible interpretations (modified inertia and modified gravity).
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    Charged black holes in generalized dilaton-axion gravity
    (2015-12-16) Sur, Sourav; Das, Saurya; SenGupta, Soumitra
    We study generic Einstein-Maxwell-Kalb-Ramond-dilaton actions, and derive conditions under which they give rise to static, spherically symmetric black hole solutions. We obtain new asymptotically flat and non-flat black hole solutions which are in general electrically and magnetically charged. They have positive definite and finite quasi-local masses. Existing non-rotating black hole solutions (including those appearing in low energy string theory) are recovered in special limits.
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    Conserved quantities in Kerr-anti-de Sitter spacetimes in various dimensions
    (SISSA, 2000) Das, Saurya; Mann, Robert B.
    We compute the conserved charges for Kerr anti-de Sitter spacetimes in various dimensions using the conformal and the counterterm prescriptions. We show that the conserved charge corresponding to the global timelike killing vector computed by the two methods di er by a constant dependent on the rotation parameter and cosmological constant in odd spacetime dimensions, whereas the charge corresponding to the rotational killing vector is the same in either approach. We comment on possible implications of our results to the AdS/CFT correspondence.
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    Cosmic coincidence or graviton mass?
    (2015-12-23) Das, Saurya
    Using the quantum corrected Friedmann equation, obtained from the quantum Raychudhuri equation, and assuming a small mass of the graviton (but consistent with observations and theory), we propose a resolution of the smallness prroblem (why is observed vacuum energy so small?) and the coincidence problem (why does it constitute most of the universe, about 70%, in the current epoch?).
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    Cosmological constant, brane tension and large hierarchy in a generalized Randall-Sundrum braneworld scenario
    (2015-12-22) Das, Saurya; Maity, Debaprasad; SenGupta, Soumitra
    We consider a generalized Randall Sundrum (RS) brane world scenario with a cosmological constant induced on the visible brane. We show that for < 0, resolution of the hierarchy problem requires an upper bound on the magnitude of . The corresponding tension on the visible brane can be positive or negative. On the other hand, there is no such bound for > 0. However, in this case, the resolution of the hierarchy problem along with the tuning of the value of the cosmological constant to its observed value closed to +10−124 (in Planck units) naturally lead to the tuning of the modulus to a small value of inverse Planck length as estimated in the original RS scenario.
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    Cosmology from quantum potential
    (2016-01-20) Ali, Ahmed Farag; Das, Saurya
    It was shown recently that replacing classical geodesics with quantal (Bohmian) trajectories gives rise to a quantum corrected Raychaudhuri equation (QRE). Here we derive the second order Friedmann equations from the QRE, and show that this also contains a couple of quantum correction terms, the first of which can be interpreted as cosmological constant (and gives a correct estimate of its observed value), or as dark matter, while the second as a radiation term in the early universe, which gets rid of the big-bang singularity and predicts an infinite age of our universe.
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    Das and Vagenas reply
    (2015-12-15) Das, Saurya; Vagenas, Elias C.
    A reply to the Comment by M. M. Ettefaghi and S. M. Fazeli
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    Discreteness of space from GUP II: relativistic wave equations
    (2015-12-22) Das, Saurya; Vagenas, Elias C.; Ali, Ahmed Farag
    Various theories of Quantum Gravity predict modifications of the Heisenberg Uncertainty Prin- ciple near the Planck scale to a so-called Generalized Uncertainty Principle (GUP). In some recent papers, we showed that the GUP gives rise to corrections to the Schrdinger equation, which in turn affect all quantum mechanical Hamiltonians. In particular, by applying it to a particle in a one- dimensional box, we showed that the box length must be quantized in terms of a fundamental length (which could be the Planck length), which we interpreted as a signal of fundamental discreteness of space itself. In this Letter, we extend the above results to a relativistic particle in a rectangular as well as a spherical box, by solving the GUP-corrected KleinGordon and Dirac equations, and for the latter, to two and three dimensions. We again arrive at quantization of box length, area and volume and an indication of the fundamentally grainy nature of space. We discuss possible implications.
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    Discreteness of space from GUP in a weak gravitational field
    (Elsevier, 2016) Deb, Soumen; Das, Saurya; Vagenas, Elias C.
    Quantum gravity effects modify the Heisenberg's uncertainty principle to a generalized uncertainty principle (GUP). Earlier work showed that the GUP-induced corrections to the Schrödinger equation, when applied to a non-relativistic particle in a one-dimensional box, led to the quantization of length. Similarly, corrections to the Klein–Gordon and the Dirac equations, gave rise to length, area and volume quantizations. These results suggest a fundamental granular structure of space. In this work, it is investigated how spacetime curvature and gravity might influence this discreteness of space. In particular, by adding a weak gravitational background field to the above three quantum equations, it is shown that quantization of lengths, areas and volumes continue to hold. However, it should be noted that the nature of this new quantization is quite complex and under proper limits, it reduces to cases without gravity. These results suggest that quantum gravity effects are universal.
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    Discreteness of space from the generalized uncertainty principle
    (2015-12-15) Ali, Ahmed Farag; Das, Saurya; Vagenas, Elias C.
    Various approaches to Quantum Gravity (such as String Theory and Doubly Special Relativity), as well as black hole physics predict a minimum measurable length, or a maximum observable momentum, and related modifications of the Heisenberg Uncertainty Principle to a so-called Generalized Uncertainty Principle (GUP). We propose a GUP consistent with String Theory, Doubly Special Relativity and black hole physics, and show that this modifies all quantum mechanical Hamiltonians. When applied to an elementary particle, it implies that the space which confines it must be quantized. This suggests that space itself is discrete, and that all measurable lengths are quantized in units of a fundamental length (which can be the Planck length). On the one hand, this signals the breakdown of the spacetime continuum picture near that scale, and on the other hand, it can predict an upper bound on the quantum gravity parameter in the GUP, from current observations. Furthermore, such fundamental discreteness of space may have observable consequences at length scales much larger than the Planck scale.
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    Effect of the generalized uncertainty principle on post-inflation preheating
    (2016-01-19) Chemissany, Wissam; Das, Saurya; Ali, Ahmed Farag; Vagenas, Elias C.
    We examine effects of the Generalized Uncertainty Principle, predicted by various theories of quantum gravity to replace the Heisenberg’s uncertainty principle near the Planck scale, on post inflation preheating in cosmology, and show that it can predict either an increase or a decrease in parametric resonance and a corresponding change in particle production. Possible implications are considered.
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    Eikonal particle scattering and dilaton gravity
    (American Physical Society, 1997) Das, Saurya; Majumdar, Parthasarathi
    Approximating light charged pointlike particles in terms of ~nonextremal! dilatonic black holes is shown to lead to certain pathologies in Planckian scattering in the eikonal approximation, which are traced to the presence of a ~naked! curvature singularity in the metric of these black holes. The existence of such pathologies is confirmed by analyzing the problem in an ‘‘external metric’’ formulation where an ultrarelativistic point particle scatters off a dilatonic black hole geometry at large impact parameters. The maladies disappear almost trivially upon imposing the extremal limit. Attempts to derive an effective three-dimensional ‘‘boundary’’ field theory in the eikonal limit are stymied by four-dimensional ~bulk! terms proportional to the light-cone derivatives of the dilaton field, leading to nontrivial mixing of electromagnetic and gravitational effects, in contrast with the case of general relativity. An eikonal scattering amplitude, showing decoupling of these effects, is shown to be derivable by resummation of graviton, dilaton, and photon exchange ladder diagrams in a linearized version of the theory for an asymptotic value of the dilaton field which makes the string coupling constant nonperturbative.
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    Electromagnetic and gravitational scattering at Planckian energies
    (American Physical Society, 1995) Das, Saurya; Majumdar, Parthasarathi
    The scattering of pointlike particles at a very large center-of-mass energies and fixed low momentum transfers, occurring due to both their electromagnetic and gravitational interactions, is reexamined in the particular case when one of the particles carries a magnetic charge. At Planckian center-of-mass energies, when gravitational dominance is usually expected, the presence of magnetic charge is show to produce dramatic modifications to the scattering cross section as well as to holomorphic structure of the scattering amplitude.
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    Electromagnetic charge-monopole versus gravitational scattering at Planckian energies
    (American Physical Society, 1994) Das, Saurya; Majumdar, Parthasarathi
    The amplitude for the scattering of a point magnetic monopole and a point charge, at center-of- mass energies much larger than the masses of the particles, and in the limit of low momentum transfer, is shown to be proportional to the (integer-valued) monopole strength, assuming the Dirac quantization condition for the monopole-charge system. It is demonstrated that, for small momentum transfer, charge-monopole electromagnetic effects remain comparable to those due to the gravitational interaction between the particles even at Planckian center-of-mass energies.
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    Entanglement as a source of black hole entropy
    (2015-12-16) Das, Saurya; Shankaranarayanan, S.
    We review aspects of black hole thermodynamics, and show how entanglement of a quantum field between the inside and outside of a horizon can account for the areaproportionality of black hole entropy, provided the field is in its ground state. We show that the result continues to hold for Coherent States and Squeezed States, while for Excited States, the entropy scales as a power of area less than unity. We also identify location of the degrees of freedom which give rise to the above entropy.
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    Entanglement entropy in all dimensions
    (2015-12-15) Braunstein, Samuel L.; Das, Saurya; Shankaranarayanan, S.
    It has long been conjectured that the entropy of quantum fields across boundaries scales as the boundary area. This conjecture has not been easy to test in spacetime dimensions greater than four because of divergences in the von Neumann entropy. Here we show that the R´enyi entropy provides a convergent alternative, yielding a quantitative measure of entanglement between quantum field theoretic degrees of freedom inside and outside hypersurfaces. For the first time, we show that the entanglement entropy in higher dimensions is proportional to the higher dimensional area. We also show that the R´enyi entropy diverges at specific values of the R´enyi parameter q in each dimension, but this divergence can be tamed by introducing a mass to the quantum field.