arXiv subjects

Tabish Qureshi

Publications and source records attributed to Tabish Qureshi.

67 records · Page 4Linked to original sources

arXiv2011

Decoherence, Time Scales and Pointer States

Certain issues regarding the time-scales over which environment-induced decoherence occurs, and the nature of emergent pointer states, are discussed. A model system, namely, a Stern-Gerlach setup coupled to a quantum mechanical "heat-bath" is studied. The emergent pointer states for this system are obtained, which are different from those discussed in the literature. It is pointed out that this difference is due to some confusion regarding the decoherence time-scale, which is clarified here.

quant-ph↗

arXiv2011

Critique of Protective Measurements

Recently proposed idea of "protective" measurement of a quantum state is critically examined, and generalized. Earlier criticisms of the idea are discussed and their relevance to the proposal assessed. Several constraints on measuring apparatus required by "protective" measurements are discussed, with emphasis on how they may restrict their experimental feasibility. Though "protective" measurements result in an unchanged system state and a shift of the pointer proportional to the expectation value of the measured observable in the system state, the actual reading of the pointer position gives rise to several subtleties. We propose several schemes for reading pointer position, both when the apparatus is treated as a classical system as well as when its quantum aspects are taken into account, that address these issues. The tiny entanglement which is always present due to deviation from extreme adiabaticity in realistic situations is argued to be the weakest aspect of the proposal. Because of this, one can never perform a protective measurement on a single quantum system with absolute certainty. This clearly precludes an ontological status for the wave function. Several other conceptual issues are also discussed.

quant-ph↗

arXiv2010

Comment on "On Visibility in the Afshar Two-Slit Experiment"

Recently Kastner has analyzed the issue of visibility in a modified two-slit experiment carried out by Afshar et al, which has been a subject of much debate. Kastner describes a thought experiment which is claimed to show interference with hundred percent visibility and also an "apparent" which-slit information. We argue that this thought experiment does not show interference at all, and is thus not applicable to the Afshar experiment.

quant-ph↗

arXiv2007

Decoherence and Matter Wave Interferometry

A two-slit interference of a massive particle in the presence of environment induced decoherence is theoretically analyzed using a fully quantum mechanical calculation. The Markovian Master equation, derived from coupling the particle to a harmonic-oscillator heat bath, is used to obtain exact solutions which show the existence of an interference. Interestingly, decoherence does not affect the pattern, but only leads to a reduction in the fringe visibility.

quant-ph↗

arXiv2005

Understanding Popper's Experiment

An experiment proposed by Karl Popper is considered by many to be a crucial test of quantum mechanics. Although many loopholes in the original proposal have been pointed out, they are not crucial to the test. We use only the standard interpretation of quantum mechanics to point out what is fundamentally wrong with the proposal, and demonstrate that Popper's basic premise was faulty.

quant-ph↗

arXiv2004

Dynamics of Rolling Massive Scalar Field Cosmology

We study the inflationary consequences of the rolling massive scalar field in the braneworld scenario with a warped metric. We find that in order to fit observational constraints the warp factor must be tuned to be $< 10^{-3}$. We also demonstrate the inflationary attractor behavior of the massive scalar field dynamics both in the standard FRW case as well as in braneworld scenario.

hep-th↗

arXiv2004

Popper's experiment, Copenhagen Interpretation and Nonlocality

A thought experiment, proposed by Karl Popper, which has been experimentally realized recently, is critically examined. A basic flaw in Popper's argument which has also been prevailing in subsequent debates, is pointed out. It is shown that Popper's experiment can be understood easily within the Copenhagen interpretation of quantum mechanics. An alternate experiment, based on discrete variables, is proposed, which constitutes Popper's test in a clearer way. It refutes the argument of absence of nonlocality in quantum mechanics.

quant-ph↗

arXiv2003

Cosmological Aspects of Rolling Tachyon

We examine the possibility of rolling tachyon to play the dual roll of inflaton at early epochs and dark matter at late times. We argue that enough inflation can be generated with the rolling tachyon either by invoking the large number of branes or brane world assisted inflation. However, reheating is problematic in this model.

hep-th↗

arXiv2002

Aspects of Tachyonic Inflation with Exponential Potential

We consider issues related to tachyonic inflation with exponential potential. We find exact solution of evolution equations in the slow roll limit in FRW cosmology. We also carry out similar analysis in case of Brane assisted tachyonic inflation. We investigate the phase space behavior of the system and show that the dust like solution is a late time attractor. The difficulties associated with reheating in the tachyonic model are also indicated.

hep-th↗

arXiv2000

Comment on "Hidden assumptions in decoherence theory"

It is shown that the conclusion of the paper "Hidden assumptions in decoherence theory" (quant-ph/0001021) is the result of a misunderstanding of the concept of pointer states. It is argued that pointer states are selected by the interaction of quantum systems with the environment, and are not based on any measurement by a conscious observer.

quant-ph↗

arXiv1998

d-electron induced icosahedral growth in strontium clusters

Strontium clusters of size 2 to 20 atoms have been studied within the local density approximation using the ab-initio molecular dynamics method with a plane wave basis in which the s and p electron wavefunctions are well represented. The resulting low lying isomers are optimized using a full-potential linear muffin tin orbital method which treats the d-electrons also on an equal footing. We find that the d electrons contribute significantly to the binding energy as the cluster size grows, expedite the onset of metallicity and induce icosahedral growth. These results are in complete agreement with the experimental finding of rare-gas like magic clusters of group IIA elements and suggest that the observation of icosahedral growth in some transition metal clusters may have a similar origin.

cond-mat.mtrl-sci↗

arXiv1995

Dynamics of a Strongly Damped Two-Level System: Beyond the DBGA

Dynamics of a dissipative two-level system is studied using quantum relaxation theory. This calculation for the first time goes beyond the commonly used dilute bounce gas approximation (DBGA), even for strong damping. The new results obtained here deviate from the DBGA results at low temperatures, however, the DBGA form is recovered at high temperatures. The results in the parameter regime $ 1/2<α<1$, where the model has connection with the Kondo Hamiltonian, are of particular significance. In this regime, the spin shows a cross-over to a slower exponential relaxation at intermediate times, which is roughly half the relaxation rate at short times, as also observed in Quantum Monte-Carlo simulation of the model. The asymptotic behavior of the spin in the Kondo regime is in agreement with the exact conformal field theory results for the Kondo model. A connection of the dissipative dynamics of the two-level system with the quantum Zeno effect is also presented.

cond-mat↗

arXiv1994

Dynamics of Tunneling Centers in Metallic Systems

Dynamics of tunneling centers (TC) in metallic systems is studied, using the technique of bosonization. The interaction of the TC with the conduction electrons of the metal involves two processes, namely, the screening of the TC by electrons, and the so-called electron assisted tunneling. The presence of the latter process leads to a different form of the renormalized tunneling frequency of the TC, and the tunneling motion is damped with a temperature dependent relaxation rate. As the temperature is lowered, the relaxation rate per temperature shows a steep rise as opposed to that in the absence of electron assisted process. It is expected that this behavior should be observed at very low temperatures in a careful experiment. The present work thus tries to go beyond the existing work on the {\it dynamics} of a two-level system in metals, by treating the electron assisted process.

cond-mat↗