114 research outputs found
Dynamical Casimir effect in curved spacetime
Get PDFA boundary undergoing relativistic motion can create particles from quantum vacuum fluctuations in a phenomenon known as the dynamical Casimir effect. We examine the creation of particles, and more generally the transformation of quantum field states, due to boundary motion in curved spacetime. We provide a novel method enabling the calculation of the effect for a wide range of trajectories and spacetimes. We apply this to the experimental scenario used to detect the dynamical Casimir effect, now adopting the Schwarzschild metric, and find novel resonances in particle creation as a result of the spacetime curvature. We discuss a potential enhancement of the effect for the phonon field of a Bose-Einstein condensate
Relativistic Quantum Clocks
No full textThe conflict between quantum theory and the theory of relativity is exemplified in their treatment of time. We examine the ways in which their conceptions differ, and describe a semiclassical clock model combining elements of both theories. The results obtained with this clock model in flat spacetime are reviewed, and the problem of generalizing the model to curved spacetime is discussed, before briefly describing an experimental setup which could be used to test of the model. Taking an operationalist view, where time is that which is measured by a clock, we discuss the conclusions that can be drawn from these results, and what clues they contain for a full quantum relativistic theory of time
Frequency spectrum of an optical resonator in a curved spacetime
Get PDFThe effect of gravity and proper acceleration on the frequency spectrum of an optical resonator—both rigid or deformable—is considered in the framework of general relativity. The optical resonator is modeled either as a rod of matter connecting two mirrors or as a dielectric rod whose ends function as mirrors. Explicit expressions for the frequency spectrum are derived for the case that it is only perturbed slightly and variations are slow enough to avoid any elastic resonances of the rod. For a deformable resonator, the perturbation of the frequency spectrum depends on the speed of sound in the rod supporting the mirrors. A connection is found to a relativistic concept of rigidity when the speed of sound approaches the speed of light. In contrast, the corresponding result for the assumption of Born rigidity is recovered when the speed of sound becomes infinite. The results presented in this article can be used as the basis for the description of optical and opto-mechanical systems in a curved spacetime. We apply our results to the examples of a uniformly accelerating resonator and an optical resonator in the gravitational field of a small moving sphere. To exemplify the applicability of our approach beyond the framework of linearized gravity, we consider the fictitious situation of an optical resonator falling into a black hole
On the feasibility of detecting quantum delocalization effects on relativistic time dilation in optical clocks
Get PDFWe derive the predicted time dilation of delocalized atomic clocks in an
optical lattice setup in the presence of a gravitational field to leading order
in quantum relativistic corrections. We investigate exotic quantum states of
motion whose relativistic time dilation is outside of the realm of classical
general relativity, finding a regime where optical lattice
clocks currently in development would comfortably be able to detect this
quantum effect (if the technical challenge of generating such states can be met
and the expected accuracy of such clocks can be attained). We provide a
detailed experimental protocol and analyse the effects of noise on our
predictions. We also show that the magnitude of our predicted quantum
relativistic time dilation effect remains just out of detectable reach for the
current generation of optical lattice clocks. Our
calculations agree with the predicted time dilation of classical general
relativity when restricting to Gaussian states
Measuring Network Effects in Mobile Telecommunications Markets with Stated‐Preference Valuation Methods
Get PDFThis paper demonstrates how stated-preference methods can be applied to modeling consumers' preferences in the field of mobile telecommunications, and to measuring and the valuation of network effects. We illustrate this with a case study of mobile phone operators in Poland. We utilize the Choice Experiment method and present the respondents with hypothetical choices of mobile phone operators, while explicitly controlling for network effects in the form of other users in the same network. Based on the hypothetical choices consumers make we construct a conditional random parameters multinomial logit model to analyze their preferences. This approach allows us to calculate welfare effects associated with alternatives, as well as marginal rates of substitution (and hence implicit prices) of the attributes used to describe the choices, such as operator brand and distribution of family and friends between available mobile networks. The latter constitutes a network effect as consumer's utility is influenced by the number (or ratio) of members of his or her family, friends and other users subscribed to the same operator. Our results confirm the existence of a strong network effect, which is related to the size of the social network group a particular subscriber belongs to, rather than the absolute size of the mobile operator's customer base. We observe that there are two sources of this 'gross' network effect - pecuniary (arising from possible price discounts for on-net calls) and non-pecuniary, and demonstrate a way to disaggregate them. In addition, we find that brand perception and brand loyalty are important determinants of operator choice. Finally, through the application of a non-market valuation method we are able to calculate monetary values of the network effect and brand loyalty, and both turn out to be relatively high. The results might be of a particular interest to mobile phone operators and regulatory authorities - we find that the capacity for vigorous price competition between mobile operators is limited due to significant non-price barriers which mitigate subscribers' mobility in the market. We demonstrate a way to measure these effects in monetary terms based on modeling of consumer preferences. --Network effects,mobile telecommunications,brand valuation,stated preference methods,non-market valuation methods,choice experiment,multinomial conditional logit model,preference heterogeneity,random parameters model
On the nature of long range electronic coupling in a medium: Distance and orientational dependence for chromophores in molecular aggregates
No full textEffect of carnosine supplementation on the plasma lipidome in overweight and obese adults: a pilot randomised controlled trial
Get PDFCorrection/corrigendum to this article published:
Baye, E., Ukropec, J., de Courten, M.P.J. et al. Author Correction: Effect of carnosine supplementation on the plasma lipidome in overweight and obese adults: a pilot randomised controlled trial. Sci Rep 10, 4384 (2020). https://doi.org/10.1038/s41598-020-61335-
Equilibration of objective observables in a dynamical model of quantum measurements
Get PDFThe challenge of understanding quantum measurement persists as a fundamental
issue in modern physics. Particularly, the abrupt and energy-non-conserving
collapse of the wave function appears to contradict classical thermodynamic
laws. The contradiction can be resolved by considering measurement itself to be
an entropy-increasing process, driven by the second law of thermodynamics. This
proposal, dubbed the Measurement-Equilibration Hypothesis, builds on the
Quantum Darwinism framework derived to explain the emergence of the classical
world. Measurement outcomes thus emerge objectively from unitary dynamics via
closed-system equilibration. Working within this framework, we construct the
set of \textit{`objectifying observables'} that best encode the measurement
statistics of a system in an objective manner, and establish a measurement
error bound to quantify the probability an observer will obtain an incorrect
measurement outcome. Using this error bound, we show that the objectifying
observables readily equilibrate on average under the set of Hamiltonians which
preserve the outcome statistics on the measured system. Using a random matrix
model for this set, we numerically determine the measurement error bound,
finding that the error only approaches zero with increasing environment size
when the environment is coarse-grained into so-called observer systems. This
indicates the necessity of coarse-graining an environment for the emergence of
objective measurement outcomes.Comment: 12 + 8 pages, 5 figure
Equivalence of Approaches to Relational Quantum Dynamics in Relativistic Settings
Get PDFWe have previously shown that three approaches to relational quantum dynamics—relational Dirac observables, the Page-Wootters formalism and quantum deparametrizations—are equivalent. Here we show that this “trinity” of relational quantum dynamics holds in relativistic settings per frequency superselection sector. Time according to a clock subsystem is defined via a positive operator-valued measure (POVM) that is covariant with respect to the group generated by its (quadratic) Hamiltonian. This differs from the usual choice of a self-adjoint clock observable conjugate to the clock momentum. It also resolves Kuchař's criticism that the Page-Wootters formalism yields incorrect localization probabilities for the relativistic particle when conditioning on a Minkowski time operator. We show that conditioning instead on the covariant clock POVM results in a Newton-Wigner type localization probability commonly used in relativistic quantum mechanics. By establishing the equivalence mentioned above, we also assign a consistent conditional-probability interpretation to relational observables and deparametrizations. Finally, we expand a recent method of changing temporal reference frames, and show how to transform states and observables frequency-sector-wise. We use this method to discuss an indirect clock self-reference effect and explore the state and temporal frame-dependence of the task of comparing and synchronizing different quantum clocks
Emerging Techniques in Cardiac Magnetic Resonance Imaging
No full textCardiovascular disease is the leading cause of death and a significant contributor of health care costs. Noninvasive imaging plays an essential role in the management of patients with cardiovascular disease. Cardiac magnetic resonance (MR) can noninvasively assess heart and vascular abnormalities, including biventricular structure/function, blood hemodynamics, myocardial tissue composition, microstructure, perfusion, metabolism, coronary microvascular function, and aortic distensibility/stiffness. Its ability to characterize myocardial tissue composition is unique among alternative imaging modalities in cardiovascular disease. Significant growth in cardiac MR utilization, particularly in Europe in the last decade, has laid the necessary clinical groundwork to position cardiac MR as an important imaging modality in the workup of patients with cardiovascular disease. Although lack of availability, limited training, physician hesitation, and reimbursement issues have hampered widespread clinical adoption of cardiac MR in the United States, growing clinical evidence will ultimately overcome these challenges. Advances in cardiac MR techniques, particularly faster image acquisition, quantitative myocardial tissue characterization, and image analysis have been critical to its growth. In this review article, we discuss recent advances in established and emerging cardiac MR techniques that are expected to strengthen its capability in managing patients with cardiovascular disease. Level of Evidence: 5. Technical Efficacy: Stage 1.Accepted Author ManuscriptImPhys/Computational ImagingImPhys/Medical Imagin
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