arXiv subjects

Nikolaos K. Kollas

Publications and source records attributed to Nikolaos K. Kollas.

6 recordsLinked to original sources

arXiv2021

Assisted harvesting and catalysis of coherence from scalar fields

Recently it has been demonstrated that it is possible to harvest quantum resources other than entanglement from a coherent scalar field. Employing time-dependent perturbation theory, we present a complete analysis of the conditions under which a spatially extended Unruh-DeWitt detector coupled to the proper time derivative of the field can harvest coherence for any initial state of the field, as well as the energy cost that is required for each harvest. By studying harvesting under repeatable extractions it is proven that when the detector interacts with the field through a delta coupling coherence is catalytic. For a Gaussian smeared detector it is shown that harvesting from a coherent field depends on the phase of its amplitude distribution and its initial energy as well as on the mean radius of the detector and the mean interaction duration between the two. For a detector moving at a constant velocity and with a mean radius of the same order as its transition wavelength, we observe that, for relativistic speeds, coherence swelling effects are present the intensity of which depends on the dimension of the underlying Minkowski spacetime.

quant-ph↗

arXiv2020

Field assisted extraction and swelling of quantum coherence for moving Unruh-DeWitt detectors

We study the effects of motion for an Unruh-DeWitt detector, modeled as a two-level system, on the amount of coherence extracted, when it interacts with a massless scalar coherent field in 1+1 Minkowski spacetime. We observe that compared to a detector at rest, for certain values of the initial energy of the field and the interaction duration, the amount is larger for both a detector moving with a constant speed or uniform acceleration. This "swelling" of coherence, which becomes more intense for increasing values of velocity or acceleration, is mostly observed for short durations, when the energy of the field is larger compared to that of the detector, and for longer durations but smaller field energies. As a consequence, the rate at which coherence is lost, is sometimes slower for a moving detector than for a detector at rest.

quant-ph↗

arXiv2020

Faithful extraction of quantum coherence

Coherence, a strictly quantum phenomenon, has found many applications, from quantum information theory and thermodynamics to quantum foundations and biology. When physical constraints are taken into consideration creation of coherence in a system is usually impossible and must therefore be extracted from another system acting as a reservoir. In this article we present two faithful extraction protocols in the sense that the interaction involved between the system and reservoir is coherence preserving. As an example we implement both in the case where the reservoir is a quantum harmonic oscillator in a coherent and a squeezed state respectively, and study the limits of repeatable extraction. For a single extraction onto qubits it is demonstrated that, perhaps surprisingly, one of the protocols manages to outperform a previous method, known as catalytic coherence, which allows the creation of an extra amount of coherence between degenerate energy eigenstates of the combined system.

quant-ph↗

arXiv2018

Optimization free measures of quantum resources

A closed expression is derived for the amount of resource present in a quantum state as quantified by a distance measure based on the \emph{Tsallis relative entropy} introduced recently in \cite{Zhao2018}, for any resource theory whose set of free states is described by the image of a unital \emph{resource destroying map}. By applying it to the resource theory of coherence it is demonstrated that the correct definition of a \emph{projective coarse grained measurement} needs to be modified in order for it to be correctly interpreted as a measurement which provides less information about a system's true state than the one obtained from a \emph{finer grained} measurement.

quant-ph↗

arXiv2017

Specular reflection on the surface of a sphere: compass and ruler constructions

We provide an explicit geometric algorithm involving only ruler and compass constructions in order to specify the specular reflection point on the surface of a reflecting sphere of radius $r$ given two focal points $A$ and $B$ lying outside of it. By numerically implementing the algorithm we compute the point in question for a number of cases. We conclude by discussing how the first iteration of the algorithm constitutes a first order approximation to the real solution by providing a closed expression for it as well as the error involved in doing so, as a function of the distances of the two focal points from the origin and the angle formed between them.

math.HO↗

arXiv2016

Thermodynamics of bipartite entanglement

A review is given on the thermodynamical structure of bipartite entanglement. By comparing it to the axiomatic formulation of thermodynamics presented by Giles it is shown that for finite dimensional systems the two theories are formally inequivalent. The same approach is used to demonstrate the full equivalence in the asymptotic limit for pure quantum states. For mixed states a different method for obtaining the second law is described applied to two different classes of operations, PPT-preserving and asymptotically non-entangling operations.

quant-ph↗