Searcharxiv⌕ Search

arXiv subjects

S. M. Barnett

Publications and source records attributed to S. M. Barnett.

17 recordsLinked to original sources
arXiv2024

Theory of paraxial optical Skyrmions

Vector light beams, characterised by a spatially varying polarisation, can exhibit localised structures reminiscent of the Skyrmions familiar from the study of magnetic media. We present a theory of such Skyrmions within paraxial optics, exploiting mathematical analogies with the study of superfluids, especially the A phase of superfluid $\textrm{He}^3$. The key feature is the Skyrmion field which, together with the underlying Skyrmion vector potential, determines the properties of the Skyrmions and, more generally, the polarisation structure of every paraxial vector beam. In addition to structures with integer Skyrmion number we find polarisation patterns with non-integer Skyrmion number; these seem to have no analogue in other fields of physics.

physics.optics↗
arXiv2023

Topological approach of characterizing optical Skyrmions and Skyrmion lattices

The Skyrmion number of paraxial optical Skyrmions can be defined solely via their polarization singularities and associated winding numbers, using a mathematical derivation that exploits Stokes's theorem. It is demonstrated that this definition provides a robust way to extract the Skyrmion number from experimental data, as illustrated for a variety of optical (Néel-type) Skyrmions and bimerons, and their corresponding lattices. This method generates not only an increase in accuracy, but also provides an intuitive geometrical approach to understanding the topology of such quasi-particles of light, and their robustness against smooth transformations.

physics.optics↗
arXiv2018

Information Erasure

Information is central to thermodynamics, providing the grounds to the formulation of the theory in powerful abstract statistical terms. One must not forget, however, that, as put by Landauer, {\it information is physical}. This means that the processing of information will be unavoidably linked to the costs of manipulating the real physical systems carrying the information. Here we will focus on the particular process of erasing information, which plays a fundamental role in the description of heat engines. We will review Landauer's principle and the associated erasure energy cost. We will also show, following the recent contributions from Vaccaro and Barnett, that cost of erasing does not need to be paid with energy, but with any other conserved quantity. Finally, we will address the issue of designing heat engines based on these new concepts.

quant-ph↗
arXiv2016

Stegosaurus chirality

We explain that Stegosaurus exhibited exterior chirality and observe that the largest plate in particular of USNM 4394, USNM 4714, DMNS 2818 and NHMUK R36730 appears to have tilted to the right rather than to the left in each case. Several instances in which Stegosaurus specimens have been confused with their distinct, hypothetical mirror-image forms are highlighted. We believe our findings to be consistent with the hypothesis that Stegosaurus's plates acted primarily as display structures. A collection of more than one Stegosaurus might be referred to henceforth as a 'handful' of Stegosaurus.

q-bio.PE↗
arXiv2014

Stability of point spectrum for three-state quantum walks on a line

Evolution operators of certain quantum walks possess, apart from the continuous part, also point spectrum. The existence of eigenvalues and the corresponding stationary states lead to partial trapping of the walker in the vicinity of the origin. We analyze the stability of this feature for three-state quantum walks on a line subject to homogenous coin deformations. We find two classes of coin operators that preserve the point spectrum. These new classes of coins are generalizations of coins found previously by different methods and shed light on the rich spectrum of coins that can drive discrete-time quantum walks.

quant-ph↗
arXiv2012

Bounds and optimisation of orbital angular momentum bandwidths within parametric down-conversion systems

The measurement of high-dimensional entangled states of orbital angular momentum prepared by spontaneous parametric down-conversion can be considered in two separate stages: a generation stage and a detection stage. Given a certain number of generated modes, the number of measured modes is determined by the measurement apparatus. We derive a simple relationship between the generation and detection parameters and the number of measured entangled modes.

quant-ph↗
arXiv2012

Increasing the dimension in high-dimensional two-photon orbital angular momentum entanglement

Any practical experiment utilising the innate D-dimensional entanglement of the orbital angular momentum (OAM) state space of photons is subject to the modal capacity of the detection system. We show that given such a constraint, the number of measured, entangled OAM modes in photon pairs generated by spontaneous parametric down-conversion (SPDC) can be maximised by tuning the phase-matching conditions in the SPDC process. We demonstrate a factor of 2 increase on the half-width of the OAM-correlation spectrum, from 10 to 20, the latter implying \approx 50 -dimensional two-photon OAM entanglement. Exploiting correlations in the conjugate variable, angular position, we measure concurrence values 0.96 and 0.90 for two phase-matching conditions, indicating bipartite, D-dimensional entanglement where D is tuneable.

quant-ph↗
arXiv2012

Asynchronous QKD on a Relay Network

We show how QKD on a multi-user, multi-path, network can be used to establish a key between any two end users in an asynchronous fashion using the technique of bit-transport. By a suitable adaptation of our previous secret-sharing scheme we show that an attacker has to compromise all of the intermediate relays on the network in order to obtain the key. Thus, two end users can establish a secret key provided they trust at least one of the network relays.

quant-ph↗
arXiv2011

Directional correlations in quantum walks with two particles

Quantum walks on the line with a single particle possess a classical analog. Involving more walkers opens up the possibility to study collective quantum effects, such as many particle correlations. In this context, entangled initial states and indistinguishability of the particles play a role. We consider directional correlations between two particles performing a quantum walk on a line. For non-interacting particles we find analytic asymptotic expressions and give the limits of directional correlations. We show that introducing $δ$-interaction between the particles, one can exceed the limits for non-interacting particles.

quant-ph↗
arXiv2010

Entangled topological features of light

We report the entanglement of topological features, namely, isolated, linked optical vortex loops in the light from spontaneous parametric down-conversion (SPDC). In three dimensions, optical vortices are lines of phase singularity and vortices of energy flow which percolate through all optical fields. This example of entanglement is between features that extend over macroscopic and finite volumes, furthermore, topological features are robust to perturbation . The entanglement of photons in complex three-dimensional(3D) topological states suggests the possibility of entanglement of similar structures in other quantum systems describable by complex scalar functions, such as superconductors, superfluids and Bose-Einstein condensates.

quant-ph↗
arXiv2007

Polarisation rotation of slow light with orbital angular momentum in ultracold atomic gases

We consider the propagation of slow light with an orbital angular momentum (OAM) in a moving atomic medium. We have derived a general equation of motion and applied it in analysing propagation of slow light with an OAM in a rotating medium, such as a vortex lattice. We have shown that the OAM of slow light manifests itself in a rotation of the polarisation plane of linearly polarised light. To extract a pure rotational phase shift, we suggest to measure a difference in the angle of the polarisation plane rotation by two consecutive light beams with opposite OAM. The differential angle $Δα_{\ell}$ is proportional to the rotation frequency of the medium $ω_{\mathrm{rot}}$ and the winding number $\ell$ of light, and is inversely proportional to the group velocity of light. For slow light the angle $Δα_{\ell}$ should be large enough to be detectable. The effect can be used as a tool for measuring the rotation frequency $ω_{\mathrm{rot}}$ of the medium.

cond-mat.other↗
arXiv2007

On the dragging of light by a rotating medium

When light is passing through a rotating medium the optical polarisation is rotated. Recently it has been reasoned that this rotation applies also to the transmitted image (Padgett et al. 2006). We examine these two phenomena by extending an analysis of Player (1976) to general electromagnetic fields. We find that in this more general case the wave equation inside the rotating medium has to be amended by a term which is connected to the orbital angular momentum of the light. We show that optical spin and orbital angular momentum account respectively for the rotation of the polarisation and the rotation of the transmitted image.

physics.optics↗
arXiv2003

Experimental demonstration of quantum source coding

We report an experimental demonstration of Schumacher's quantum noiseless coding theorem. Our experiment employs a sequence of single photons each of which represents three qubits. We initially prepare each photon in one of a set of 8 non-orthogonal codeword states corresponding to the value of a block of three binary letters. We use quantum coding to compress this quantum data into a two-qubit quantum channel and then uncompress the two-qubit channel to restore the original data with a fidelity approaching the theoretical limit.

quant-ph↗
arXiv2002

Antisymmetric multi-partite quantum states and their applications

Entanglement is a powerful resource for processing quantum information. In this context pure, maximally entangled states have received considerable attention. In the case of bipartite qubit-systems the four orthonormal Bell-states are of this type. One of these Bell states, the singlet Bell-state, has the additional property of being antisymmetric with respect to particle exchange. In this contribution we discuss possible generalizations of this antisymmetric Bell-state to cases with more than two particles and with single-particle Hilbert spaces involving more than two dimensions. We review basic properties of these totally antisymmetric states. Among possible applications of this class of states we analyze a new quantum key sharing protocol and methods for comparing quantum states.

quant-ph↗
arXiv2001

Theory of Pseudomodes in Quantum Optical Processes

This paper deals with non-Markovian behaviour in atomic systems coupled to a structured reservoir of quantum EM field modes, with particular relevance to atoms interacting with the field in high Q cavities or photonic band gap materials. In cases such as the former, we show that the pseudo mode theory for single quantum reservoir excitations can be obtained by applying the Fano diagonalisation method to a system in which the atomic transitions are coupled to a discrete set of (cavity) quasimodes, which in turn are coupled to a continuum set of (external) quasimodes with slowly varying coupling constants and continuum mode density. Each pseudomode can be identified with a discrete quasimode, which gives structure to the actual reservoir of true modes via the expressions for the equivalent atom-true mode coupling constants. The quasimode theory enables cases of multiple excitation of the reservoir to now be treated via Markovian master equations for the atom-discrete quasimode system. Applications of the theory to one, two and many discrete quasimodes are made. For a simple photonic band gap model, where the reservoir structure is associated with the true mode density rather than the coupling constants, the single quantum excitation case appears to be equivalent to a case with two discrete quasimodes.

quant-ph↗
arXiv1998

Quantum local-field corrections and spontaneous decay

A recently developed scheme [S. Scheel, L. Knoll, and D.-G. Welsch, Phys. Rev. A 58, 700 (1998)] for quantizing the macroscopic electromagnetic field in linear dispersive and absorbing dielectrics satisfying the Kramers-Kronig relations is used to derive the quantum local-field correction for the standard virtual-sphere-cavity model. The electric and magnetic local-field operators are shown to be consistent with QED only if the polarization noise is fully taken into account. It is shown that the polarization fluctuations in the local field can dramatically change the spontaneous decay rate, compared with the familiar result obtained from the classical local-field correction. In particular, the spontaneous emission rate strongly depends on the radius of the local-field virtual cavity.

quant-ph↗