Internet authentication based on personal history - a feasibility test

On the Internet, there is an uneasy tension between the security and usability of authentication mechanisms. An easy three-part classification is: 'something you know' (e.g. password); 'something you hold' (e.g. device holding digital certificate), and 'who you are' (e.g. biometric assessment) [9]. Each of these has well-known problems; passwords are written down, guessable, or forgotten; devices are lost or stolen, and biometric assays alienate users. We have investigated a novel strategy of querying the user based on their personal history (a 'Rip van Winkle' approach.) The sum of this information is large and well-known only to the individual. The volume is too large for impostors to learn; our observation is that, in the emerging environment, it is possible to collate and automatically query such information as an authentication test. We report a proof of concept study based on the automatic generation of questions from electronic 'calendar' information. While users were, surprisingly, unable to answer randomly generated questions any better than impostors, if questions are categorized according to appropriate psychological parameters then significant results can be obtained. We thus demonstrate the potential viability of this concept

Angular momentum of a strongly focussed Gaussian beam

A circularly polarized rotationally symmetric paraxial laser beams carries hbar angular momentum per photon as spin. Focussing the beam with a rotationally symmetric lens cannot change this angular momentum flux, yet the focussed beam must have spin less than hbar per photon. The remainder of the original spin is converted to orbital angular momentum, manifesting itself as a longitudinal optical vortex at the focus. This demonstrates that optical orbital angular momentum can be generated by a rotationally symmetric optical system which preserves the total angular momentum of the beam.Comment: 4 pages, 3 figure

Unveiling the nature of bright z ~ 7 galaxies with the Hubble Space Telescope

We present new Hubble Space Telescope/Wide Field Camera 3 imaging of 25 extremely luminous (-23.2 < M_ UV < -21.2) Lyman-break galaxies (LBGs) at z ~ 7. The sample was initially selected from 1.65 deg^2 of ground-based imaging in the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman-alpha emitters, `Himiko' and `CR7'. A deconfusion analysis of the deep Spitzer photometry available suggests that these galaxies exhibit strong rest-frame optical nebular emission lines (EW_0(H_beta + [OIII]) > 600A). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common (f_multi > 0.4) in bright z ~ 7 galaxies, and ubiquitous at the very bright end (M_UV < -22.5). The galaxies have half-light radii in the range r_1/2 ~ 0.5-3 kpc. The size measurements provide the first determination of the size-luminosity relation at z ~ 7 that extends to M_UV ~ -23. We find the relation to be steep with r_1/2 ~ L^1/2. Excluding clumpy, multi-component galaxies however, we find a shallower relation that implies an increased star-formation rate surface density in bright LBGs. Using the new, independent, HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ~ 7 favours a power-law decline at the bright-end, compared to an exponential Schechter function drop-off. Finally, these results have important implications for the Euclid mission, which we predict will detect > 1000 similarly bright galaxies at z ~ 7. Our new HST imaging suggests that the vast majority of these galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf star contamination.Comment: 26 pages, 11 figures and 5 tables. Updated to match MNRAS accepted versio

F stars, metallicity, and the ages of red galaxies at z > 1

We explore whether the rest-frame near-UV spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-UV spectra of F stars of known (sub-solar and super-solar) metallicity. F stars are of particular interest because the rest-frame near-UV spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turnoff. We find that, in the case of the F stars, where the HST ultraviolet spectra have high signal:noise, model-fitting with metallicity allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turnoff ages of these stars yielded by the model fitting are well constrained. Encouraged by this we have fitted these same variable- metallicity models to the deep, optical spectra of the z \simeq 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age-metallicity degeneracy is not so easily lifted for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still \geq 3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED (Heavens et al., 2000) leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein-de Sitter universe, and favour a lambda-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3 - 5.Comment: 10 pages, LaTeX, uses MNRAS style file, incorporates 14 postscript figures, submitted to MNRAS. Changes include: inclusion of single stellar atmosphere model fits; more rigorous calculation of confidence regions; some re-structurin

Orientation of biological cells using plane-polarized Gaussian beam optical tweezers

Optical tweezers are widely used for the manipulation of cells and their internal structures. However, the degree of manipulation possible is limited by poor control over the orientation of trapped cells. We show that it is possible to controllably align or rotate disc shaped cells - chloroplasts of Spinacia oleracea - in a plane polarised Gaussian beam trap, using optical torques resulting predominantly from circular polarisation induced in the transmitted beam by the non-spherical shape of the cells.Comment: 9 pages, 6 figure

Nanotrapping and the thermodynamics of optical tweezers

Particles that can be trapped in optical tweezers range from tens of microns down to tens of nanometres in size. Interestingly, this size range includes large macromolecules. We show experimentally, in agreement with theoretical expectations, that optical tweezers can be used to manipulate single molecules of polyethylene oxide suspended in water. The trapped molecules accumulate without aggregating, so this provides optical control of the concentration of macromolecules in solution. Apart from possible applications such as the micromanipulation of nanoparticles, nanoassembly, microchemistry, and the study of biological macromolecules, our results also provide insight into the thermodynamics of optical tweezers.Comment: 5 pages, 3 figures, presented at 17th AIP Congress, Brisbane, 200