Adiabatic two-qubit gates in capacitively coupled quantum dot hybrid qubits

The ability to tune qubits to flat points in their energy dispersions ("sweet spots") is an important tool for mitigating the effects of charge noise and dephasing in solid-state devices. However, the number of derivatives that must be simultaneously set to zero grows exponentially with the number of coupled qubits, making the task untenable for as few as two qubits. This is a particular problem for adiabatic gates, due to their slower speeds. Here, we propose an adiabatic two-qubit gate for quantum dot hybrid qubits, based on the tunable, electrostatic coupling between distinct charge configurations. We confirm the absence of a conventional sweet spot, but show that controlled-Z (CZ) gates can nonetheless be optimized to have fidelities of $\sim$99% for a typical level of quasistatic charge noise ($\sigma_\varepsilon$$\simeq$1 $\mu$eV). We then develop the concept of a dynamical sweet spot (DSS), for which the time-averaged energy derivatives are set to zero, and identify a simple pulse sequence that achieves an approximate DSS for a CZ gate, with a 5$\times$ improvement in the fidelity. We observe that the results depend on the number of tunable parameters in the pulse sequence, and speculate that a more elaborate sequence could potentially attain a true DSS.Comment: 14 pages, 9 figure

Tight-binding study of high-pressure phase transitions in titanium: alpha to omega and beyond

We use a tight-binding total energy method, with parameters determined from a fit to first-principles calculations, to examine the newly discovered gamma phase of titanium. Our parameters were adjusted to accurately describe the alpha Ti-omega Ti phase transition, which is misplaced by density functional calculations. We find a transition from omega Ti to gamma Ti at 102 GPa, in good agreement with the experimental value of 116 GPa. Our results suggest that current density functional calculations will not reproduce the omega Ti-gamma Ti phase transition, but will instead predict a transition from omega Ti to the bcc beta Ti phase.Comment: 3 encapsulated Postscript figures, submitted to Phyical Review Letter

Tight-binding study of structure and vibrations of amorphous silicon

We present a tight-binding calculation that, for the first time, accurately describes the structural, vibrational and elastic properties of amorphous silicon. We compute the interatomic force constants and find an unphysical feature of the Stillinger-Weber empirical potential that correlates with a much noted error in the radial distribution function associated with that potential. We also find that the intrinsic first peak of the radial distribution function is asymmetric, contrary to usual assumptions made in the analysis of diffraction data. We use our results for the normal mode frequencies and polarization vectors to obtain the zero-point broadening effect on the radial distribution function, enabling us to directly compare theory and a high resolution x-ray diffraction experiment

Anomalous increase in nematic-isotropic transition temperature in dimer molecules induced by magnetic field

We have determined the nematic-isotropic transition temperature as a function of applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15K when subjected to 22T magnetic field. The increase is conjectured to be caused by a magnetic field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers

Dynamical properties of Au from tight-binding molecular-dynamics simulations

We studied the dynamical properties of Au using our previously developed tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K were determined by computing the dynamical-matrix using a supercell approach. In addition, we performed molecular-dynamics simulations at various temperatures to obtain the temperature dependence of the lattice constant and of the atomic mean-square-displacement, as well as the phonon density-of-states and phonon-dispersion curves at finite temperature. We further tested the transferability of the model to different atomic environments by simulating liquid gold. Whenever possible we compared these results to experimental values.Comment: 7 pages, 9 encapsulated Postscript figures, submitted to Physical Review

Discursive quads: new kinds of lexical co‐occurrence data with linguistic concept modelling

This paper introduces linguistic concept modelling, a new computational approach to humanities-driven analysis of meaning in large text collections, and presents illustrative examples of the approach applied to over one billion words of printed Early Modern English contained in Early English Books Online (Text Creation Partnership edition). Linguistic concept modelling methods and innovations are described in detail, and justified as a unique, new, powerful means for studying meaning in texts in relation to individual lexical items, sets of lexical items and entire text collections. Linguistic concept modelling is compared to established approaches of distributional semantics and topic modelling, to show how linguistic concept modelling overcomes the limitations of those approaches to support humanities research. Examples demonstrate how our approach can be used to explore and analyse semantic, pragmatic and discursive meaning in texts, highlighting the novel observations that our approach affords, and its benefits for humanities scholars

A fibre forming smectic twist-bent liquid crystalline phase

We demonstrate the nanostructure and filament formation of a novel liquid crystal phase of a dimeric mesogen below the twist–bend nematic phase. The new fibre-forming phase is distinguished by a short-correlated smectic order combined with an additional nanoscale periodicity that is not associated with density modulation

What we talk about when we talk about corpus frequency: The example of polysemous verbs with light and concrete senses

Gilquin (2008, What you think ain't what you get: Highly polysemous verbs in mind and language. In Jean-Remi Lapaire, Guillaume Desagulier & Jean-Baptiste Guignard (eds.), From gram to mind: Grammar as cognition, 235-255. Bordeaux: Presse Universitaires de Bordeaux) reported that light uses of verbs (e.g. make use) tend to outnumber concrete uses of the same verbs (e.g. make furniture) in corpora, whereas concrete senses tend to outnumber light senses in responses to elicitation tests. The differences between corpus frequency and cognitive salience remain an important and much-discussed question (cf. Arppe et al. 2010, Cognitive corpus linguistics: Five points of debate on current theory and methodology. Corpora 5(1). 1-27). The question is particularly complicated because both corpus frequency and cognitive salience are difficult to define, and are often left undefined. Operationalising and defining corpus frequencies are the issues at the heart of the present paper, which includes a close, manual semantic analysis of nearly 6,000 instances of three polysemous verbs with light and concrete uses, make, take, and give, in the British component of the International Corpus of English. The paper compares semasiological frequencies like those measured by Gilquin (2008) to onomasiological frequency measurements (cf. Geeraerts 1997, Diachronic prototype semantics: A contribution to historical lexicology. Oxford: Clarendon Press). Methodologically, the paper demonstrates that these approaches address fundamentally different research questions, and offer dramatically different results. Findings indicate that corpus frequencies in speech may correlate with elicitation test results, if the corpus frequencies are measured onomasiologically rather than semasiologically; I refer to Geeraerts's (2010, Theories of lexical semantics. Oxford: Oxford University Press) hypothesis of onomasiological salience in explaining this observation