Unusual light spectra from a two-level atom in squeezed vacuum

We investigate the interaction of an atom with a multi-channel squeezed vacuum. It turns out that the light coming out in a particular channel can have anomalous spectral properties, among them asymmetry of the spectrum, absence of the central peak as well as central hole burning for particular parameters. As an example plane-wave squeezing is considered. In this case the above phenomena can occur for the light spectra in certain directions. In the total spectrum these phenomena are washed out.Comment: 16 pages, LaTeX, 3 figures (included via epsf

Vegetation database of Great Britain: Countryside Survey

This paper describes the vegetation database created as part of the Countryside Survey (CS) of Great Britain (GIVD ID EU-GB-003) which was established to monitor ecological and land use change in 1978 (http://www.countrysidesurvey.org.uk). The sample design is based on a series of stratified, randomly selected 1 km squares, which numbered 256 in the 1978 survey, 500 in the 1990 survey, 569 in the 1998 survey and 591 in the 2007 survey. Stratification of sample squares was based on predefined strata (called land classes) which have been derived from a classification of all 1 km squares in Britain based on their topographic, climatic and geological attributes obtained from published maps. A series of vegetation plots were located within each 1 km square using a restricted randomisation procedure designed to reduce aggregation. Linear features (road verges, watercourse banks, hedges, arable margins and field boundaries) and areal features (fields, unenclosed land and small semi-natural biotope patches) were sampled. Linear plots were 1 x 10 m laid out along a feature whilst unenclosed land and small biotopes were sampled using 2 m x 2 m plots. Larger randomly-placed plots were nested 14 m² plots with an inner nest of 2 m x 2 m. Within each 1 km Countryside Survey sample square the land cover and all landscape features were mapped and each parcel of land (and vegetation plot) has been assigned to a Broad Habitat/EUNIS habitat type. This database of vegetation plots is a very useful resource. The data is freely available from the website, however, there are restrictions on the release of the spatial location of the plots. There is now a considerable time-series of plots within the database going back to 1978 representing different habitat types and landscape features that can be analysed to determine changes in vegetation metrics (e.g. Ellenberg scores) and individual species. Vegetation changes can be linked to environmental drivers and the spatial scale (across GB) is sufficiently large to analyse gradients in most driving variables

How well is current plant trait composition predicted by modern and historical forest spatial configuration?

There is increasing evidence to suggest that a delayed response of many forest species to habitat loss and fragmentation leads to the development of extinction debts and immigration credits in affected forest habitat. These time lags result in plant communities which are not well predicted by present day landscape structure, reducing the accuracy of biodiversity assessments and predictions for future change. Here, species richness data and mean values for five life history characteristics within deciduous broadleaved forest habitat across Great Britain were used to quantify the degree to which aspects of present day forest plant composition are best explained by modern or historical forest patch area. Ancient forest specialist richness, mean rarity and mean seed terminal velocity were not well predicted by modern patch area, implying the existence of a degree of lag in British forest patches. Mean seedbank persistence values were more closely related to modern patch area than historical, particularly in larger patches. The variation in response for different mean trait values suggests that species respond to landscape change at different rates depending upon their combinations of different trait states. Current forest understorey communities are therefore likely to consist of a mixture of declining species whose extinction debt is still to be paid, and faster colonising immigrant species. These results indicate that without management action, rare and threatened species of plant are likely to be lost in the future as a result of changes in forest spatial configuration that have already taken place. The lag seen here for rare specialist plants suggests however that there may still be scope to protect such species before they are lost from forest patches

The use of spin-pure and non-orthogonal Hilbert spaces in Full Configuration Interaction Quantum Monte-Carlo

Full Configuration Interaction Quantum Monte–Carlo (FCIQMC) al- lows for exact results to be obtained for the ground state of a system within a finite-basis approximation of the Schrödinger equation. Work- ing within imposed symmetry constraints permits dramatic reductions in the size and internal connectivity of the Hilbert space considered, with associated reductions in the computational cost involved, as well as permitting exclusion of the natural ground state to extract a se- ries of excited states of the system. As all converged solutions are ˆ eigenfunctions of the square of the total spin operator, S 2 , as well as the Hamiltonian and the projected spin, imposing spin-purity as an additional ‘symmetry’ is a natural extension. In this thesis, the use of various spin-pure spaces is compared to the previously used determinental spaces. Variations on the FCIQMC al- gorithm which work in non-orthogonal (and non-normalised) basis sets, and with the arbitrary discretisation of imaginary time removed, are considered along with the implications of the differences to the normal FCIQMC algorithm

An excited-state approach within full configuration interaction quantum Monte Carlo.

We present a new approach to calculate excited states with the full configuration interaction quantum Monte Carlo (FCIQMC) method. The approach uses a Gram-Schmidt procedure, instantaneously applied to the stochastically evolving distributions of walkers, to orthogonalize higher energy states against lower energy ones. It can thus be used to study several of the lowest-energy states of a system within the same symmetry. This additional step is particularly simple and computationally inexpensive, requiring only a small change to the underlying FCIQMC algorithm. No trial wave functions or partitioning of the space is needed. The approach should allow excited states to be studied for systems similar to those accessible to the ground-state method due to a comparable computational cost. As a first application, we consider the carbon dimer in basis sets up to quadruple-zeta quality and compare to existing results where available.N.S.B. gratefully acknowledges Trinity College, Cambridge for funding. G.H.B. gratefully acknowledges the Royal Society for funding via a university research fellowship. This work has been supported by the EPSRC under grant no. EP/J003867/1.This is the accepted manuscript. The final version is available at http://scitation.aip.org/content/aip/journal/jcp/143/13/10.1063/1.4932595

Traits of plant communities in fragmented forests: the relative influence of habitat spatial configuration and local abiotic conditions

1. The plant trait composition of forest fragments is thought to be partly determined by forest spatial properties, although the relative importance of habitat configuration and local abiotic drivers is poorly understood. 2. To address this issue, large-scale habitat extent data were combined with detailed field survey information for temperate broad-leaved deciduous forest patches to quantify the relative effects of spatial and abiotic filters on plant community mean trait values. 3. Local conditions such as shade and soil fertility had the largest effect on mean trait values, but aspects of habitat configuration also had significant partial effects on a number of traits. 4. Mean trait values within older forest patches were more strongly influenced by forest spatial configuration than in younger patches. 5. Synthesis. Results indicate that, in addition to the effects of greater light availability and competition in small patches and at forest edges, aspects of habitat configuration such as patch size and isolation are themselves important factors limiting the occurrence of forest specialist species. Large areas of core forest habitat contain a greater proportion of rare, poor dispersing species, although these effects were less visible in more recently established forest. This highlights the importance of maintaining existing large and old forest patches as a refuge for forest specialist plants. The results of this comparison of spatial and abiotic variables suggest that controlling the spatial properties of forest patches is likely to prove an effective way of managing plant species diversity, provided that sites with appropriate abiotic conditions are chosen

Improving Uniformity of Potato Crops

Within potato crops some tubers are too small or too large to be marketable. There are economic and environmental incentives to improve uniformity, thereby increasing the fraction of yield that is marketable. The potato crop has a complex, three-tiered population structure consisting of plants, stems and tubers, each of which is a branch of the previous tier. Plant-to-plant variation in the number of stems, number of tubers and yield has long been noted to occur but the causes of this variability and how it impacts the uniformity of the crop are uncertain. This thesis investigated how variability in seed tuber weight, within-row spacing and date of emergence affected the growth of individual plants in field-grown crops and how this affected the uniformity of the crops. Stem-to-stem variation in yield was described for the first time and shown to be considerably more variable than plant-to-plant variation. The causes of this stem-to-stem variation in yield and its influence on tuber size were investigated through detailed surveying of Desiree, King Edward, Maris Piper and Russet Burbank. Commercial crops grown from different seed tuber sizes and on contrasting soil types were surveyed to establish whether plant-to-plant variation was similar to that encountered in field experiments. Within crops, plants with a higher average yield per stem were found to produce larger tubers and mean tuber size varied by c. 20 mm between plants. The number of tubers increased as the average yield per stem increased, but was insufficient to counter the influence of increased yield per stem on tuber size. These findings demonstrate that the same principles apply for explaining variation in mean tuber size between plants and between crops. Although the average seed tuber weight correlates linearly with the average number of stems produced, for individual plants, seed tuber weight accounted for little of the variation in the number of stems per plant. In 2013 reducing variation in seed tuber weight did not reduce variation in stem number per plant nor yield per plant. In 2014 variation in the number of stems and yield per plant were decreased by reducing variation in seed tuber weight, but this did not affect uniformity. In Maris Piper, high levels of variability in within-row plant spacing did not affect uniformity in 2013, whilst in 2014 it caused a small but significant decrease of uniformity. In a separate experiment, variability in within-row plant spacing had a larger negative effect on uniformity in Markies and Marfona and possible reasons for the different responses are discussed. Variation in emergence was affected by differences in sprout development at planting, but only relatively small differences were created between treatments that had no detectable effect on uniformity. In all four varieties where stem-to-stem variation was surveyed, there was a strong correlation between yield per stem and mean tuber weight. The number of tubers per stem was highly variable and did not increase with yield sufficiently to compensate for the effect of yield on mean tuber weight. The proportion of plant yield produced by each stem and the allocation of yield to tubers on the same stem were highly variable. Reducing variation in seed tuber weight reduced variation in plant yield, and during the initial phases of development plants growing from larger seed tubers grew at a faster rate. The implication for stems is that variation in the supply of seed tuber substrate is a substantial cause of stem-to-stem variation in yield. Models were developed that provide a framework for analysing the contribution of variation at different scales to the overall tuber size distribution. The underlying mechanism governing uniformity is that decreasing variation in mean tuber weight between plants or stems improves uniformity. Differences in plant-to-plant variation do not have any effect on uniformity if both variation in yield and number of tubers change by similar amounts. Models predicted that reducing variation in the yield per stem would improve uniformity, particularly in varieties where there is only a weak relationship between yield and number of tubers. The implications of these findings are discussed in relation to growers improving uniformity of present varieties and for breeders developing new varieties.Funding was provided by Cambridge University Potato Growers Research Association (CUPGRA