Proposal for Implementing Device-Independent Quantum Key Distribution based on a Heralded Qubit Amplification

In device-independent quantum key distribution (DIQKD), the violation of a Bell inequality is exploited to establish a shared key that is secure independently of the internal workings of the QKD devices. An experimental implementation of DIQKD, however, is still awaited, since hitherto all optical Bell tests are subject to the detection loophole, making the protocol unsecured. In particular, photon losses in the quantum channel represent a fundamental limitation for DIQKD. Here, we introduce a heralded qubit amplifier based on single-photon sources and linear optics that provides a realistic solution to overcome the problem of channel losses in Bell tests.Comment: 5 pages, 4 figures, 6 page appendi

The size of quantum superpositions as measured with "classical" detectors

We propose a criterion which defines whether a superposition of two photonic components is macroscopic. It is based on the ability to discriminate these components with a particular class of "classical" detectors, namely a photon number measurement with a resolution coarse-grained by noise. We show how our criterion can be extended to a measure of the size of macroscopic superpositions by quantifying the amount of noise that can be tolerated and taking the distinctness of two Fock states differing by N photons as a reference. After applying our measure to several well-known examples, we demonstrate that the superpositions which meet our criterion are very sensitive to phase fluctuations. This suggests that quantifying the macroscopicity of a superposition state through the distinguishability of its components with "classical" detectors is not only a natural measure but also explains why it is difficult to observe superpositions at the macroscopic scale.Comment: 5 pages, 3 figures, updated versio

Propagation of a Dark Soliton in a Disordered Bose-Einstein Condensate

We consider the propagation of a dark soliton in a quasi 1D Bose-Einstein condensate in presence of a random potential. This configuration involves nonlinear effects and disorder, and we argue that, contrarily to the study of stationary transmission coefficients through a nonlinear disordered slab, it is a well defined problem. It is found that a dark soliton decays algebraically, over a characteristic length which is independent of its initial velocity, and much larger than both the healing length and the 1D scattering length of the system. We also determine the characteristic decay time.Comment: 4 pages, 2 figure

Bell-type inequalities for non-local resources

We present bipartite Bell-type inequalities which allow the two partners to use some non-local resource. Such inequality can only be violated if the parties use a resource which is more non-local than the one permitted by the inequality. We introduce a family of N-inputs non-local machines, which are generalizations of the well-known PR-box. Then we construct Bell-type inequalities that cannot be violated by strategies that use one these new machines. Finally we discuss implications for the simulation of quantum states.Comment: 8 pages, 4 figure

Detailed temporal modelling of carbon and water fluxes from pastures in New Zealand : case study of an experimental dairy farm in the Waikato region : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, Palmerston North, New Zealand

The terrestrial biosphere is an important pool of carbon, with its size governed by the opposing processes of CO2 uptake through photosynthesis and release through respiration. It is therefore critically important to understand and reliably and accurately model these processes and predict changes in carbon exchange in response to key drivers. Pasture-based livestock production is particularly important for the New Zealand’s economy but it is also a main contributor to NZ’s greenhouse gas budget. My Ph.D. work used half-hourly eddy-covariance (EC) data, previously collected over 2 consecutive years from a grazed pasture in the Waikato region. The main aims of this study were to assess whether there was any bias in gap-filled eddy covariance measurements, to assess whether incomplete capture of cow respiration during grazing events could have led to biased observations, and to quantify the resulting difference on the net carbon budget of the farm. I approached the work by developing a new process-based model, CenW_HH, running at a half-hourly time step, to predict the energy and CO2 exchange of grazed pastures. I implemented and evaluated different photosynthesis models and upscaling schemes and modelled the energy budget separately for the canopy, litter layer, and the soil. CenW_HH was then parameterised and validated with the available EC measurements. The paddocks surrounding the EC tower were rotationally grazed, which caused heterogeneities in respiratory pulses when grazing events were in the flux footprint and subsequent vegetation cover on the different paddocks. To deal with that heterogeneity, the model was run independently for each individual paddock and a footprint model was used to estimate resultant net fluxes at the EC tower. Modelled fluxes agreed well with half-hourly observed fluxes as seen by model efficiencies of 0.81 for net ecosystem productivity, 0.75 for gross primary production, 0.70 for ecosystem respiration, 0.87 for latent heat flux, 0.76 for sensible heat flux, 0.94 for net radiation, and 0.92 for soil temperature. CenW_HH was then used to test for any biases in gap-filled data for times without the presence of grazing animals, but identified no consistent systematic deviations. Eddy covariance measurements often failed to capture carbon losses due to cattle respiration, especially when measurements had to rely on gap-filled data. By replacing gap-filled NEP fluxes affected by grazing cattle by estimates generated by CenW_HH, the farm carbon budget was reduced by 31% and 113% (and turning from a positive into a slight negative balance) in 2008 and 2009, respectively