Universal oscillations in counting statistics

Noise is a result of stochastic processes that originate from quantum or classical sources. Higher-order cumulants of the probability distribution underlying the stochastic events are believed to contain details that characterize the correlations within a given noise source and its interaction with the environment, but they are often difficult to measure. Here we report measurements of the transient cumulants > of the number n of passed charges to very high orders (up to m=15) for electron transport through a quantum dot. For large m, the cumulants display striking oscillations as functions of measurement time with magnitudes that grow factorially with m. Using mathematical properties of high-order derivatives in the complex plane we show that the oscillations of the cumulants in fact constitute a universal phenomenon, appearing as functions of almost any parameter, including time in the transient regime. These ubiquitous oscillations and the factorial growth are system-independent and our theory provides a unified interpretation of previous theoretical studies of high-order cumulants as well as our new experimental data.Comment: 19 pages, 4 figures, final version as published in PNA

The influence of charge detection on counting statistics

We consider the counting statistics of electron transport through a double quantum dot with special emphasis on the dephasing induced by a nearby charge detector. The double dot is embedded in a dissipative enviroment, and the presence of electrons on the double dot is detected with a nearby quantum point contact. Charge transport through the double dot is governed by a non-Markovian generalized master equation. We describe how the cumulants of the current can be obtained for such problems, and investigate the difference between the dephasing mechanisms induced by the quantum point contact and the coupling to the external heat bath. Finally, we consider various open questions of relevance to future research.Comment: 15 pages, 2 figures, Contribution to 5-th International Conference on Unsolved Problems on Noise, Lyon, France, June 2-6, 200

Non-equilibrium Entanglement and Noise in Coupled Qubits

We study charge entanglement in two Coulomb-coupled double quantum dots in thermal equilibrium and under stationary non-equilibrium transport conditions. In the transport regime, the entanglement exhibits a clear switching threshold and various limits due to suppression of tunneling by Quantum Zeno localisation or by an interaction induced energy gap. We also calculate quantum noise spectra and discuss the inter-dot current correlation as an indicator of the entanglement in transport experiments.Comment: 4 pages, 4 figure

Optical properties of graphene antidot lattices

Undoped graphene is semi-metallic and thus not suitable for many electronic and optoelectronic applications requiring gapped semiconductor materials. However, a periodic array of holes (antidot lattice) renders graphene semiconducting with a controllable band gap. Using atomistic modelling, we demonstrate that this artificial nanomaterial is a dipole-allowed direct gap semiconductor with a very pronounced optical absorption edge. Hence, optical infrared spectroscopy should be an ideal probe of the electronic structure. To address realistic experimental situations, we include effects due to disorder and the presence of a substrate in the analysis.Comment: 11 pages, 9 figures, accepted for publication in Phys. Rev.

Frequency-dependent counting statistics in interacting nanoscale conductors

We present a formalism to calculate finite-frequency current correlations in interacting nanoscale conductors. We work within the n-resolved density matrix approach and obtain a multi-time cumulant generating function that provides the fluctuation statistics, solely from the spectral decomposition of the Liouvillian. We apply the method to the frequency-dependent third cumulant of the current through a single resonant level and through a double quantum dot. Our results, which show that deviations from Poissonian behaviour strongly depend on frequency, demonstrate the importance of finite-frequency higher-order cumulants in fully characterizing interactions.Comment: 4 pages, 2 figures, improved figures & discussion. J-ref adde

Measurement of finite-frequency current statistics in a single-electron transistor

Electron transport in nano-scale structures is strongly influenced by the Coulomb interaction which gives rise to correlations in the stream of charges and leaves clear fingerprints in the fluctuations of the electrical current. A complete understanding of the underlying physical processes requires measurements of the electrical fluctuations on all time and frequency scales, but experiments have so far been restricted to fixed frequency ranges as broadband detection of current fluctuations is an inherently difficult experimental procedure. Here we demonstrate that the electrical fluctuations in a single electron transistor (SET) can be accurately measured on all relevant frequencies using a nearby quantum point contact for on-chip real-time detection of the current pulses in the SET. We have directly measured the frequency-dependent current statistics and hereby fully characterized the fundamental tunneling processes in the SET. Our experiment paves the way for future investigations of interaction and coherence induced correlation effects in quantum transport.Comment: 7 pages, 3 figures, published in Nature Communications (open access

Electron Waiting Times in Mesoscopic Conductors

Electron transport in mesoscopic conductors has traditionally involved investigations of the mean current and the fluctuations of the current. A complementary view on charge transport is provided by the distribution of waiting times between charge carriers, but a proper theoretical framework for coherent electronic systems has so far been lacking. Here we develop a quantum theory of electron waiting times in mesoscopic conductors expressed by a compact determinant formula. We illustrate our methodology by calculating the waiting time distribution for a quantum point contact and find a cross-over from Wigner-Dyson statistics at full transmission to Poisson statistics close to pinch-off. Even when the low-frequency transport is noiseless, the electrons are not equally spaced in time due to their inherent wave nature. We discuss the implications for renewal theory in mesoscopic systems and point out several analogies with energy level statistics and random matrix theory.Comment: 4+ pages, 3 figure

The Design and Performance of a Distributed Flow Water-Cooled Solar Collector

Design of a flat plate collector which reduces the temperature differential between the absorber plate and the fluid is described. The reduced temperature differences are shown to yield increase collector performance. Flow characteristics of the collector are examined. Collector thermal performance is illustrated for typical operating and environmental conditions. A cost analysis is presented to demonstrate that material and assembly costs are substantially lower than for any collector presently on the market

Impact of eutrophication on the life cycle, population dynamics and production of Ampithoe valida (Amphipoda) along an estuarine spatial gradient (Mondego estuary, Portugal)

The life cycle, population dynamics and production of Ampithoe valida was studied from an intertidal mudflat in central Portugal, close to the northern limit of the species¹ distributional range in the eastern Atlantic Ocean. Sampling was carried out in eutrophicated areas, where macroalgae blooms of Enteromorpha spp. occur usually from January to early summer, and also in non-eutrophicated areas, with Zostera noltii meadows. A. valida showed a contagious distribution and the population density clearly changed during the study period along the eutrophication gradient. No migratory patterns were detected between the estuary and the sea, but migrations inside the estuary might have occurred. Females were morphologically recognisable at smaller sizes than males. Females reached sexual maturity before males, but males may live slightly longer than females. Females are iteroparous, producing 2, perhaps 3, broods. A 2-generation life cycle involving a short-lived (7 mo), fast-growing summer generation and a longer-lived (9 mo), slower-growing generation that overwinters is hypothesised. Ovigerous females were present year-round. Eggs, depending on the season, increase differently in volume during marsupial development. No correlations were found between fecundity (number of eggs) and the size of females. Along the eutrophication gradient no differences were found regarding the biology of the species. Besides these features, differences were observed between eutrophicated and non-eutrophicated areas with regard to productivity. Growth production (P) of A. valida in the most eutrophicated area was 0.098 g m-2 18 mo-1 and 0.64 g m-2 18 mo-1 in the Z. noltii meadows. P/B- and E/B- ratios (where E is the elimination production and B- is the average population biomass) ranged from 1.42 and 3.06 in the most eutrophicated area to 5.98 and 12.41 in the Z. noltii beds. To a certain extent, the increase of macroalgae biomass may favour A. valida populations, but extensive blooms affecting the whole area of distribution of this species will determine its disappearanc