Bogoliubov theory for atom scattering into separate regions

We review the Bogoliubov theory in the context of recent experiments, where atoms are scattered from a Bose-Einstein Condensate into two well-separated regions. We find the full dynamics of the pair-production process, calculate the first and second order correlation functions and show that the system is ideally number-squeezed. We calculate the Fisher information to show how the entanglement between the atoms from the two regions changes in time. We also provide a simple expression for the lower bound of the useful entanglement in the system in terms of the average number of scattered atoms and the number of modes they occupy. We then apply our theory to a recent "twin-beam" experiment [R. B\"ucker {\it et al.}, Nat. Phys. {\bf 7}, 608 (2011)]. The only numerical step of our semi-analytical description can be easily solved and does not require implementation of any stochastic methods.Comment: 11 pages, 6 figure

Физика пробоя жидких диэлектриков (История и современное состояние, вклад советских и российских ученых)

Дана краткая история развития учения об электрическом пробое диэлектрических жидкостей, совершенствования техники и методики исследования этого явления. Показано, что по целому ряду направлений советские (российские) ученые и, в частности, сотрудники Томского политехнического университета опережали зарубежных коллег и внесли определяющий вклад в понимание механизмов пробоя жидкостей. Представлены современные физические модели инициирования и развития разряда при импульсном воздействии напряжения

Two-point density correlations of quasicondensates in free expansion

We measure the two-point density correlation function of freely expanding quasicondensates in the weakly interacting quasi-one-dimensional (1D) regime. While initially suppressed in the trap, density fluctuations emerge gradually during expansion as a result of initial phase fluctuations present in the trapped quasicondensate. Asymptotically, they are governed by the thermal coherence length of the system. Our measurements take place in an intermediate regime where density correlations are related to near-field diffraction effects and anomalous correlations play an important role. Comparison with a recent theoretical approach described by Imambekov et al. yields good agreement with our experimental results and shows that density correlations can be used for thermometry of quasicondensates.Comment: 4 pages, 4 figures, minor change

Two-point phase correlations of a one-dimensional bosonic Josephson junction

We realize a one-dimensional Josephson junction using quantum degenerate Bose gases in a tunable double well potential on an atom chip. Matter wave interferometry gives direct access to the relative phase field, which reflects the interplay of thermally driven fluctuations and phase locking due to tunneling. The thermal equilibrium state is characterized by probing the full statistical distribution function of the two-point phase correlation. Comparison to a stochastic model allows to measure the coupling strength and temperature and hence a full characterization of the system

Stochastic optimization of a cold atom experiment using a genetic algorithm

We employ an evolutionary algorithm to automatically optimize different stages of a cold atom experiment without human intervention. This approach closes the loop between computer based experimental control systems and automatic real time analysis and can be applied to a wide range of experimental situations. The genetic algorithm quickly and reliably converges to the most performing parameter set independent of the starting population. Especially in many-dimensional or connected parameter spaces the automatic optimization outperforms a manual search.Comment: 4 pages, 3 figure

Single-particle-sensitive imaging of freely propagating ultracold atoms

We present a novel imaging system for ultracold quantum gases in expansion. After release from a confining potential, atoms fall through a sheet of resonant excitation laser light and the emitted fluorescence photons are imaged onto an amplified CCD camera using a high numerical aperture optical system. The imaging system reaches an extraordinary dynamic range, not attainable with conventional absorption imaging. We demonstrate single-atom detection for dilute atomic clouds with high efficiency where at the same time dense Bose-Einstein condensates can be imaged without saturation or distortion. The spatial resolution can reach the sampling limit as given by the 8 \mu m pixel size in object space. Pulsed operation of the detector allows for slice images, a first step toward a 3D tomography of the measured object. The scheme can easily be implemented for any atomic species and all optical components are situated outside the vacuum system. As a first application we perform thermometry on rubidium Bose-Einstein condensates created on an atom chip.Comment: 24 pages, 10 figures. v2: as publishe

Optimal control of a step-by-step transmission of vehicles

Рассматривается актуальная задача оптимального выбора передаточных чисел трансмиссии транспортных средств (ТС), оснащенных двигателями внутреннего сгорания (ДВС) со ступенчатой коробкой передач, по критерию минимального расхода топлива. Анализ научно-технической литературы показал отсутствие простых и достаточно обоснованных алгоритмов управления энергоустановкой автомобиля, представляющей собой систему, состоящую из ДВС и регулируемой трансмиссии. Целью настоящей работы является теоретическое обоснование закона переключения передаточных чисел ступенчатой трансмиссии, обеспечивающего максимальную топливную экономичность для любого мощного и скоростного режима ТС. В основу предлагаемого метода положено однопараметрическое множество линейных преобразований, связывающих момент двигателя и его частоту вращения с тяговым усилием и скоростью ТС. Множеству передаточных чисел трансмиссии соответствует множество одноэкстремальных функций удельных расходов топлива. Множество точек линий равных уровней расходов топлива, соответствующих двум соседним значениям передаточных чисел трансмиссии, образует линию переключения передач. Предложен соответствующий алгоритм переключения передаточных чисел, обеспечивающий наиболее экономичное функционирование энергоустановки ТС в условиях переменных силовых и скоростных режимов.The actual problem of optimal selection of means of transport (MT) gear ratios, equipped with internal combustion engines (ICE) with a step-by-step gearbox according to the criterion of minimum fuel consumption is considered in the article. An analysis of the scientific and technical literature has showed the absence of simple and sufficiently justified algorithms for controlling the vehicle’s power plant, which is a system that consists of an ICE and an adjustable transmission. The aim of this work is the theoretical justification of the law of gear ratios shifting of a step-by-step transmission, ensuring maximum fuel efficiency for any power and speed mode of MT. The proposed method is based on set of linear transformations that connect the engine moment and engine speed with a traction force and a speed of MT. The set of gear ratios correspond to the set of one-extremal functions of specific fuel consumptions. The set of points of equal levels lines corresponding to two neighboring values of the gear ratios of the transmission forms a gear shift line. An appropriate algorithm for switching gear ratios is proposed, which ensures the most efficient operation of a power plant of MT in conditions of variable power and speed modes

Matter-wave recombiners fro trapped Bose-Einstein condensates

Interferometry with trapped atomic Bose-Einstein condensates (BECs) requires the development of techniques to recombine the two paths of the interferometer and map the accumulated phase difference to a measurable atom number difference. We have implemented and compared two recombining procedures in a double-well-based BEC interferometer. The first procedure utilizes the bosonic Josephson effect and controlled tunneling of atoms through the potential barrier, similar to laser light in an optical fiber coupler. The second one relies on the interference of the reflected and transmitted parts of the BEC wave function when impinging on the potential barrier, analogous to light impinging on a half-silvered mirror. Both schemes were implemented successfully, yielding an interferometric contrast of similar to 20% and 42% respectively. Building efficient matter-wave recombiners represents an important step towards the coherent manipulation of external quantum superposition states of BECs

Spatial and temporal distribution of sperm whales (Physeter macrocephalus) within the Kaikoura submarine canyon in relation to oceanographic variables

The Kaikoura area is a valuable feeding spot for sperm whales with the presence of a submarine canyon close to shore. Male sperm whales can be found there year around, close to the shore and exhibiting almost constant foraging activities. This thesis investigates the distribution and habitat use, both spatially and temporally, of sperm whales (Physeter macrocephalus) within the Kaikoura submarine canyon, New Zealand. The primary aim was to determine which oceanographic variables and bathymetric features influence the sperm whale distribution patterns off Kaikoura. A theodolite was used to track surfacing and movement of sperm whales from a shore-based station. The accuracy of positions recorded by the theodolite was investigated by comparing theodolite measurements of an object of known position. A calibration technique was then developed as the vertical angle was not accurately determined by the theodolite. In addition to investigating the distribution of sperm whales, the daily abundance of sperm whales within the Kaikoura submarine canyon was estimated. Distance sampling and mark-resight models showed an average of 4 (SEM = 0.13) individuals present in the study area at any given time. The mark-resight technique using photo-identification was not possible from a shore-based station so a spatio-temporal model was built in order to track the identity of individuals. The model was tested using photo-identification of sperm whales collected from a boat-based station. Results showed that 88% of the modeled identifications corresponded to the photo-identification database. Sperm whales off Kaikoura were strongly associated with depth, slope and distance from the nearest coast. They were found in waters between 500 m to 1250 m deep and preferred shallower waters in winter. In spring, sperm whales occurred further from the coast, mainly in the Hikurangi Trough, north-east of the shore-based station. Generalized Additive Models (GAM) were used to identify significant oceanographic variables predicting the presence of sperm whales off Kaikoura. Models indicated that sea surface temperature (SST), chlorophylla (Chla) and distance from sea surface temperature fronts were all important parameters in predicting sperm whales presence. Results showed that sperm whales aggregated in the section of the study area with the lowest SST and near SST fronts. This study provides a detailed insight into the use of the Kaikoura submarine canyon by male sperm whales