A Dynamic 'Double Slit' Experiment in a Single Atom

A single-atom 'double-slit' experiment is realized by photo-ionizing Rubidium atoms using two independent low power lasers. The photoelectron wave of well-defined energy recedes to the continuum either from the 5P or 6P states in the same atom, resulting in two-path interference imaged in the far field using a photoelectron detector. Even though the lasers are independent and not phase locked, the transitions within the atom impart the phase relationship necessary for interference. The experiment is designed so that either 5P or 6P states are excited by one laser, before ionization by the second beam. The measurement cannot determine which excitation path is taken, resulting in interference in wave-vector space analogous to Young's double-slit studies. As the lasers are tunable in both frequency and intensity, the individual excitation-ionization pathways can be varied, allowing dynamic control of the interference term. Since the electron wave recedes in the Coulomb potential of the residual ion, a quantum model is used to capture the dynamics. Excellent agreement is found between theory and experiment.Comment: 8 pages, 4 figures, accepted in Phys. Rev. Let

Design of a processor to support the teaching of computer systems

Teaching computer systems, including computer architecture, assembly language programming and operating system implementation, is a challenging occupation. At the University of Waikato this is made doubly true because we require all computer science and information systems students study this material at second year. The challenges of teaching difficult material to a wide range of students have driven us to find ways of making the material more accessible. The corner stone of our strategy for delivering this material is the design and implementation of a custom CPU that meets the needs of teaching. This paper describes our motivation and these needs. We present the CPU and board design and describe the implementation of the CPU in an FPGA. The paper also includes some reflections on the use of a real CPU rather than a simulation environment. We conclude with a discussion of how the CPU can be used for advanced classes in computer architecture and a description of the current status of the project

A low-energy MHz repetition rate short-pulse electron gun

A pulsed electron gun that can produce MHz repetition rate nanosecond pulses is described. The gun uses a Pierce grid in combination with an anode to extract electrons from a tungsten filament cathode. The electrons emerging from the anode are accelerated and focused using two triple-aperture lenses to form a beam. By applying a high speed grid pulse that transitions through the extraction voltage region of the grid/anode combination, pulses of electrons are produced from the gun that have temporal widths less than 5 ns. The pulsed beams are produced at both rising and falling edges of the driving pulse. The characteristics of the emerging electron beams have been determined using an (e,2e) coincidence spectrometer and examples where they are used for time of flight decay measurements are given.Comment: 10 pages, 7 figure

Skeletal muscle energy metabolism in environmental hypoxia: climbing towards consensus.

Skeletal muscle undergoes metabolic remodelling in response to environmental hypoxia, yet aspects of this process remain controversial. Broadly, environmental hypoxia has been suggested to induce: (i) a loss of mitochondrial density; (ii) a substrate switch away from fatty acids and towards other substrates such as glucose, amino acids and ketone bodies; and (iii) a shift from aerobic to anaerobic metabolism. There remains a lack of a consensus in these areas, most likely as a consequence of the variations in degree and duration of hypoxic exposure, as well as the broad range of experimental parameters used as markers of metabolic processes. To attempt to resolve some of the controversies, we performed a comprehensive review of the literature pertaining to hypoxia-induced changes in skeletal muscle energy metabolism. We found evidence that mass-specific mitochondrial function is decreased prior to mass-specific mitochondrial density, implicating intra-mitochondrial changes in the response to environmental hypoxia. This loss of oxidative capacity does not appear to be matched by a loss of glycolytic capacity, which on the whole is not altered by environmental hypoxia. Environmental hypoxia does however induce a selective attenuation of fatty acid oxidation, whilst glucose uptake is maintained or increased, perhaps to support glycolysis in the face of a downregulation of oxidative metabolism, optimising the pathways of ATP synthesis for the hypoxic environment.JAH receives a PhD studentship from the BBSRC. AJM thanks the Research Councils UK for supporting his academic fellowship and Action Medical Research, the British Heart Foundation and the BBSRC for supporting research projects in his laboratory.This is the final version of the article. It first appeared from BioMed Central via http://dx.doi.org/10.1186/2046-7648-3-19

Mitochondrial function at extreme high altitude.

At high altitude, barometric pressure falls and with it inspired P(O2), potentially compromising O2 delivery to the tissues. With sufficient acclimatisation, the erythropoietic response increases red cell mass such that arterial O2 content (C(aO2)) is restored; however arterial P(O2)(P(aO2)) remains low, and the diffusion of O2 from capillary to mitochondrion is impaired. Mitochondrial respiration and aerobic capacity are thus limited, whilst reactive oxygen species (ROS) production increases. Restoration of P(aO2) with supplementary O2 does not fully restore aerobic capacity in acclimatised individuals, possibly indicating a peripheral impairment. With prolonged exposure to extreme high altitude (>5500 m), muscle mitochondrial volume density falls, with a particular loss of the subsarcolemmal population. It is not clear whether this represents acclimatisation or deterioration, but it does appear to be regulated, with levels of the mitochondrial biogenesis factor PGC-1α falling, and shows similarities to adapted Tibetan highlanders. Qualitative changes in mitochondrial function also occur, and do so at more moderate high altitudes with shorter periods of exposure. Electron transport chain complexes are downregulated, possibly mitigating the increase in ROS production. Fatty acid oxidation capacity is decreased and there may be improvements in biochemical coupling at the mitochondrial inner membrane that enhance O2 efficiency. Creatine kinase expression falls, possibly impairing high-energy phosphate transfer from the mitochondria to myofibrils. In climbers returning from the summit of Everest, cardiac energetic reserve (phosphocreatine/ATP) falls, but skeletal muscle energetics are well preserved, possibly supporting the notion that mitochondrial remodelling is a core feature of acclimatisation to extreme high altitude.Dr Murray thanks the Research Councils UK for supporting his Academic Fellowship, and the British Heart Foundation, BBSRC, Action Medical Research, Isaac Newton Trust and Oroboros Instruments for supporting research in his laboratory. Mr Horscroft thanks the BBSRC for funding his PhD Studentship.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1113/JP27007

Parametrization of Electron-Impact Ionization Cross Sections from Laser-Excited and Aligned Atoms

A set of parameters describing electron-impact ionization from laser-aligned atoms are reported, which define the “length”, “width”, and “direction” of the quadruple differential cross section (QDCS) as a function of target alignment kB for fixed ingoing electron momentum k0 and outgoing momenta k1, k2. 24Mg was used, with k0, k1, k2, and kB in the same plane. The parameters are derived for a range of k2 angles, with k1 set at 30° to k0. The QDCS is then determined for all kB. The parameters are very angle sensitive, the QDCS direction varying more than 90° as the length to width ratio varied more than an order of magnitude

Mechanistic Target of Rapamycin (mTOR) in the Cancer Setting

This special issue on mammalian target of rapamycin (mTOR) explores the importance of mTOR in cell growth control and cancer. Cancer cells often exploit mTOR as a mechanism to enhance their capacity to grow. While protein synthesis is by far the best-characterized mTOR-driven process, this special issue also describes a wider array of mTOR-driven biological processes that cancer cells benefit from, including autophagy, cell cycle control, metabolic transformation, angiogenic signaling, and anabolic processes such as nucleotide biosynthesis and ribosomal biogenesis. Other areas of mTOR signaling covered in these reviews delve into cell migration, inflammation, and regulation of transcription factors linked to cancer progression

LASED: A Laser-Atom Interaction Simulator using Quantum Electrodynamics

A laser-atom interaction simulator using quantum electrodynamics (LASED) is presented, which has been developed in the python programming language. LASED allows a user to calculate the time evolution of a laser-excited atomic system. The model allows for any laser polarization, a Gaussian laser beam profile, a rotation of the reference frame chosen to define the states, and an averaging over the Doppler profile of an atomic beam. Examples of simulations using LASED are presented for excitation of calcium from the 4$^1$S$_0$ state to the 4$^1$P$_1$ state, for laser excitation from a D-state excited by electron impact to a P-state, and for laser excitation of caesium via the D$_2$ line.Comment: 14 pages, 8 figures, for source code go to https://github.com/mvpmanish/LASED. Changes in V2: added Sec. III D detailing the visualisation of angular shape, replaced Fig. 6 and Sec. IV B with new results from modelling a realistic 3D to 10P transition in He with the evolution of angular shap