Degenerate Quantum Gases in Microgravity

Clouds of ultra-cold atoms and especially Bose–Einstein condensates (BEC) provide a source for coherent matter-waves in numerous earth bound experiments. Analogous to optical interferometry, matter-wave interferometers can be used for precision measurements allowing for a sensitivity orders of magnitude above their optical counterparts. However, in some respects the presence of gravitational forces in the lab limits experimental possibilities. In this article, we report about a compact and robust experiment generating Bose–Einstein condensates in the drop tower facility in Bremen, Germany. We also present the progress of building the succeeding experiment in which a two species atom interferometer will be implemented to test the weak equivalence principle with quantum matter.     

EXPERIMENTAL AND ANALYTICAL ESTIMATES OF FATIGUE CRACK CLOSURE IN AN ALUMINIUM-COPPER ALLOY PART II: A FINITE ELEMENT ANALYSIS

Abstract— A numerical investigation of the fatigue crack closure phenomenon has been performed by an elastic-plastic finite element analysis. Computer software was developed to consider many aspects affecting plasticity-induced crack closure. Linear and power-law hardening models are considered in the finite element analysis. The paper presents results from the study carried out on compact tension (CT) coupons at various crack lengths corresponding to different loading conditions. Finally the results of the analysis are compared with the experimental estimates of fatigue crack closure levels obtained from laser interferometry, scanning-electron and transmission-electron fractography presented in Part I of this paper on identical specimens.     

Lithium isotope separation with tunable diode lasers

A laser-isotope-separation study of lithium has been performed with two-step excitation involving UV laser radiation and a visible tunable-diode laser. The method yields a high degree of selectivity by tuning the narrow-linewidth diode laser to the D1 or D2 levels of the lithium atom. Selective laser excitation is simplified by the use of the tunable diode laser and the overall approach benefits from the application of a compact mass selector that includes a precision magnetic sector and an ion beam that is designed specifically for light atoms such as lithium.     

Compact, passively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury

A compact, passively Q-switched Nd:YAG laser has been developed for the Mercury Laser Altimeter, an instrument on the Mercury Surface, Space Environment, Geochemistry, and Ranging mission to the planet Mercury. The laser achieves 5.4% efficiency with a near-diffraction-limited beam. It passed all space-flight environmental tests at subsystem, instrument, and satellite integration testing and successfully completes a postlaunch aliveness check en route to Mercury. The laser design draws on a heritage of previous laser altimetry missions, specifically the Ice Cloud and Elevation Satellite and the Mars Global Surveyor, but incorporates thermal management features unique to the requirements of an orbit of the planet Mercury.     

Space interferometry application of laser frequency stabilization with molecular iodine

A number of planned space interferometry missions, including the Laser Interferometer Space Antenna (LISA) gravitational wave detector, require a laser system with high-frequency stability over long time scales. A 1064 nm wavelength nonplanar ring oscillator (NPRO) laser stabilized to a resonant transition in molecular iodine is suitable for these missions, providing high-frequency stability at an absolute reference frequency. The iodine stabilized laser also offers low sensitivity to temperature and alignment fluctuations and allows frequency tuning. We have evaluated the noise performance of a NPRO laser stabilized to iodine using frequency modulation spectroscopy and have found an Allan standard deviation of 10(-14) over 100 s. Simplified optical configurations and the radiation hardness of the frequency-doubling crystals have also been investigated.     

Single-frequency diode-pumped Nd:YAG prism laser with use of a composite laser crystal

A compact, stable, diode-pumped Nd:YAG laser suitable for high-power single-frequency operation is investigated theoretically as well as experimentally. Residual spatial hole burning has been eliminated with a unidirectional ring-laser design with a specially designed intracavity prism and a composite YAG laser crystal. A detailed Jones matrix analysis is performed, leading to design criteria for high loss difference and high-frequency stability.     

Quantum Optics with One Atom in an Optical Cavity

Abstract

The quantum mechanical master equation for a single two-level atom in a single-mode optical cavity is numerically solved in both the quantum and the semiclassical limits. The quantum limit of few cavity photons shows semiclassically forbidden behaviour such as steady state two-level population inversion. Qualitatively new fluorescent spectra, having sidebands broadened by the cavity interaction, also occur. The quantum theory of the single-atom laser with injected signal is presented. At the interface between its quantum and semiclassical dynamics we elucidate the signature of semiclassical limit cycles.     

Phase-stepped gauge block interferometry using a frequency-tunable visible laser diode

Frequency changes induced by bias or temperature modulation of injection diode lasers can provide an economical and effective method of applying phase-stepping interferometry to optical metrology. However, the intrinsic frequency instability of these devices limits their use in gauge block interferometry where precise and repeatable phase steps must be maintained simultaneously on two discontinuous surfaces and over relatively long path lengths. We demonstrate a method using a visible injection diode laser, the frequency of which is locked by using a Fabry-Perot interferometer. Small changes to the length of the Fabry-Perot interferometer shift the frequency of the laser producing proportional and repeatable phase steps to the gauge block interferogram. This method has been successfully implemented with a Fizeau-type gauge block interferometer with a phase measurement resolution of 0.005 lambda. The phase data are then processed to map the surface form of gauge blocks up to 100 mm in length and to objectively assess surface shape parameters.     

Simple multifrequency and phase-shifting fringe-projection system based on two-wavelength lateral shearing interferometry for three-dimensional profilometry

We propose a simple multifrequency spatial-carrier and phase-shifting fringe-projection system based on two-wavelength lateral shearing interferometry (LSI). In this system a wedge-shaped plate lateral shearing interferometer is used and, owing to the presence of tilt, a finite number of fringes parallel to the direction of the shear appears; hence a significant spatial-carrier frequency is generated at the focus position. We further enhance the spatial-carrier frequency either by changing the wavelength of the laser light or by slight defocusing. A synthetic interferogram with low spatial-carrier frequency is obtained by use of laser light of two wavelengths simultaneously in the lateral shear interferometer. We obtain the phase-shifted fringe patterns from the same setup by simply moving the wedge plate in an in-plane parallel direction, using a linear translator. The fringe projection system was tested for measurement of the three-dimensional shape of a discontinuous object. The present system has many advantages; e.g., it is a common-path interferometry and hence is insensitive to external vibrations, is compact in size, and is relatively inexpensive.     

Extreme Adiabatic Expansion in Micro-gravity: Modeling for the Cold Atomic Laboratory

The upcoming Cold Atom Laboratory mission for the International Space Station will allow the investigation of ultracold gases in a microgravity environment. Cold atomic samples will be produced using evaporative cooling in a magnetic chip trap. We investigate here the possibility to release atoms from the trap via adiabatic expansion. We discuss both general considerations and a detailed model of the planned apparatus. We find that it should be possible to reduce the mean trap confinement frequency to about 0.2 Hz, which will correspond to a three-dimensional sample temperature of about 150 pK and a mean atom velocity of 0.1 mm/s.     

Compact detector for proteins based on two-photon excitation of native fluorescence

A compact, inexpensive detector for proteins has been constructed based on two-photon excitation of fluorescence from phenylalanine, tyrosine, and tryptophan. The fluorescence was excited by a solid-state microchip laser operating at 532 nm. Detection limits for phenylalanine, tyrosine, and tryptophan were 62 microM, 2.0 microM, and 470 nM, respectively, in a volume of 3 fL. The detection limit for a test protein, bovine serum albumin, was 130 nM.     

光盘基片平整度的相移干涉测量方法

使用工作波长为10.6mm的红外激光平面干涉仪对光盘基片的表面面形进行了移相式干涉法测试,对测试的波面数据进行多项式拟合,并用计算机处理测试结果,实现了光盘平整度的自动化测量。所测光盘基片平整度的峰谷值为7.88mm,均方根值为2.0mm。     

Limitations of interferometry due to the flicker noise of laser diodes

Interferometry with laser diodes is a cost-effective way to perform displacement measurement. The tunability of laser diodes is also of great interest in multiple-wavelength interferometry. However, the additional flicker noise in the frequency-noise spectrum of semiconductor lasers may become a limiting factor. Investigations on the limitations due to the 1/f noise of laser diodes are presented for both classical and multiple-wavelength interferometry. Measurements at the limit of the coherence length of laser diodes with the corresponding phase fluctuations are reported. The theoretical results are verified experimentally.     

Optically controlled semiconductor microwave modulator with nanosecond response

A microwave modulator optically controlled by laser pulses has been theoretically studied, constructed, and tested. Results of numerical simulations performed by the finite-difference method are presented, design parameters are reported, and coincidence between the calculated and experimentally measured characteristics is demonstrated. The switching time of the modulator is about 1 ns, the characteristic energy of control laser pulses sufficient for the optimum switching is 10 mJ, and the range of mechanical tuning of the microwave frequency is about 10% (66–72 GHz). It is experimentally demonstrated that the modulator reliably operates when the control laser radiation parameters vary within broad limits.     

An experimental study of a schrödinger cat decoherence with atoms and cavities

Abstract

The dynamics of quantum decoherence have been experimentally studied for the first time. A single circular Rydberg atom prepares in a high-quality superconducting cavity a ?Schrödinger cat‘ state of the radiation field. This highly non-classical state is a quantum superposition of two coherent components with different classical phases. A second atom probes the cavity state after a tunable delay. The decay of the quantum correlations between the two atoms reveals the evolution of the initial quantum superposition into a mere statistical mixture. The time scale of this process decreases when the separation beween the two components increases. A simple theoretical model of the experiment is presented. The excellent agreement with the experimental signals confirms in a striking way the decoherence approach.     

Laser phase noise reduction for industrial interferometric applications

Laser ultrasound is a technique used for the ultrasonic inspection of composites during manufacturing of advanced jet fighters. With this technique laser interferometry is used to detect ultrasonic displacements generated by a laser. In theory, the signal-to-noise ratio is proportional to the square root of the collected detection light. In practice, laser phase noise limits the signal-to-noise ratio above a certain collected light level. Two techniques are presented to decrease effects due to laser noise. In one technique the dual-cavity Fabry-Perot currently used is replaced by an interferometer based on a photorefractive crystal. The other technique has a high-finesse Sagnac cavity that filters the phase noise from the detection laser. Experimental results demonstrate that these two techniques significantly reduce limitations due to laser noise.     

The approach to equilibrium during tempering of a bulk nanocrystalline steel: an atom probe investigation

A local electrode atom probe has been used to analyze the solute partitioning during bainite transformation in a novel, nanocrystalline bainitic steel. Atom probe results show the absence of any partitioning of substitutional elements between the phases involved. The results are fully consistent with the diffusionless transformation of austenite to bainite. However, substitutional elements are expected to redistribute approaching an equilibrium phase boundary as the mixture of bainitic ferrite and retained austenite is tempered. The compositional analysis of the austenite/ferrite interface by atom probe tomography indicates that retained austenite decomposes during tempering before equilibrium is reached at the interface.     

Analytical modeling of the periodic nonlinearity in heterodyne interferometry

The periodic nonlinearity that arises from nonideal laser sources and imperfections of optical components limits the accuracy of displacement measurements in heterodyne interferometry at the nanometer level. An analytical approach to investigating the nonlinearity is presented. Frequency mixing, polarization mixing, polarization-frequency mixing, and ghost reflections are all included in this investigation. A general form for the measurement signal, including that of the distortions, is given. The analytical approach is also applicable to homodyne interferometry.     

Shielding against space debris. A comparison between different shields: The effect of materials on their performances

The space environment requires the Space Station to be shielded against orbital debris. A technological programme undertaken by the European Space Agency has led to a preliminary definition of the shield configuration for the European Attached Pressurized Module. The envisaged shield is a modified Whipple shield. A second bumper is located midway between the first bumper and the backwall.

The work described has been initiated to quantify experimentally the merits of different shields compatible with the APM system requirements. For this technological investigation, two requirements had to be satisfied. The spacing between the front bumper and the backwall had to be limited to 120 mm. The backwall thickness could not be reduced to technological limits as it has structural functions as well. In addition, the long life requirements of the Space Station precludes the use of unproved materials for the external parts of the shield.

Different materials have been tried as second bumper. The effect of the first bumper thickness on the projectile fragmentation has been explored as well. Shields based on Aluminium, Kevlar and Glare have been investigated. Kevlar 29 fabrics impregnated with epoxy resin were used for this work. Glare is a material developed to improve the fatigue strength of metal structures. It is primarily intended for aircraft skin applications. Glare consists of a 60 percent fibre volume adhesive prepreg with high-strength unidirectional or cross-ply R-glass fibres. A variety of lay-up sequences is available ranging from 2/1 (two layers of aluminium alloy sheet bonded by one layer of prepeg) to any number of layers. The 2/1 layers version of the Glare material has been used for this work.

The tests results indicate the performances of materials can change significantly with the impact conditions. Glare shows the best performances in the low velocity regime while Kevlar is very promising in the high velocity regime. It is concluded the use of Kevlar can improve substantially the performances of the APM shield.     

水介质中原子转移自由基聚合的研究进展

作为一种新兴的可控/活性聚合方法,原子转移自由基聚合(ATRP)兼具了自由基聚合与可控/活性聚合的优点。水作为一种环境友好性的溶剂,使其作为ATRP的反应介质有着强烈的吸引力。文中从均相水溶液体系以及悬浮、乳液、微乳液等非均相水溶液体系,分别介绍了近年来水介质中ATRP的研究进展,并对其发展方向进行了展望。