Simple reflection anisotropy microscopy set-up for CO oxidation studies

Reflection anisotropy microscopy (RAM) is a tool to monitor the optical anisotropy of surfaces with spatial resolution (Rotermund et al 1995 Science 270 608–10). It has been applied to pattern formation during CO oxidation on Pt(110), where it provides a high sensitivity for surface reconstruction and partially also for the coverage with reaction educts (Heumann 2000 Dissertation TU-Berlin). However, the spatial resolution of RAM and the alignment procedure of the optical components were not satisfactory. Here, we give a detailed description of a new set-up, which employs a simple polarizing beam splitter cube as an analyser instead of a Foster prism, offering a higher spatial resolution (<10 μm) and easier alignment of the optical components while retaining the high sensitivity for surface structure. Polarization contrast and spatial resolution of the new set-up are systematically measured, and applications to CO oxidation on uniform and microstructured Pt(110) single crystals are presented

Vacuum-ultraviolet frequency-modulation spectroscopy

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing ($\nu_{\mathrm{VUV}}=2\nu_{\mathrm{UV}}+\nu_2$) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies $\nu_{\mathrm{UV}}$ and $\nu_2$. Sidebands generated in the output of the second laser ($\nu_2$) using an electro-optical modulator operating at the frequency $\nu_{\mathrm{mod}}$ are directly transfered to the VUV and used to record FM spectra. Demodulation is demonstrated both at $\nu_{\mathrm{mod}}$ and $2\nu_{\mathrm{mod}}$. The main advantages of the method are that its sensitivity is not reduced by pulse-to-pulse fluctuations of the VUV laser intensity, compared to VUV absorption spectroscopy is its background-free nature, the fact that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages we present VUV FM spectra of Ar, Kr, and N$_2$ in selected regions between 105000cm$^{-1}$ and 122000cm$^{-1}$.Comment: 23 pages, 10 figure

Imaging electric fields in the vicinity of cryogenic surfaces using Rydberg atoms

The ability to characterize static and time-dependent electric fields in situ is an important prerequisite for quantum-optics experiments with atoms close to surfaces. Especially in experiments which aim at coupling Rydberg atoms to the near field of superconducting circuits, the identification and subsequent elimination of sources of stray fields is crucial. We present a technique that allows the determination of stray-electric-field distributions $(F^\text{str}_\text{x}(\vec{r}),F^\text{str}_\text{y}(\vec{r}),F^\text{str}_\text{z}(\vec{r}))$ at distances of less than $2~\text{mm}$ from (cryogenic) surfaces using coherent Rydberg-Stark spectroscopy in a pulsed supersonic beam of metastable $1\text{s}^12\text{s}^1~{}^{1}S_{0}$ helium atoms. We demonstrate the capabilities of this technique by characterizing the electric stray field emanating from a structured superconducting surface. Exploiting coherent population transfer with microwave radiation from a coplanar waveguide, the same technique allows the characterization of the microwave-field distribution above the surface.Comment: 6 pages, 4 figure

Measuring the dispersive frequency shift of a rectangular microwave cavity induced by an ensemble of Rydberg atoms

In recent years the interest in studying interactions of Rydberg atoms or ensembles thereof with optical and microwave frequency fields has steadily increased, both in the context of basic research and for potential applications in quantum information processing. We present measurements of the dispersive interaction between an ensemble of helium atoms in the 37s Rydberg state and a single resonator mode by extracting the amplitude and phase change of a weak microwave probe tone transmitted through the cavity. The results are in quantitative agreement with predictions made on the basis of the dispersive Tavis-Cummings Hamiltonian. We study this system with the goal of realizing a hybrid between superconducting circuits and Rydberg atoms. We measure maximal collective coupling strengths of 1 MHz, corresponding to 3*10^3 Rydberg atoms coupled to the cavity. As expected, the dispersive shift is found to be inversely proportional to the atom-cavity detuning and proportional to the number of Rydberg atoms. This possibility of measuring the number of Rydberg atoms in a nondestructive manner is relevant for quantitatively evaluating scattering cross sections in experiments with Rydberg atoms

Magnetic trapping of hydrogen after multistage zeeman deceleration

We report the first experimental realization of magnetic trapping of a sample of cold radicals following multistage Zeeman deceleration of a pulsed supersonic beam. Hatoms seeded in a supersonic expansion of Kr have been decelerated from an initial velocity of 520m/s to 100m/s in a 12-stage Zeeman decelerator and loaded into a magnetic quadrupole trap by rapidly switching the fields of the trap solenoids. © 2008 The American Physical Society

Surface-electrode Rydberg-stark decelerator

Hydrogen atoms in Rydberg states with principal quantum numbers between 23 and 70 have been accelerated, decelerated, and electrostatically trapped using a surface-electrode Rydberg-Stark decelerator. By applying a set of oscillating electrical potentials to a two-dimensional array of electrodes on a printed circuit board (PCB), a continuously moving, three-dimensional electric trap with a predefined velocity and acceleration is generated. From an initial longitudinal velocity of 760m/s, final velocities of the Rydberg atoms ranging from 1200m/s to zero velocity in the laboratory-fixed frame of reference were achieved. Accelerated or decelerated atoms were detected directly by pulsed electric-field ionization. Atoms trapped at zero mean velocity above the PCB were reaccelerated off the PCB before field ionization. © 2012 American Physical Society

Energy Safety Management: A Training Model to Improve Flight Safety

Failing to properly manage an airplane’s energy state can be unforgiving. Mismanagement of mechanical energy (altitude and/or airspeed) is a contributing factor to three common types of fatal accidents in aviation: loss of control in flight, approach and landing accidents, and controlled flight into terrain. Recognizing the importance of energy management, the Federal Aviation Administration has incorporated new elements into the Airman Certification Standards, emphasizing knowledge of energy management concepts and the consequences of mishandling an airplane’s energy state. Unfortunately, no adequate guidance has been available in terms of defining key energy management concepts or suggesting how these should be taught to the average pilot and applied to everyday flying. This article introduces energy safety management (ESM) as a best practice for incorporating energy management into pilot training. First, ESM integrates three well-tested energy management theories developed independently in engineering, military science, and biology. Second, ESM relies on the power of simple analogies and a pilot-oriented approach to make energy management principles accessible and practical to any airplane pilot operating standard propulsion/flight control systems and existing cockpit displays. Third, to organize and optimize learning, ESM incorporates a well-known human performance framework that establishes how humans learn to perform new tasks. In sum, this article offers both the rationale and the road map for an outside-the-box instructional approach illustrating how established complex scientific concepts can be taught to any pilot. The ESM training model has successfully been applied to design a new college course and, in collaboration with the Federal Aviation Administration, is being used to support and develop new energy management guidance materials for pilots

Collisional and radiative processes in adiabatic deceleration, deflection, and off-axis trapping of a Rydberg atom beam

A supersonic beam of Rydberg hydrogen atoms has been adiabatically deflected by 90°, decelerated to zero velocity in less than 25μs, and loaded into an electric trap. The deflection has allowed the suppression of collisions with atoms in the trailing part of the gas pulse. The processes leading to trap losses, i.e., fluorescence to the ground state, and transitions and ionization induced by blackbody radiation have been monitored over several milliseconds and quantitatively analyzed. © 2011 American Physical Society

RoLoMa: robust loco-manipulation for quadruped robots with arms

Deployment of robotic systems in the real world requires a certain level of robustness in order to deal with uncertainty factors, such as mismatches in the dynamics model, noise in sensor readings, and communication delays. Some approaches tackle these issues reactively at the control stage. However, regardless of the controller, online motion execution can only be as robust as the system capabilities allow at any given state. This is why it is important to have good motion plans to begin with, where robustness is considered proactively. To this end, we propose a metric (derived from first principles) for representing robustness against external disturbances. We then use this metric within our trajectory optimization framework for solving complex loco-manipulation tasks. Through our experiments, we show that trajectories generated using our approach can resist a greater range of forces originating from any possible direction. By using our method, we can compute trajectories that solve tasks as effectively as before, with the added benefit of being able to counteract stronger disturbances in worst-case scenarios

Multilevel blocking approach to the fermion sign problem in path-integral Monte Carlo simulations

A general algorithm toward the solution of the fermion sign problem in finite-temperature quantum Monte Carlo simulations has been formulated for discretized fermion path integrals with nearest-neighbor interactions in the Trotter direction. This multilevel approach systematically implements a simple blocking strategy in a recursive manner to synthesize the sign cancellations among different fermionic paths throughout the whole configuration space. The practical usefulness of the method is demonstrated for interacting electrons in a quantum dot.Comment: 4 pages RevTeX, incl. two figure