1,238 research outputs found
Three-dimensional nuclear spin positioning using coherent radio-frequency control
Distance measurements via the dipolar interaction are fundamental to the
application of nuclear magnetic resonance (NMR) to molecular structure
determination, but they only provide information on the absolute distance
and polar angle between spins. In this Letter, we present a protocol
to also retrieve the azimuth angle . Our method relies on measuring the
nuclear precession phase after application of a control pulse with a calibrated
external radio-frequency coil. We experimentally demonstrate three-dimensional
positioning of individual carbon-13 nuclear spins in a diamond host crystal
relative to the central electronic spin of a single nitrogen-vacancy center.
The ability to pinpoint three-dimensional nuclear locations is central for
realizing a nanoscale NMR technique that can image the structure of single
molecules with atomic resolution.Comment: 5 pages, 4 figure
High-bandwidth microcoil for fast nuclear spin control
The active manipulation of nuclear spins with radio-frequency (RF) coils is
at the heart of nuclear magnetic resonance (NMR) spectroscopy and spin-based
quantum devices. Here, we present a microcoil transmitter system designed to
generate strong RF pulses over a broad bandwidth, allowing for fast spin
rotations on arbitrary nuclear species. Our design incorporates: (i) a planar
multilayer geometry that generates a large field of 4.35 mT per unit current,
(ii) a 50 Ohm transmission circuit with a broad excitation bandwidth of
approximately 20 MHz, and (iii) an optimized thermal management for removal of
Joule heating. Using individual 13C nuclear spins in the vicinity of a diamond
nitrogen-vacancy (NV) center as a test system, we demonstrate Rabi frequencies
exceeding 70 kHz and nuclear pi/2 rotations within 3.4 us. The extrapolated
values for 1H spins are about 240 kHz and 1 us, respectively. Beyond enabling
fast nuclear spin manipulations, our microcoil system is ideally suited for the
incorporation of advanced pulse sequences into micro- and nanoscale NMR
detectors operating at low (<1 T) magnetic field.Comment: 8 pages, 5 figures. Submitted to Rev. Sci. Inst
Quantum sensing with arbitrary frequency resolution
Quantum sensing takes advantage of well controlled quantum systems for
performing measurements with high sensitivity and precision. We have
implemented a concept for quantum sensing with arbitrary frequency resolution,
independent of the qubit probe and limited only by the stability of an external
synchronization clock. Our concept makes use of quantum lock-in detection to
continuously probe a signal of interest. Using the electronic spin of a single
nitrogen vacancy center in diamond, we demonstrate detection of oscillating
magnetic fields with a frequency resolution of 70 uHz over a MHz bandwidth. The
continuous sampling further guarantees an excellent sensitivity, reaching a
signal-to-noise ratio in excess of 10,000:1 for a 170 nT test signal measured
during a one-hour interval. Our technique has applications in magnetic
resonance spectroscopy, quantum simulation, and sensitive signal detection.Comment: Manuscript resubmitted to Science. Includes Supplementary Material
High resolution quantum sensing with shaped control pulses
We investigate the application of amplitude-shaped control pulses for
enhancing the time and frequency resolution of multipulse quantum sensing
sequences. Using the electronic spin of a single nitrogen vacancy center in
diamond and up to 10,000 coherent microwave pulses with a cosine square
envelope, we demonstrate 0.6 ps timing resolution for the interpulse delay.
This represents a refinement by over 3 orders of magnitude compared to the 2 ns
hardware sampling. We apply the method for the detection of external AC
magnetic fields and nuclear magnetic resonance signals of carbon-13 spins with
high spectral resolution. Our method is simple to implement and especially
useful for quantum applications that require fast phase gates, many control
pulses, and high fidelity.Comment: 5 pages, 4 figures, plus supplemental materia
Mechanisms of vascular smooth muscle contraction and the basis for pharmacologic treatment of smooth muscle disorders
The smooth muscle cell directly drives the contraction of the vascular wall and hence regulates the size of the blood vessel lumen. We review here the current understanding of the molecular mechanisms by which agonists, therapeutics, and diseases regulate contractility of the vascular smooth muscle cell and we place this within the context of whole body function. We also discuss the implications for personalized medicine and highlight specific potential target molecules that may provide opportunities for the future development of new therapeutics to regulate vascular function.Accepted manuscrip
Evidence-based Economic Policy - Three Essays in Applied Microeconometrics
Over thirty years ago, Leamer (1983) - among many others - expressed doubts about the quality and usefulness of empirical analyses for the economic profession by stating that "hardly anyone takes data analyses seriously. Or perhaps more accurately, hardly anyone takes anyone else's data analyses seriously" (p.37). Improvements in data quality, more robust estimation methods and the evolution of better research designs seem to make that assertion no longer justifiable (see Angrist and Pischke (2010) for a recent response to Leamer's essay). The economic profes- sion and policy makers alike often rely on empirical evidence as a means to investigate policy relevant questions. The approach of using scientifically rigorous and systematic evidence to identify policies and programs that are capable of improving policy-relevant outcomes is known under the increasingly popular notion of evidence-based policy.
Evidence-based economic policy often relies on randomized or quasi-natural experiments in order to identify causal effects of policies. These can require relatively strong assumptions or raise concerns of external validity. In the context of this thesis, potential concerns are for example endogeneity of policy reforms with respect to the business cycle in the first chapter, the trade-off between precision and bias in the regression-discontinuity setting in chapter 2 or non-representativeness of the sample due to self-selection in chapter 3. While the identification strategies are very useful to gain insights into the causal effects of specific policy questions, transforming the evidence into concrete policy conclusions can be challenging. Policy develop- ment should therefore rely on the systematic evidence of a whole body of research on a specific policy question rather than on a single analysis. In this sense, this thesis cannot and should not be viewed as a comprehensive analysis of specific policy issues but rather as a first step towards a better understanding of certain aspects of a policy question.
The thesis applies new and innovative identification strategies to policy-relevant and topical questions in the fields of labor economics and behavioral environmental economics. Each chapter relies on a different identification strategy. In the first chapter, we employ a difference- in-differences approach to exploit the quasi-experimental change in the entitlement of the max- imum unemployment benefit duration to identify the medium-run effects of reduced benefit durations on post-unemployment outcomes. Shortening benefit duration carries a double- dividend: It generates fiscal benefits without deteriorating the quality of job-matches. On the
contrary, shortened benefit durations improve medium-run earnings and employment possibly through containing the negative effects of skill depreciation or stigmatization.
While the first chapter provides only indirect evidence on the underlying behavioral channels, in the second chapter I develop a novel approach that allows to learn about the relative impor- tance of the two key margins of job search - reservation wage choice and search effort. In the framework of a standard non-stationary job search model, I show how the exit rate from un- employment can be decomposed in a way that is informative on reservation wage movements over the unemployment spell. The empirical analysis relies on a sharp discontinuity in unem- ployment benefit entitlement, which can be exploited in a regression-discontinuity approach to identify the effects of extended benefit durations on unemployment and survivor functions. I find evidence that calls for an important role of reservation wage choices for job search be- havior. This can have direct implications for the optimal design of unemployment insurance policies.
The third chapter - while thematically detached from the other chapters - addresses one of the major policy challenges of the 21st century: climate change and resource consumption. Many governments have recently put energy efficiency on top of their agendas. While pricing instru- ments aimed at regulating the energy demand have often been found to be short-lived and difficult to enforce politically, the focus of energy conservation programs has shifted towards behavioral approaches - such as provision of information or social norm feedback. The third chapter describes a randomized controlled field experiment in which we discuss the effective- ness of different types of feedback on residential electricity consumption. We find that detailed and real-time feedback caused persistent electricity reductions on the order of 3 to 5 % of daily electricity consumption. Also social norm information can generate substantial electricity sav- ings when designed appropriately. The findings suggest that behavioral approaches constitute effective and relatively cheap way of improving residential energy-efficiency
Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe
Chiral magnetic interactions induce complex spin textures including helical
and conical spin waves, as well as particle-like objects such as magnetic
skyrmions and merons. These spin textures are the basis for innovative device
paradigms and give rise to exotic topological phenomena, thus being of interest
for both applied and fundamental sciences. Present key questions address the
dynamics of the spin system and emergent topological defects. Here we analyze
the micromagnetic dynamics in the helimagnetic phase of FeGe. By combining
magnetic force microscopy, single-spin magnetometry, and
Landau-Lifschitz-Gilbert simulations we show that the nanoscale dynamics are
governed by the depinning and subsequent motion of magnetic edge dislocations.
The motion of these topologically stable objects triggers perturbations that
can propagate over mesoscopic length scales. The observation of stochastic
instabilities in the micromagnetic structure provides new insight to the
spatio-temporal dynamics of itinerant helimagnets and topological defects, and
discloses novel challenges regarding their technological usage
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