The problem of eliminating the effects of critical races on asynchronous machines is considered in a control theoretic context. State feedback controllers that eliminate the effects of critical races are developed. The results include necessary and sufficient conditions for the existence of such controllers and algorithms for their design. When the controllers exist, they eliminate the race effects and control the machine to match a given race-free model.

We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using near-ultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in laser-triggered and DC-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80±0.05) nm in laser-triggered and (0.55±0.02) nm in DC-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.Coherent electron sources are central to studying microscopic objects with highest spatial resolution. They provide electron beams with flat wavefronts that can be focused to the fundamental physical limit given by matter wave diffraction [1]. Currently, time-resolved electron based imaging is pursued with large efforts, both in realspace microscopy [2, 3] and in diffraction [4,5]. However, the spatial resolution in time-resolved electron microscopy is about two orders of magnitude worse than its DC counterpart [6], which reaches below 0.1Å [7]. Combining highest spatial resolution with time resolution in the picosecond to (sub-) femtosecond range requires spatially coherent electron sources driven by ultrashort laser pulses. Although laser-driven metal nanotips promise to provide coherent electron pulses with highest time resolution, a quantitative study of their spatial coherence has been elusive. Here we demonstrate that photoemitted electrons from a tungsten nanotip are highly coherent.So far no time-resolved electron based imaging instrument fully utilizes the coherence capabilities provided by nanotip electron sources. Meanwhile, nanotips operated in DC-field emission are known and employed in practical applications for almost half a century for their paramount spatial coherence properties [8]. Thence, highest resolution microscopy as well as coherent imaging, such as holography and interferometry, have long been demonstrated in DC-field emission [1, 9, 10]. Here we investigate whether these concepts can be inherited to laserdriven nanotip sources by comparing the spatial coherence of photoemitted electron beams to their DC counterparts. This would enable time-resolved high resolution imaging, but may also herald fundamental studies based on the generation of quantum degenerate electron beams [11].The spatial coherence of electron sources is commonly quantified by means of their effective source radius r eff . It eq...

One of the astounding consequences of quantum mechanics is that it allows the detection of a target using an incident probe, with only a low probability of interaction of the probe and the target. This 'quantum weirdness' could be applied in the field of electron microscopy to generate images of beam-sensitive specimens with substantially reduced damage to the specimen. A reduction of beam-induced damage to specimens is especially of great importance if it can enable imaging of biological specimens with atomic resolution. Following a recent suggestion that interaction-free measurements are possible with electrons, we now analyze the difficulties of actually building an atomic resolution interaction-free electron microscope, or "quantum electron microscope". A quantum electron microscope would require a number of unique components not found in conventional transmission electron microscopes. These components include a coherent electron beam-splitter or two-state-coupler, and a resonator structure to allow each electron to interrogate the specimen multiple times, thus supporting high success probabilities for interaction-free detection of the specimen. Different system designs are presented here, which are based on four different choices of two-state-couplers: a thin crystal, a grating mirror, a standing light wave and an electro-dynamical pseudopotential. Challenges for the detailed electron optical design are identified as future directions for development. While it is concluded that it should be possible to build an atomic resolution quantum electron microscope, we have also identified a number of hurdles to the development of such a microscope and further theoretical investigations that will be required to enable a complete interpretation of the images produced by such a microscope.

The problem of eliminating the effects of infinite cycles on asynchronous sequential machines is considered in a control theoretic context. The main objective is to develop state feedback controllers that stop infinite cycles in an existing asynchronous machine, while controlling the machine to match a prescribed model. Necessary and sufficient conditions for the existence of such controllers are derived in terms of an inequality condition between two numerical matrices. The results include an algorithm for the characterization of all infinite cycles of a given machine as well as an algorithm for the construction of appropriate controllers, whenever they exist.