Finding the Prepotent Sire

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Magnetism in SQUIDs at Millikelvin Temperatures

We have characterized the temperature dependence of the flux threading dc SQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data is compatible with the thermal polarization of surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin-glass freezing at low temperature. These results suggest an explanation for the "universal" 1/f flux noise in SQUIDs and superconducting qubits.Comment: 4 pages, 4 figure

Resonance fluorescence from an artificial atom in squeezed vacuum

We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments

Tracking random finite objects using 3D-LIDAR in marine environments

This paper presents a random finite set theoretic formulation for multi-object tracking as perceived by a 3D-LIDAR in a dynamic environment. It is mainly concerned with the joint detection and estimation of the unknown and time varying number of objects present in the environment and the dynamic state of these objects, given a set of measurements. This problem is particularly challenging in cluttered dynamic environments such as in urban settings or marine environments, because, given a measurement set, there is absolutely no knowledge of which object generated which measurement, and the detected measurements are indistinguishable from false alarms. The proposed approach to multi-object tracking is based on the rigorous theory of finite set statistics (FISST). The optimal Bayesian multi-object tracking is not yet practical due to its computational complexity. However, a practical alternative to the optimal filter is the probability hypothesis density (PHD) filter, that propagates the first order statistical moment of the full multi-object posterior distribution. In contrast to classical approaches, this random finite set framework does not require any explicit data associations. In this paper, a Gaussian mixture approximation of the PHD filter is applied to track variable number of objects from 3D-LIDAR measurements by estimating both the number of objects and their respective locations in each scan. Experimental results obtained in marine environments demonstrate the efficacy and tracking performance of the proposed approach.MIT-Singapore Allianc

Development and application of distributed MEMS pressure sensor array for AUV object avoidance

A novel sensory system is being developed for AUVs to augment current sensory systems for navigation and operation in difficult environments. These environments are frequently cluttered and murky with substantial flow from currents or waves, frustrating sonar and vision systems while posing an increased risk to AUVs. In order to manage such situations, a better ability to locate and identify physical objects is needed. This gap could be filled by small low frequency pressure sensors distributed over the surface of the AUV in dense arrays.United States. National Oceanic and Atmospheric Administration (Grant NA06OAR4170019 Project R/RT-2/RCM-17

Infrastructure for 3D model reconstruction of marine structures

3D model reconstruction of marine structures, such as dams, oil-rigs, and sea caves, is both important and challenging. An important application includes structural inspection. Manual inspection of marine structures is tedious and even a small oversight can have severe consequences for the structure and the people around it. A robotic system that can construct 3D models of marine structures would hopefully reduce the chances of oversight, and hence improve the safety of marine environment. Due to the water currents and wakes, developing a robotic system to construct 3D models of marine structures is a challenge, as it is difficult for a robot to reach the desired scan configurations and take a scan of the environment while remaining stationary. This paper presents our preliminary work in developing a robotic and software system for construction of 3D models of marine structures. We have successfully tested our system in a sea water environment in the Singapore Straits

Lateral-Line Inspired MEMS-Array Pressure Sensing for Passive Underwater Navigation

This paper presents work toward the development of a novel MEMS sensing technology for AUVs. The proposed lateral line-inspired sensor system is a high-density array of pressure sensors for measuring hydrodynamic disturbances. By measuring pressure variations on a vehicle surface, a dense pressure sensor array will allow the AUV to detect, classify, and locate nearby obstacles and optimize its motion in unsteady environments. This approach is very similar to the canal lateral line system found in all fish, which allow them to function in dark or clouded environments. In order to lay the groundwork for developing the MEMS sensor and interpreting the pressure distributions, the paper also presents experiments demonstrating the discrimination between cylindrical obstacles of round and square cross sections with an array of off-the-shelf pressure sensors. Test objects with 5.1 cm and 7.6 cm diameters passed stationary sensors at 0.5 m/s and 0.75 m/s and with 1.3 and 5.1 mm separation. Hand chosen features and features chosen through a Principal Component Analysis are used to discriminate between object shapes under a variety of conditions. A classification error rate of under 2% is achieved across all velocities, sizes, and separations. These results lead to requirements for the density, sensitivity, and frequency response of the MEMS sensors, which fall well in the MEMS domain. The pressure sensor array proposed here consists of hundreds of MEMS pressure sensors with diameters near 1 mm spaced a few millimeters apart fabricated on etched silicon and Pyrex wafers; a fabrication process for producing the array is described. A strain-gauge pressure sensor is analyzed and shown to satisfy specifications as required by the results from the afore-mentioned experiments. The sensing element is a strain gauge mounted on a flexible diaphragm, which is a thin (20 µm) layer of silicon attached at the edges to a square silicon cavity 2000 µm wide on a side. A source voltage of 10 V produces a sensor with a sensitivity on the order of 1µV/Pa. Since the thermal noise voltage is near 0.7 µV, the pressure resolution of the sensors is on the order of 1 Pa.United States. National Oceanic and Atmospheric Administration (Grant NA06OAR4170019 Project R/RT-2/RMC-17

Foraging Ecology of Fall-Migrating Shorebirds in the Illinois River Valley

Populations of many shorebird species appear to be declining in North America, and food resources at stopover habitats may limit migratory bird populations. We investigated body condition of, and foraging habitat and diet selection by 4 species of shorebirds in the central Illinois River valley during fall migrations 2007 and 2008 (Killdeer [Charadrius vociferus], Least Sandpiper [Calidris minutilla], Pectoral Sandpiper [Calidris melanotos], and Lesser Yellowlegs [Tringa flavipes]). All species except Killdeer were in good to excellent condition, based on size-corrected body mass and fat scores. Shorebird diets were dominated by invertebrate taxa from Orders Diptera and Coleoptera. Additionally, Isopoda, Hemiptera, Hirudinea, Nematoda, and Cyprinodontiformes contribution to diets varied by shorebird species and year. We evaluated diet and foraging habitat selection by comparing aggregate percent dry mass of food items in shorebird diets and core samples from foraging substrates. Invertebrate abundances at shorebird collection sites and random sites were generally similar, indicating that birds did not select foraging patches within wetlands based on invertebrate abundance. Conversely, we found considerable evidence for selection of some diet items within particular foraging sites, and consistent avoidance of Oligochaeta. We suspect the diet selectivity we observed was a function of overall invertebrate biomass (51.264.4 [SE] kg/ha; dry mass) at our study sites, which was greater than estimates reported in most other food selection studies. Diet selectivity in shorebirds may follow tenants of optimal foraging theory; that is, at low food abundances shorebirds forage opportunistically, with the likelihood of selectivity increasing as food availability increases. Nonetheless, relationships between the abundance, availability, and consumption of Oligochaetes for and by waterbirds should be the focus of future research, because estimates of foraging carrying capacity would need to be revised downward if Oligochaetes are truly avoided or unavailable for consumption