913 research outputs found
Application and comparison of three tomographic techniques for detection of decay in trees
This paper reports application of electric, ultrasonic, and georadar tomography for detection of decay in trees and their comparison with the traditional penetrometer. Their feasibility in arboriculture is also evaluated, critically considering some "open problems." The experiments were carried out in an urban environment on two plane (Platanus hybrida Brot.) trees. Both trees, after felling, showed extensive white rot in the central cylinder. The electric tomography revealed low resistivity zones roughly centered in the trunk. A comparison with the successively cut sections showed a fine correspondence to decayed areas and a strong correspondence between high moisture zones and low resistivity zones. Ultrasonic tomography demonstrated to be a very effective tool for the detection of internal decay, accurately locating the position of the anomalies and estimating their size, shape, and characteristic in terms of mechanical properties. With the georadar technique, the high contrast of electromagnetic impedance measured between the inner decayed section and the outside sound section allowed the detection of the interface between the sound and decayed section of the tree, using radar acquisition in reflection modality. The penetrometer profiles detected the low-resistance areas inside the two trunk
Decoherence in a Josephson junction qubit
The zero-voltage state of a Josephson junction biased with constant current
consists of a set of metastable quantum energy levels. We probe the spacings of
these levels by using microwave spectroscopy to enhance the escape rate to the
voltage state. The widths of the resonances give a measurement of the coherence
time of the two states involved in the transitions. We observe a decoherence
time shorter than that expected from dissipation alone in resonantly isolated
20 um x 5 um Al/AlOx/Al junctions at 60 mK. The data is well fit by a model
including dephasing effects of both low-frequency current noise and the escape
rate to the continuum voltage states. We discuss implications for quantum
computation using current-biased Josephson junction qubits, including the
minimum number of levels needed in the well to obtain an acceptable error limit
per gate.Comment: 4 pages, 6 figure
Quantum logic with weakly coupled qubits
There are well-known protocols for performing CNOT quantum logic with qubits
coupled by particular high-symmetry (Ising or Heisenberg) interactions.
However, many architectures being considered for quantum computation involve
qubits or qubits and resonators coupled by more complicated and less symmetric
interactions. Here we consider a widely applicable model of weakly but
otherwise arbitrarily coupled two-level systems, and use quantum gate design
techniques to derive a simple and intuitive CNOT construction. Useful
variations and extensions of the solution are given for common special cases.Comment: 4 pages, Revte
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
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