38 research outputs found
Superconductivity in nanostructures: Andreev billiards and Josephson junction qubits
UBL - phd migration 201
Adiabatic quantization of Andreev quantum billiard levels
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
Noiseless scattering states in a chaotic cavity
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
Quantum-to-classical crossover of mesoscopic conductance fluctuations
We calculate the system-size-over-wave-length () dependence of
sample-to-sample conductance fluctuations, using the open kicked rotator to
model chaotic scattering in a ballistic quantum dot coupled by two -mode
point contacts to electron reservoirs. Both a fully quantum mechanical and a
semiclassical calculation are presented, and found to be in good agreement. The
mean squared conductance fluctuations reach the universal quantum limit of
random-matrix-theory for small systems. For large systems they increase
at fixed mean dwell time . The universal
quantum fluctuations dominate over the nonuniversal classical fluctuations if
. When expressed as a ratio of time scales, the
quantum-to-classical crossover is governed by the ratio of Ehrenfest time and
ergodic time.Comment: 5 pages, 5 figures: one figure added, references update
Nonequilibrium stabilization of charge states in double quantum dots
We analyze the decoherence of charge states in double quantum dots due to
cotunneling. The system is treated using the Bloch-Redfield generalized master
equation for the Schrieffer-Wolff transformed Hamiltonian. We show that the
decoherence, characterized through a relaxation and a dephasing time
, can be controlled through the external voltage and that the
optimum point, where these times are maximum, is not necessarily in
equilibrium. We outline the mechanism of this nonequilibrium-induced
enhancement of lifetime and coherence. We discuss the relevance of our results
for recent charge qubit experiments.Comment: 5 pages, 5 figure
Low-energy quasiparticle excitations in dirty d-wave superconductors and the Bogoliubov-de Gennes kicked rotator
We investigate the quasiparticle density of states in disordered d-wave
superconductors. By constructing a quantum map describing the quasiparticle
dynamics in such a medium, we explore deviations of the density of states from
its universal form (), and show that additional low-energy
quasiparticle states exist provided (i) the range of the impurity potential is
much larger than the Fermi wavelength [allowing to use recently developed
semiclassical methods]; (ii) classical trajectories exist along which the
pair-potential changes sign; and (iii) the diffractive scattering length is
longer than the superconducting coherence length. In the classically chaotic
regime, universal random matrix theory behavior is restored by quantum
dynamical diffraction which shifts the low energy states away from zero energy,
and the quasiparticle density of states exhibits a linear pseudogap below an
energy threshold .Comment: 4 pages, 3 figures, RevTe
NEMA NU4-2008 Performance Evaluation of Albira: A Two-Ring Small-Animal PET System Using Continuous LYSO Crystals
Goals: This paper presents the performance review based on a dual-ring Positron Emission Tomography (PET) scanner being a part
of Bruker Albira: a multi-modal small-animal imaging platform. Each ring of Albira PET contains eight detectors arranged as
octagon, and each detector is built using a single continuous lutetium-yttrium oxyorthosilicate crystal and multi-anode photo
multiplier tube. In two-ring configuration, the scanner covers 94.4 mm in axial- and 80´80 mm in trans-axial direction, which is
sufficient to acquire images of small animals (e.g. mice) without the need of moving the animal bed during the scan.
Methods: All measurements and majority of data processing were performed according to the NEMA NU4-2008 standard with one
exception. Due to the scanner geometry, the spatial resolution test was reconstructed using iterative algorithm instead of the
analytical one. The main performance characteristics were compared with those of the other PET sub-systems of tri-modal smallanimal
scanners.
Results: The measured spatial resolution at the centre of the axial field of view in radial, tangential and axial directions was 1.72,
1.70 and 2.45 mm, respectively. The scatter fraction for the mouse-like phantom was 9.8% and for the rat-like phantom, 21.8%. The
maximum absolute sensitivity was 5.30%. Finally, the recovery co-efficients for 5, 4, 3, 2, 1 mm diameter rods in image quality
phantom were: 0.90, 0.77, 0.66, 0.30 and 0.05, respectively.
Conclusion: The Bruker Albira is a versatile small-animal multi-modal device that can be used for variety of studies. Overall the
PET sub-system provides a good spatial resolution coupled with better-than average sensitivity and the ability to produce good
quality animal images when administering low activities
Quantum information processing with superconducting qubits in a microwave field
We investigate the quantum dynamics of a Cooper-pair box with a
superconducting loop in the presence of a nonclassical microwave field. We
demonstrate the existence of Rabi oscillations for both single- and
multi-photon processes and, moreover, we propose a new quantum computing scheme
(including one-bit and conditional two-bit gates) based on Josephson qubits
coupled through microwaves.Comment: 7 pages, 1 figur
Monitoring phenylalanine concentrations in the follow-up of phenylketonuria patients:An inventory of pre-analytical and analytical variation
Background: Reliable measurement of phenylalanine (Phe) is a prerequisite for adequate follow-up of phenylketonuria (PKU) patients. However, previous studies have raised concerns on the intercomparability of plasma and dried blood spot (DBS) Phe results. In this study, we made an inventory of differences in (pre-)analytical methodology used for Phe determination across Dutch laboratories, and compared DBS and plasma results. Methods: Through an online questionnaire, we assessed (pre-)analytical Phe measurement procedures of seven Dutch metabolic laboratories. To investigate the difference between plasma and DBS Phe, participating laboratories received simultaneously collected plasma-DBS sets from 23 PKU patients. In parallel, 40 sample sets of DBS spotted from either venous blood or capillary fingerprick were analyzed. Results: Our data show that there is no consistency on standard operating procedures for Phe measurement. The association of DBS to plasma Phe concentration exhibits substantial inter-laboratory variation, ranging from a mean difference of −15.5% to +30.6% between plasma and DBS Phe concentrations. In addition, we found a mean difference of +5.8% in Phe concentration between capillary DBS and DBS prepared from venous blood. Conclusions: The results of our study point to substantial (pre-)analytical variation in Phe measurements, implicating that bloodspot Phe results should be interpreted with caution, especially when no correction factor is applied. To minimize variation, we advocate pre-analytical standardization and analytical harmonization of Phe measurements, including consensus on application of a correction factor to adjust DBS Phe to plasma concentrations