PubMed Health⌕ Search

Biomedical subjects

M P Hanson

Publications and source records attributed to M P Hanson.

17 recordsLinked to original sources
PubMed2006

Effect of exchange interaction on spin dephasing in a double quantum dot.

We measure singlet-triplet dephasing in a two-electron double quantum dot in the presence of an exchange interaction which can be electrically tuned from much smaller to much larger than the hyperfine energy. Saturation of dephasing and damped oscillations of the spin correlator as a function of time are observed when the two interaction strengths are comparable. Both features of the data are compared with predictions from a quasistatic model of the hyperfine field.

Journal Article↗
PubMed2006

Resonant-optical-cavity photoconductive switch with 0.5% conversion efficiency and 1.0 W peak power.

We report a new photoconductive switch having an average output power of 44 microW, an instantaneous bandwidth of approximately 300 GHz, an output pulse width of approximately 2.2 ps, a peak output power of approximately 1.0 W, and an optical-to-electrical conversion efficiency of approximately 0.5% when pumped by a palm-sized mode-locked free-space laser at lambda=780.6 nm with an average power of 8.7 mW and an optical pulse width of approximately -230 fs. The switch is made from an ErAs:GaAs epitaxial layer inside a resonant optical cavity and coupled to a planar three-turn square spiral antenna.

Journal Article↗
PubMed2006

Asymmetry of nonlinear transport and electron interactions in quantum dots.

The symmetry properties of transport beyond the linear regime in chaotic quantum dots are investigated experimentally. A component of differential conductance that is antisymmetric in both applied source-drain bias V and magnetic field B, absent in linear transport, is found to exhibit mesoscopic fluctuations around a zero average. Typical values of this component allow a measurement of the electron interaction strength.

Journal Article↗
PubMed2005

Coherent manipulation of coupled electron spins in semiconductor quantum dots.

We demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of approximately 10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.

Journal Article↗
PubMed2005

Triplet-singlet spin relaxation via nuclei in a double quantum dot.

The spin of a confined electron, when oriented originally in some direction, will lose memory of that orientation after some time. Physical mechanisms leading to this relaxation of spin memory typically involve either coupling of the electron spin to its orbital motion or to nuclear spins. Relaxation of confined electron spin has been previously measured only for Zeeman or exchange split spin states, where spin-orbit effects dominate relaxation; spin flips due to nuclei have been observed in optical spectroscopy studies. Using an isolated GaAs double quantum dot defined by electrostatic gates and direct time domain measurements, we investigate in detail spin relaxation for arbitrary splitting of spin states. Here we show that electron spin flips are dominated by nuclear interactions and are slowed by several orders of magnitude when a magnetic field of a few millitesla is applied. These results have significant implications for spin-based information processing.

Journal Article↗
PubMed2004

Cotunneling spectroscopy in few-electron quantum dots.

Few-electron quantum dots are investigated in the regime of strong tunneling to the leads. Inelastic cotunneling is used to measure the two-electron singlet-triplet splitting above and below a magnetic field driven singlet-triplet transition. Evidence for a nonequilibrium two-electron singlet-triplet Kondo effect is presented. Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-electron singlet ground state to be extracted from dc transport.

Journal Article↗
PubMed2004

Manipulation of a single charge in a double quantum dot.

We manipulate a single electron in a fully tunable double quantum dot using microwave excitation. Under resonant conditions, microwaves drive transitions between the (1,0) and (0,1) charge states of the double dot. Local quantum point contact charge detectors enable a direct measurement of the photon-induced change in occupancy of the charge states. From charge sensing measurements, we find T1 approximately 16 ns and a lower bound estimate for T*(2) of 400 ps for the charge two-level system.

Journal Article↗
PubMed2004

Coulomb-modified Fano resonance in a one-lead quantum dot.

We investigate a tunable Fano interferometer consisting of a quantum dot coupled via tunneling to a one-dimensional channel. In addition to Fano resonance, the channel shows strong Coulomb response to the dot, with a single electron modulating channel conductance by factors up to 100. Where these effects coexist, line shapes with up to four extrema are found. A model of Coulomb-modified Fano resonance is developed and gives excellent agreement with experiment.

Journal Article↗
PubMed2004

Differential charge sensing and charge delocalization in a tunable double quantum dot.

We report measurements of a tunable double quantum dot, operating in the quantum regime, with integrated local charge sensors. The spatial resolution of the sensors allows the charge distribution within the double dot system to be resolved at fixed total charge. We use this readout scheme to investigate charge delocalization as a function of temperature and strength of tunnel coupling, demonstrating that local charge sensing can be used to accurately determine the interdot coupling in the absence of transport.

Journal Article↗
PubMed2004

Tunable nonlocal spin control in a coupled-quantum dot system.

The effective interaction between magnetic impurities in metals that can lead to various magnetic ground states often competes with a tendency for electrons near impurities to screen the local moment (known as the Kondo effect). The simplest system exhibiting the richness of this competition, the two-impurity Kondo system, was realized experimentally in the form of two quantum dots coupled through an open conducting region. We demonstrate nonlocal spin control by suppressing and splitting Kondo resonances in one quantum dot by changing the electron number and coupling of the other dot. The results suggest an approach to nonlocal spin control that may be relevant to quantum information processing.

Journal Article↗
PubMed1993

prlA suppression of defective export of maltose-binding protein in secB mutants of Escherichia coli.

An Escherichia coli strain containing a signal sequence mutation in the periplasmic maltose-binding protein (MBP) (malE18-1) and a point mutation in the soluble export factor SecB (secBL75Q) is completely defective in export of MBP and unable to grow on maltose (Mal- phenotype). We isolated 95 spontaneous Mal+ revertants and characterized them genetically. Three types of extragenic suppressors were identified: informational (missense) suppressors, a bypass suppressor conferring the Mal+ phenotype in the absence of MBP, and suppressors affecting the prlA gene, which encodes a component of the protein export apparatus. In this study, a novel prlA allele, designated prlA1001 and mapping in the putative second transmembrane domain of the PrlA (SecY) protein, was found. In addition, we isolated a mutation designated prlA1024 which is identical to prlA4-2, the mutation responsible for the signal sequence suppression in the prlA4 (prlA4-1 prlA4-2) double mutant (T. Sako and T. Iino, J. Bacteriol. 170:5389-5391, 1988). Comparison of the prlA1024 mutant and the prlA4 double mutant provides a possible explanation for the isolation of these prlA alleles.

ATP-Binding Cassette Transporters↗
PubMed1991

Loss of chromosome 22 alleles in human sporadic spinal schwannomas.

Acoustic neuromas occur either as sporadic solitary tumors in the general population or as inherited bilateral tumors typically in patients with neurofibromatosis type 2. Loss of heterozygosity for markers on the long arm of chromosome 22 has been reported in both instances, and neurofibromatosis type 2 has been genetically linked to a marker on the long arm of this autosome, suggesting that a unique locus on chromosome 22 is implicated in tumorigenesis of both sporadic and inherited acoustic neuromas. To determine whether the locus for neurofibromatosis type 2 might also be responsible for tumorigenesis of those schwannomas distinct from acoustic neuromas in people without neurofibromatosis type 2, we studied the DNA content of three sporadic spinal schwannomas. In all three, we found loss of heterozygosity for at least three markers on the long arm of chromosome 22, indicating a partial or total monosomy 22 in the tumor. Our results suggest that a locus on chromosome 22 is responsible for tumorigenesis in schwann cells regardless of their location in the central nervous system, and that some other mechanism (genetic or nongenetic) might account for the relative high proportion of schwannomas developing from the eighth cranial nerve.

Adult↗
PubMed1991

Parental origin of chromosome 22 loss in sporadic and NF2 neuromas.

It has recently been proposed that the maternally derived chromosome might be preferentially lost in nonfamilial cases of embryonal or early onset malignant tumors. This observation pointed to a potential role of the parental imprinting of the genome during gametogenesis which would be at least partly maintained in the somatic cells. Neuromas are benign tumors that develop from Schwann cells. They occur either sporadically or in individuals that have a genetic predisposition due to neurofibromatosis type 2 (NF2) and usually are multiple. Regardless of the context of occurrence, in approximately 40% of the investigated cases a loss of a chromosome 22 has been documented either by karyotype analysis or by monitoring somatic loss of heterozygosity. We have now examined the parental origin of the chromosome 22 lost in 19 cases of neuromas of patients with unaffected parents among which 11 were non-NF2 patients (sporadic and unique neuroma) and 8 were NF2 patients (bilateral acoustic or multiple neuromas). In both sets of tumors, the lost chromosome 22 can be of either parental origin. A close to threefold preference for the loss of the maternally derived chromosome was observed and should be either confirmed or disproved by studying a larger number of patients.

Adult↗
PubMed1990

Hyperkalemic periodic paralysis and the adult muscle sodium channel alpha-subunit gene.

Hyperkalemic periodic paralysis (HYPP) is an autosomal dominant disorder characterized by episodes of muscle weakness due to depolarization of the muscle cell membrane associated with elevated serum potassium. Electrophysiological studies have implicated the adult muscle sodium channel. Here, portions of the adult muscle sodium channel alpha-subunit gene were cloned and mapped near the human growth hormone locus (GH1) on chromosome 17. In a large pedigree displaying HYPP with myotonia, these two loci showed tight linkage to the genetic defect with no recombinants detected. Thus, it is likely that the sodium channel alpha-subunit gene contains the HYPP mutation.

Adult↗
PubMed1990

Equal parental origin of chromosome 22 losses in human sporadic meningioma: no evidence for genomic imprinting.

Inactivation of tumor suppressor genes can occur either by mutation at the gene locus or by loss of part or all of the chromosome region containing the gene. The latter is most frequently detected by DNA markers as loss of heterozygosity in the tumor tissue. In several reports, the paternal homologue was preferentially retained in embryonal tumors associated with loss of particular chromosomal regions, suggesting genomic imprinting of the corresponding tumor suppressor loci. To explore the generality of these findings and the possible role of genomic imprinting in adult tumors of the nervous system, we have determined the parental origin of chromosome 22 loss in sporadic meningioma. Of nine cases studied, five tumors retained the maternally derived chromosome 22 homologue while four retained the paternally derived chromosome 22. Thus, in contrast to the embryonal tumors, the meningioma locus on chromosome 22 is inactivated by random mutation in sporadic adult meningiomas.

Alleles↗
PubMedDate not supplied

Synthesis and conformational studies of peptides containing TOAC, a spin-labelled C alpha, alpha-disubstituted glycine.

A variety of host L-alanine homo-peptides (to the pentamer) containing one or two spin-labelled TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) residues were synthesized by solution methods and fully characterized. The conformational features of the terminally blocked, doubly spin-labelled TOAC-(Ala)2-TOAC-Ala-pentapeptide were examined in the crystal state by X-ray diffraction and in solution using a combination of techniques (Fourier transform infrared, circular dichroism, cyclic voltammetry and electron spin resonance) in comparison with singly labelled shorter peptides. The 3(10)-helical structure of the pentapeptide, promoted by the two C alpha, alpha-disubstituted glycines under favourable experimental conditions, allows an interaction to take place between the two nitroxide TOAC side chains spaced by one turn of the helix. Taken together, these results suggest that TOAC is an excellent probe for exploring bends and helices in doubly labelled peptides.

Circular Dichroism↗