Biomedical subjects

Hans-Andreas Engel

Publications and source records attributed to Hans-Andreas Engel.

6 recordsLinked to original sources

PubMed2006

The contrast-imaging function for tilted specimens.

A theoretical description of the contrast-imaging function is derived for tilted specimens that exhibit weak-phase object characteristics. We show that the tilted contrast-imaging function (TCIF) is a linear transformation, which can be approximated by the convolution operation for small tilt angles or for small specimens. This approximation is not valid for electron tomography, where specimen tilts are above 60 degrees and specimen dimensions amount to some 10 microm. The approximation also breaks down for electron crystallography, where atomic resolution is to be achieved. Therefore, we do not make this approximation and propose a generalized algorithm for inverting the TCIF. The implications of our description are discussed in the context of electron tomography, single particle analysis, and electron crystallography, and the improved resolution is quantitatively demonstrated.

Crystallography↗

PubMed2005

Theory of spin hall conductivity in n-doped GaAs.

We develop a theory of extrinsic spin currents in semiconductors, resulting from spin-orbit coupling at charged scatterers, which leads to skew-scattering and side-jump contributions to the spin-Hall conductivity. Applying the theory to bulk n-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with experiments by Kato et al. [Science 306, 1910 (2004)].

Journal Article↗

PubMed2005

Fermionic bell-state analyzer for spin qubits.

We propose a protocol and physical implementation for partial Bell-state measurements of Fermionic qubits, allowing for deterministic quantum computing in solid-state systems without the need for two-qubit gates. Our scheme consists of two spin qubits in a double quantum dot where the two dots have different Zeeman splittings and resonant tunneling between the dots is only allowed when the spins are antiparallel. This converts spin parity into charge information by means of a projective measurement and can be implemented with established technologies. This measurement-based qubit scheme greatly simplifies the experimental realization of scalable quantum computers in electronic nanostructures.

Journal Article↗

PubMed2004

Asymmetric quantum shot noise in quantum dots.

We analyze the frequency-dependent noise of a current through a quantum dot which is coupled to Fermi leads and which is in the Coulomb blockade regime. We show that the asymmetric shot noise, as a function of detection frequency, shows steps and becomes super-Poissonian. This provides experimental access to the quantum fluctuations of the current. We present an exact calculation of the noise for a single dot level and a perturbative evaluation of the noise in Born approximation (sequential tunneling regime but without Markov approximation) for the general case of many levels with charging interaction.

Journal Article↗

PubMed2004

Measurement efficiency and n-shot readout of spin qubits.

We consider electron spin qubits in quantum dots and define a measurement efficiency e to characterize reliable measurements via n-shot readouts. We propose various implementations based on a double dot and a quantum point contact (QPC) and show that the associated efficiencies e vary between 50% and 100%, allowing single-shot readout in the latter case. We model the readout microscopically and derive its time dynamics in terms of a generalized master equation, calculate the QPC current, and show that it allows spin readout under realistic conditions.

Journal Article↗

PubMed2002

Sampling the conformational space of membrane protein surfaces with the AFM.

The atomic force microscope acquires topographs of single native membrane proteins at subnanometer resolution. Owing to the high signal-to-noise ratio, such images allow the conformational space of membrane protein surfaces to be sampled. This is demonstrated by topographs of porin OmpF, aquaporin-Z, and bacteriorhodopsin, all recorded at a lateral resolution of <7 A and a vertical resolution of ~1 A. The amplitudes of the domain movements were estimated from a large number of single molecule topographs and the corresponding energy landscapes calculated. To visualize the motion of protein domains, movies were generated by similarity ranking of the observed protein configurations. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00249-001-0197-8

Aquaporins↗