Mats Nylén

Demonstration of a Controllable Three-Dimensional Brownian Motor in Symmetric Potentials

We demonstrate a Brownian motor, based on cold atoms in optical lattices, where isotropic random fluctuations are rectified in order to induce controlled atomic motion in arbitrary directions. In contrast to earlier demonstrations of ratchet effects, our Brownian motor operates in potentials that are spatially and temporally symmetric, but where spatiotemporal symmetry is broken by a phase shift between the potentials and asymmetric transfer rates between them. The Brownian motor is demonstrated in three dimensions and the noise-induced drift is controllable in our system.

A user-centric cluster and grid computing portal

The HPC2N grid portal is a user-centric environment that provides a homogeneous interface to a set of heterogeneous high-performance computing resources from standard Web-browsers. The interface includes support for most everyday activities for a regular user, such as to submit, manipulate and delete jobs, monitor queues and job status, obtain user-, project-, and resource statistics and information, view job output, etc. This contribution reviews the portal functionalities and presents the design and implementation of the underlying system architecture. Some major design considerations, features and limitations are discussed and future extensions are outlined. The portal currently gives access to all major resources at HPC2N, in total comprising over 700 CPUs.

Design and evaluation of a TOP100 Linux Super Cluster system

The High Performance Computing Center North (HPC2N) Super Cluster is a truly self‐made high‐performance Linux cluster with 240 AMD processors in 120 dual nodes, interconnected with a high‐bandwidth, low‐latency SCI network. This contribution describes the hardware selected for the system, the work needed to build it, important software issues and an extensive performance analysis. The performance is evaluated using a number of state‐of‐the‐art benchmarks and software, including STREAM, Pallas MPI, the Atlas DGEMM, High‐Performance Linpack and NAS Parallel benchmarks. Using these benchmarks we first determine the raw memory bandwidth and network characteristics; the practical peak performance of a single CPU, a single dual‐node and the complete 240‐processor system; and investigate the parallel performance for non‐optimized dusty‐deck Fortran applications. In summary, this

Assessment of a time-of-flight detection technique for measuring small velocities of cold atoms

500 000 system is extremely cost‐effective and shows the performance one would expect of a large‐scale supercomputing system with distributed memory architecture. According to the TOP500 list of June 2002, this cluster was the 94th fastest computer in the world. It is now fully operational and stable as the main computing facility at HPC2N. The systems utilization figures exceed 90%, i.e. all 240 processors are on average utilized over 90% of the time, 24 hours a day, seven days a week. Copyright

A nonadiabatic semi-classical method for dynamics of atoms in optical lattices

A low noise time-of-flight detection system for laser cooled atoms has been constructed and incrementally optimized. Here, a thorough description of the construction is presented along with an analysis of the capabilities of the system. The quality of the detection (the resolution, the reproducibility, the sensitivity, etc.) is crucial for, e.g., the ability to see details in the velocity distribution profile, which is of interest for fundamental studies of statistical physics and of the laser cooling processes, and for detection of small initial velocities of an atomic cloud, important, e.g., when studying small drifts induced by Brownian motors and ratchets. We estimate the signal-to-noise ratio of our signal to be better than 1000:1 for a typical single shot, and we discuss the effect of the initial atomic cloud size, the probe size, and the effects of the wave packet spread during the fall time on the measured quantities. We show that the shape of the velocity distribution is well conserved during the mapping done in the detection, i.e., in the convolution with the probe beam, and that velocities as small as a few percent of the single photon recoil velocity can be resolved.

Controllable 3D atomic Brownian motor in optical lattices

Abstract.We develop a semi-classical method to simulate the motion of atoms in a dissipative optical lattice. Our method treats the internal states of the atom quantum mechanically, including all nonadiabatic couplings, while position and momentum are treated as classical variables. We test our method in the one-dimensional case. Excellent agreement with fully quantum mechanical simulations is found. Our results are much more accurate than those of earlier semi-classical methods based on the adiabatic approximation.

New critical point for two dimensional XY-type models.

Abstract.We study a Brownian motor, based on cold atoms in optical lattices, where atomic motion can be induced in a controlled manner in an arbitrary direction, by rectification of isotropic random fluctuations. In contrast with ratchet mechanisms, our Brownian motor operates in a potential that is spatially and temporally symmetric, in apparent contradiction to the Curie principle. Simulations, based on the Fokker-Planck equation, allow us to gain knowledge on the qualitative behaviour of our Brownian motor. Studies of Brownian motors, and in particular ones with unique control properties, are of fundamental interest because of the role they play in protein motors and their potential applications in nanotechnology. In particular, our system opens the way to the study of quantum Brownian motors.