Algorithms and Throughput Analysis for MDS-Coded Switches

Network switches and routers need to serve packet writes and reads at rates that challenge the most advanced memory technologies. As a result, scaling the switching rates is commonly done by parallelizing the packet I/Os using multiple memory units. For improved read rates, packets can be coded with an [n,k] MDS code, thus giving more flexibility at read time to achieve higher utilization of the memory units. In the paper, we study the usage of [n,k] MDS codes in a switching environment. In particular, we study the algorithmic problem of maximizing the instantaneous read rate given a set of packet requests and the current layout of the coded packets in memory. The most interesting results from practical standpoint show how the complexity of reaching optimal read rate depends strongly on the writing policy of the coded packets.Comment: 6 pages, an extended version of a paper accepted to the 2015 IEEE International Symposium on Information Theory (ISIT

Radio-frequency atomic magnetometry with a rubidium Bose-Einstein condensate

This thesis details progress in radio-frequency atomic magnetometry with ultracold rubidium atoms. Motivations and context are first covered, before an introduction of the main concepts required to understand the underlying physics is given. At first, a cold atom magnetometer is designed, built and characterised. Consistent 20 µK atoms are produced. Radio-frequency (RF) atomic magnetometry (AM) is performed by placing the atoms in a bias magnetic field and generating coherent precession with an external AC field. A noise floor at 330 pT/√Hz defines the sensor’s sensitivity, with a range of applications. RF-AM is then performed with a Bose-Einstein condensate (BEC). The 20 µK atoms are loaded into a magnetic trap, where RF evaporation increases their phase space density (PSD = nλ^3dB, n is the density and λdB is the thermal de Broglie wavelength of the atoms). Next, atoms are transferred into a hybrid dipole trap, collecting in a dimple created at the intersect of two high power laser beams. Production and stabilisation of these beams is described, which are focused down to a 75 µm beam waist at the trap position with a total power of 7 W. Optimisation of the evaporation process in both traps leads to consistent BEC production. A pure condensate with 4x10^4 atoms at 25 nK is reported. Radio-frequency magnetometry is performed at various probe volumes. With systematic optimisation a best AC sensitivity of 24 pT/√Hz with 3.4 × 10^8 atoms in the magnetic trap before evaporation is achieved. This is extended to the BEC with 4 × 10^4 atoms, where an AC sensitivity of 84 nT/√Hz and DC sensitivity of 14 nT/√Hz is reported, bringing previously achieved atomic magnetometry into the micrometer regime. A trade-off must be considered due to reduction in sensitivity at lower probe volumes. Volumes between 1.4×10−7 m^3 and 1.6×10−14 m^3 can be accessed, highlighting the sensors adaptability and tunability for different applications. The results are contextualised in the background of previously achieved magnetometers of various types. Finally, proof-of-concept electromagnetic induction imaging (EMI) measurements are made to confirm the sensor’s viability for high resolution imaging

Exploring an Infinite Space with Finite Memory Scouts

Consider a small number of scouts exploring the infinite $d$-dimensional grid with the aim of hitting a hidden target point. Each scout is controlled by a probabilistic finite automaton that determines its movement (to a neighboring grid point) based on its current state. The scouts, that operate under a fully synchronous schedule, communicate with each other (in a way that affects their respective states) when they share the same grid point and operate independently otherwise. Our main research question is: How many scouts are required to guarantee that the target admits a finite mean hitting time? Recently, it was shown that $d + 1$ is an upper bound on the answer to this question for any dimension $d \geq 1$ and the main contribution of this paper comes in the form of proving that this bound is tight for $d \in \{ 1, 2 \}$.Comment: Added (forgotten) acknowledgement