Spatial coherence of room-temperature monolayer WSe2_2 exciton-polaritons in a trap

The emergence of spatial and temporal coherence of light emitted from solid-state systems is a fundamental phenomenon, rooting in a plethora of microscopic processes. It is intrinsically aligned with the control of light-matter coupling, and canonical for laser oscillation. However, it also emerges in the superradiance of multiple, phase-locked emitters, and more recently, coherence and long-range order have been investigated in bosonic condensates of thermalized light, as well as in exciton-polaritons driven to a ground state via stimulated scattering. Here, we experimentally show that the interaction between photons in a Fabry-Perot microcavity and excitons in an atomically thin WSe$_2$ layer is sufficient such that the system enters the hybridized regime of strong light-matter coupling at ambient conditions. Via Michelson interferometry, we capture clear evidence of increased spatial and temporal coherence of the emitted light from the spatially confined system ground-state. The coherence build-up is accompanied by a threshold-like behaviour of the emitted light intensity, which is a fingerprint of a polariton laser effect. Valley-physics is manifested in the presence of an external magnetic field, which allows us to manipulate K and K' polaritons via the Valley-Zeeman-effect. Our findings are of high application relevance, as they confirm the possibility to use atomically thin crystals as simple and versatile components of coherent light-sources, and in valleytronic applications at room temperature.Comment: 13 pages, 4 figure

Search for heavy resonances decaying to two Higgs bosons in final states containing four b quarks

A search is presented for narrow heavy resonances X decaying into pairs of Higgs bosons (H) in proton-proton collisions collected by the CMS experiment at the LHC at root s = 8 TeV. The data correspond to an integrated luminosity of 19.7 fb(-1). The search considers HH resonances with masses between 1 and 3 TeV, having final states of two b quark pairs. Each Higgs boson is produced with large momentum, and the hadronization products of the pair of b quarks can usually be reconstructed as single large jets. The background from multijet and t (t) over bar events is significantly reduced by applying requirements related to the flavor of the jet, its mass, and its substructure. The signal would be identified as a peak on top of the dijet invariant mass spectrum of the remaining background events. No evidence is observed for such a signal. Upper limits obtained at 95 confidence level for the product of the production cross section and branching fraction sigma(gg -> X) B(X -> HH -> b (b) over barb (b) over bar) range from 10 to 1.5 fb for the mass of X from 1.15 to 2.0 TeV, significantly extending previous searches. For a warped extra dimension theory with amass scale Lambda(R) = 1 TeV, the data exclude radion scalar masses between 1.15 and 1.55 TeV

Measurement of the top quark mass using charged particles in pp collisions at root s=8 TeV

Peer reviewe

Search for anomalous couplings in boosted WW/WZ -> l nu q(q)over-bar production in proton-proton collisions at root s=8TeV

Peer reviewe

Network-aware big data processing

The scale-out approach of modern data-parallel frameworks such as Apache Flink or Apache Spark has enabled them to deal with large amounts of data. These applications are often deployed in large-scale data centres with many resources. However, as deployments and data continue to grow, more network communication is incurred during a data processing query. At the same time, data centre networks (DCNs) are becoming increasingly more complex in terms of the physical network topology, the variety of applications that are sharing the network, and the different requirements of these applications on the network. The high complexity of DCNs combined with the increased traffic demands of applications has made the network a bottleneck for query performance. In this thesis, we explore ways of making data-parallel frameworks network-aware, i.e. we combine specific knowledge about the application and the physical network to reduce query completion times. We identify three main types of traffic that occur during query processing and add network-awareness to each of them to optimise network usage. 1) Traffic reduction for aggregatable traffic exploits the physical network topology and the associativity and commutativity of aggregation queries to reduce traffic as early as possible. In-network aggregation trees utilise existing networking hardware and the tree topology of DCNs to partially aggregate and thereby reduce data as it flows through the network. 2) Traffic balancing for non-aggregatable traffic monitors the network throughput of an application and uses knowledge about the query to optimise the overall network utilisation. By dynamically changing the destinations of parts of the transferred data, network hotspots, which can occur when many applications share the network, can be avoided. 3) Traffic elimination for storage traffic gives control over data placement to the application instead of the distributed storage system. This allows the application to optimise where data is stored across the cluster based on application properties and thereby eliminate unnecessary network traffic.Open Acces

Cryptanalysis of the GPRS Encryption Algorithms GEA-1 and GEA-2

International audienceThis paper presents the first publicly available cryptanalytic attacks on the GEA-1 and GEA-2 algorithms. Instead of providing full 64-bit security, we show that the initial state of GEA-1 can be recovered from as little as 65 bits of known keystream (with at least 24 bits coming from one frame) in time $2^{40}$ GEA-1 evaluations and using 44.5 GiB of memory. The attack on GEA-1 is based on an exceptional interaction of the deployed LFSRs and the key initialization, which is highly unlikely to occur by chance. This unusual pattern indicates that the weakness is intentionally hidden to limit the security level to 40 bit by design. In contrast, for GEA-2 we did not discover the same intentional weakness. However, using a combination of algebraic techniques and list merging algorithms we are still able to break GEA-2 in time $2^{45.1}$ GEA-2 evaluations. The main practical hurdle is the required knowledge of 1600 bytes of keystream