Temps universel coordonné | EPFL Graph Search

vignette|upright=1.5|L’heure dans le monde par rapport à UTC. Le temps universel coordonné ou UTC (coordinated universal time) est une échelle de temps adoptée comme base du temps civil international par la majorité des pays du globe. Description UTC est une échelle de temps comprise entre le temps atomique international (TAI) qui est stable mais déconnecté de la rotation de la Terre et le temps universel (TU), directement lié à la rotation de la Terre et donc lentement variable. Le terme « coordonné » indique que le temps universel coordonné est identique au temps atomique international dont il a la stabilité et l’exactitude à un nombre entier de secondes près, ce qui lui permet de coller au temps universel à moins de près. L'appellation correcte en anglais du temps universel coordonné (TUC) serait « coordinated universal time », abrégé en CUT. Les experts de l’Union internationale des télécommunications étaient d’accord pour définir une abréviation com

Classical soft graviton theorem rewritten

Biswajit Sahoo

Classical soft graviton theorem gives the gravitational wave-form at future null infinity at late retarded time u for a general classical scattering. The large u expansion has three known universal terms: the constant term, the term proportional to 1/u and the term proportional to ln u/u(2), whose coefficients are determined solely in terms of the momenta of incoming and the outgoing hard particles, including the momenta carried by outgoing gravitational and electromagnetic radiation produced during scattering. For the constant term, also known as the memory effect, the dependence on the momenta carried away by the final state radiation / massless particles is known as non-linear memory or null memory. It was shown earlier that for the coefficient of the 1/u term the dependence on the momenta of the final state massless particles / radiation cancels and the result can be written solely in terms of the momenta of the incoming particles / radiation and the final state massive particles. In this note we show that the same result holds for the coefficient of the ln u/u(2) term. Our result implies that for scattering of massless particles the coefficients of the 1/u and ln u/u(2) terms are determined solely by the incoming momenta, even if the particles coalesce to form a black hole and massless radiation. We use our result to compute the low frequency flux of gravitational radiation from the collision of massless particles at large impact parameter.

SPRINGER2022

Efficient Protocols for Enforcing Causal Consistency in Geo-Replicated Key-Value Data Stores

Kristina Spirovska

Modern large-scale data platforms manage colossal amount of data, generated by the ever-increasing number of concurrent users. Geo-replicated and sharded key-value data stores play a central role when building such platforms. As the strongest consistency model proven not to compromise availability, causal consistency (CC) is perfectly positioned to address the needs of such large-scale systems, while preventing some ordering anomalies. Transactional Causal Consistency (TCC) augments CC by providing richer transactional semantics, simplifying the development of distributed applications. However, achieving CC/TCC in an efficient manner is very challenging. In this thesis we introduce several protocols and designs for high performance causally consistent geo-replicated key-value data stores in several different settings. First, we present a new approach to implementing CC in geo-replicated data stores, called Optimistic Causal Consistency (OCC). By introducing a technique that we call client-assisted lazy dependency resolution, OCC makes it possible for updates replicated to a remote data center to become visible immediately, without checking if their causal dependencies have been received. We further propose a recovery mechanism that allows an OCC system to fall back on a pessimistic protocol to continue operating during network partitions. We show that OCC improves data freshness, while offering performance that is comparable or better than its pessimistic counterpart. Next, we address the problem of providing low-latency TCC reads under full replication. We present Wren, the first TCC system that at the same time achieves low latency by implementing nonblocking read operations and efficiently scales out by sharding. Wren introduces new protocols for transaction execution, dependency tracking and stabilization. The transaction protocol supports nonblocking reads by providing a transaction snapshot as a union of a fresh causal snapshot installed by every partition in the local data center and a client-side cache for writes that are not yet included in the snapshot. The dependency tracking and stabilization protocols require only two scalar timestamps, resulting in efficient resource utilization and providing scalability in terms of shards and replication sites, under a full replication setting. In return for these benefits, Wren slightly increases the visibility latency of updates. Finally, we present PaRiS, the first TCC system that supports partial replication and provides low latency by implementing non-blocking parallel read operations. PaRiS relies on a novel protocol to track dependencies, called Universal Stable Time (UST). By means of a lightweight background gossip process, UST identifies a snapshot of the data that has been installed by every data center in the system. PaRiS equips clients with a private cache, in which they store their own updates that are not yet reflected in the snapshot. The combination of the UST-defined snapshot with client-side cache enables interactive transactions that can consistently read from any replication site without blocking. Moreover, PaRiS requires only one timestamp to track dependencies and define transactional snapshots, thereby achieving resource efficiency and scalability in terms of shards and replication sites, in a partial replication setting.

EPFL2020

Time-resolved X-ray and Optical Studies of Metalloproteins and Iridium Complexes

Mahsa Silatani

A wide variety of physicochemical processes at a molecular level, in particular charge or energy transfer, electronic and vibrational relaxation, are at the origin of biological functionality of proteins and organometallic compounds. The work reported in this thesis is devoted to the study of these molecular dynamics in selected organometallic systems. In particular, it focuses on the role of these dynamics in the ultrafast photophysics and photochemistry of Myoglobin and iridium complexes having a transition metal active center of d6 configuration. The nature of ligand recombination to the active centre has a strong impact on the reactivity and activation of heme proteins. Using picosecond (ps) Fe K-edge X-ray absorption spectroscopy (XAS) we probed the NO-heme recombination with direct sensitivity to the Fe-NO binding in physiological solution. The transient XAS at 70 ps and 300 ps are identical, but they deviate from the difference between the static spectra of deoxyMb and nitrosyl myoglobin demonstrating the formation of an intermediate species, supposedly the 6-coordinated domed form that is populated on a time scale of ~200 ps and relaxes in ~30 ps. A broadband femtosecond (fs) luminescence study of Ir(ppy)3 (Ir1), Ir(ppy)2(pic) (Ir2), [Ir(ppy)2(bpy)]+ (Ir3) and Ir(ppz)3 (Ir4) provides the first extensive picture of their ligand dependence. After excitation to a ligand-centered state (266 nm), we directly clock the relaxation cascade of Ir1 leading to the lowest metal-to-ligand charge transfer (3MLCT) state. The cascade proceeds within â€ 10 fs, being faster than some of the high-frequency modes of the system. In Ir4 the 3MLCT state decays (530 fs) by non-radiative channels. For Ir1-3, the early emission (100s ps the emission evolves to the steady-state showing evidence of dual emission for Ir1,2 due to a double-well minimum of the lowest 3MLCT state. For Ir3, the final emission stems from a ppy to bpy inter-ligand CT state. The studies of the electronic and geometric structure of Ir1-4 by Ir L3 XAS in solution reveals overall identical above-edge multiple scattering resonances and EXAFS indicating similar bond lengths and angles. Simulation of the XAS using TDDFT and FEFF9 shows good overall agreement with the experiment. The near-edge region of the spectra shows distinct differences among the studied complexes. Ir4 has stronger WL intensity than Ir1, indicating weaker Ï-back donation. The XAS spectra of Ir2,3 indicate larger 5d-electron density on the iridium. The XAS spectra of Ir1 and Ir4 show weak pre-edge resonance due to metal (2p3/2)-to-ligand charge transfer excitations. The absence of the pre-edge in Ir2,3 is due to the reduced covalency of the bonds. However, the existence of such transitions remains open for debate since Ir+3 has only occupied t2g orbitals in the low spin configuration. Using ps XAS, we have successfully captured the transient spectrum at the Ir L3-edge of both Ir1 and Ir4 probed 150 ps after laser excitation in solution. Upon excitation of a MLCT band (355 nm), we observed oxidation state change of Ir from +3 to +4. In DCM, the transient spectra are almost identical for both Ir1,4. The only significant changes in the transient spectra are those observed around the WL. The calculations reproduce the blue shift of the absorption spectrum due to the reduced electron density on the Ir, consistent with the MLCT excitation.

EPFL2015