Sommerfeld Theory Colloquium (ASC)

All the information we will ever obtain from the early universe is imprinted in the spatial correlations of primordial fluctuations at the hot Big Bang. I will explain how an influx of ideas from various areas of fundamental physics is providing us with new conceptual and practical tools to decode the physics of these primordial fluctuations. A thorough understanding of the fluctuations will give us insight into particle physics at high energies and may provide a window into the nature of spacetime itself.

One of the great insights of cryptography and computational complexity is the notion of computationally-bounded algorithms, which allow us to reason about properties that would otherwise be impossible to achieve. Could these ideas also be relevant to quantum information and quantum mechanics? In this talk, we discuss how computational thinking changes the landscape of quantum cryptography: What primitives can we construct, how definitions change, and what are the outstanding open problems. If time permits, we will also discuss new foundational questions on quantum information, motivated by cryptographic applications.

Sommerfeld Theory Colloquium

Sommerfeld Theory Colloquium

Over the past ten years, there has been a growing interest among physicists

for ‘active matter’, a codename that encompasses systems in which energy is

taken from the environment to generate self-propulsion at the single particle

level. Active particles, such as run-and-tumble bacteria, self-diffusiophoretic

colloids or actin filaments in motility assays, are strongly out-of-equilibrium

and exhibit much richer behaviours than their passive counterpart.

In this talk I will review recent progresses regarding the physics of active

particles. I will show how simple concepts like pressure, the force density

exerted by assemblies of particles on their container, play a new role for

active systems because of the lack of equation of state. I will also show how

new collective phenomena emerge, from the transition to collective motion

to the existence of cohesive matter without cohesive forces, that have no

counterpart in thermal equilibrium.

Sommerfeld Theory Colloquium

Sommerfeld Theory Colloquium

Since the emergence of information theory in the middle of the 20th century, this new scientific field has been deeply entangled with statistical physics, as attested from the beginning by the use of entropy to quantify information content. The talk will explore the major

impact of these transdisciplinary exchanges, with a particular focus on the important new research topic called "compressed sensing". Starting from the observation that interesting signals can be

compressed, and thus are sparse in some representation, compressed sensing aims at acquiring data directly in a compressed way, using

then computational methods to reconstruct the original signal. It

opens the way to faster, less destructive, and more e�ective signal acquisition, with possible applications in many branches of science,

from magnetic resonance imaging to astronomy, tomography, or gene

interaction network reconstruction. The talk will describe the spectacular progress that can be made using various statistical physics

ideas, from spin glass theory to crystal nucleation.

I describe an algebraic scheme for quantizing the Ruijsenaars-Schneider

models in the R-matrix formalism. It is based on a special parametrization

of the cotangent bundle over GL(n,C). In new variables

the standard symplectic structure is described by a classical (Frobenius)

r-matrix and by a new dynamical r¯-matrix. Quantizing these

r-matrices, I will exhibit the quantum L-operator algebra and construct

its particular representation corresponding to the RuijsenaarsSchneider

system. I will also indicate a couple of open problems.

In this seminar, I shall describe recent work by our group on iden- tifying pockets of gas at high redshift that have undergone mini- mum processing through stars. The chemical composition of such gas still bears the imprints of the first few generations of stars that formed only a few hundred million years after the Big Bang, and thereby gives us clues to the physical properties of these still mys- terious objects which heralded the so-called ‘epoch of reionisation’. Near-pristine gas at high redshift is also the astrophysical environ- ment where the primordial abundance of deuterium can be measured most precisely. I will show how determinations of the cosmic den- sity of baryons from Big-Bang Nucleosynthesis and from the Cosmic Microwave Background have now reached comparable precision, in both cases of order of a few percent. The excellent agreement be- tween these two measures at widely different cosmic epochs places interesting limits on the existence of relativistic particles beyond the standard model of physics.