# Calendar

## April 2018

### Statistical Mechanics of Repeated Quantum Measurements

#### Vojkan Jaksic (Department of Mathematics and Statistics, McGill University, Canada)

### Séminaire de Physique Statistique & Matière Condensée

#### Alexis Dubreuil (ENS, Paris)

**Titre**: A sensory-motor hub driving photo-taxis in zebrafish

**Résumé**: Animals continuously gather sensory cues in order to move towards favorable environments. Efficient goal-directed navigation requires sensory perception and motor command to be intertwined in a feedback loop, yet the neural substrate underlying this sensorimotor task in the vertebrate brain remains elusive. Here we present analysis of whole-brain calcium recordings together with circuit modeling to reveal the neural mechanisms through which zebrafish performs phototaxis, i.e. actively orients towards a light source. Key to this process is a self-oscillating hindbrain network that acts as a pacemaker for ocular saccades and controls the orientation of successive swim-bouts. It further integrates visual stimuli in a state dependent manner, i.e. its response to visual inputs varies with the motor context, a mechanism that manifests itself in the phase-locked entrainment of the circuit by periodic stimuli. A rate model is developed that reproduces our observations and demonstrates how this sensorimotor processing eventually biases the animal trajectory towards bright regions.

### Towards a classification of healthy scalar-tensor theories

#### Karim Noui (Tours)

Once again, Einstein was right! The recent detection of gravitational waves is exceptional, but it confirms what Einstein predicted 100 years ago... However, there are reasons to think that general relativity may not be the "final" theory of space-time: it could be modified at very "large" scale (to account for dark energy) or at very "short" scale (to account for quantum gravity).

Scalar-tensor theories form a very large class of modified theories of gravity where, in addition to the usual two tensorial modes, a scalar mode propagates. Such theories have been studied a lot these last years to model dark energy or some aspects of quantum gravity for instance. In particular, there has been a huge activity around scalar-tensor theories with higher-order derivatives of the scalar field in the action, which appear to have very interesting physical properties.

In this seminar, I will show how we managed to classify healthy higher-order scalar-tensor theories in a systematic way. Then, I will also illustrate physical applications of these theories in cosmology and in quantum gravity. Finally, in the context of cosmology, I will show how the recent observation of the gravitational wave event GW170817 and of its electromagnetic counterpart GRB 170817A, which shows that the speed of gravitational waves is the same as

the speed of light (within deviations of order 10^*-15*), strongly constrains these theories.

### Concentration de familles de fonctions propres du laplacien sur des variétés compactes

#### Julien Sabin (Département de Mathématiques, Université Paris-Sud)

### Quantum measurements and simulation with Rydberg atoms

#### Jean-Michel Raimond Sorbonne Université, Paris Laboratoire Kastler Brossel, Collège de France, ENS, CNRS

The exploration of the puzzling features of the quantum, like state superpositions, entanglement or measurement processes is a thriving experimental field. It is driven by fascinating applications of the quantum, like quantum metrology, quantum information processing or quantum simulation.

Rydberg atoms, long-lived and extremely strongly coupled to external fields, are ideal tools to explore these basic quantum features. We will review a few explorations of the quantum performed at Collège de France using these remarkable states.

The strong interaction of circular Rydberg states with microwave fields in high quality superconducting cavities lead to ideal measurements of the photon number, illustrating all postulate of quantum measurement, or to the generation of field states reminiscent of the famous Schrödinger cat. Coherent manipulations in the complex Rydberg manifold lead to the observation of quantum Zeno dynamics and to the realization of ultra-sensitive electric field measurements. Finally, we will present a quantum simulator project emulating spin chains with laser-trapped circular Rydberg atoms.

### Conference Semi-classical and Geometric Asymptotics in Mathematical physics

Cette Conférence est organisée dans le cadre d’un Projet International de Coopération scientifique (PICS) avec la Russie, avec comme partenaires, d’une part le Centre de Physique Théorique (UMR 7332, Université d’Aix-Marseille, Université de Toulon, CNRS) ; d’autre part le Centre Interdisciplinaire J.-V. Porcelet (UMI CNRS, Moscou), et l’Institut Ishlinski pour les Problèmes de la Mécanique (Moscou).