Il est bien connu que les propriétés d'un émetteur de lumière ponctuel, tel qu'un atome ou une boîte quantique, peuvent être drastiquement modifiées à l'aide d'une cavité optique. Je passerai en revue les principales stratégies pour y parvenir, en évoquant notamment le cas des nano-antennes métalliques avec lesquelles nous travaillons au laboratoire pour façonner l'émission de LEDs à boîtes quantiques colloïdales. Nous verrons à travers ces différents exemples que l'intuition que possède la communauté sur ces systèmes repose pour l'essentiel sur des modèles et expériences impliquant des émetteurs uniques. Je présenterai ensuite des mesures nouvelles qui montrent que cette intuition n'est en fait pas pertinente dès lors que le système comprend un ensemble d'émetteurs en interaction. Je rationaliserai ces observations et discuterai des potentialités de ce régime d'émission pour l'optique et l'optoélectronique à l'échelle nano.

The strength of the electroweak symmetry breaking at high temperatures of the early Universe could substantially differ from the present day value. The Standard Model predicts the electroweak symmetry to get restored above the temperatures of about 160 GeV. I will discuss the recently proposed scenarios where the situation is reversed, and the breaking survives even at high temperatures. I will also discuss the implications of such a behaviour for one of the popular scenarios explaining the matter-antimatter asymmetry -- electroweak baryogenesis.

$T^4$ visio-conf*

Karsten will entertain us with the possibility that primordial magnetic fields could resolve the present-day Hubble tension

https://arxiv.org/abs/2004.09487

*bring your own tea/coffee/biscuits; visio-break around 3:30pm

$T^4$ visio-conf*

PART II of:
Karsten will entertain us with the possibility that primordial magnetic fields could resolve the present-day Hubble tension

https://arxiv.org/abs/2004.09487

*bring your own tea/coffee/biscuits; visio-break around 3:30pm

The most impressive aspect of 2D materials is not the fact that they include vast variety of electronic and optical properties (metals, large band gap semiconductors, superconductors, ferromagnetic, ...). Their specificity is that they can be combined or stacked on top of other materials, quantum light sources or photonic elements, they can even be twisted in plane from another 2D materials. In my talk I will present some illustrations of that unique degree of freedom within the context of material science, optomechanics and quantum nano-photonics.
In particular I will present a graphene NEMS suspended a few tens of nanometers above nitrogen-vacancy centres (NVCs), which are stable single-photon emitters embedded in nanodiamonds [1]. The optomechanical coupling between the graphene displacement and the NVC emission is based on near-field dipole–dipole interaction, making it suitable for nanoscale devices. Semiconducting NEMS (WSe2, MoSe2) also show high mechanical Q-factors, which can be used to study the thermal properties of such materials[4]. Then, I will show a slightly different hybrid system consisting in lifetime-limited single emitters [2,3] (linewidth ~40 MHz) and 2D materials at sub-wavelength separation without degradation of the emission properties [4]. Our device’s nanoscale dimensions enable ultra-broadband tuning (tuning range > 400 GHz) and fast modulation (frequency ~ 100 MHz) of the emission energy, which renders it an integrated, ultra-compact tuneable SPS [5]. Conversely, this offers a novel approach to optical sensing of 2D material properties using a single emitter as a nanoprobe.

References
[1] Reserbat-Plantey A., Schädler K., Gaudreau L., Navickaite G., Güttinger J., Chang D.,Toninelli C., Bachtold A., and Koppens F. Electromechanical control of nitrogen- vacancy defect emission using graphene NEMS. Nature Communication. 7, 10218. 2016
[2] N. Morell, S. Tepsic, A. Reserbat-Plantey, A. Cepellotti, M. Manca, I. Epstein, A. Isacsson, X. Marie, F. Mauri, and Adrian Bachtold. Optomechanical Measurement of Thermal Transport in Two-Dimensional MoSe2 Lattices. Nano Letters. 19-5, 3141. 2019
[3] Morell*, N., Reserbat-Plantey*, A., Tsioutsios, I., Schädler, K., Dubin, F., Koppens, F., Bachtold, A. High Quality Factor Mechanical Resonators Based on WSe2 Monolayers.
Nano Letters. 16, 5102–5108. 2016
[4] K. Schädler,C. Ciancico, S.Pazzagli, P. Lombardi, A. Bachtold, C. Toninelli, A. Reserbat- Plantey, and F. Koppens. Electrical Control of Lifetime-Limited Quantum Emitters Using 2D Materials. Nano Letters. 19-6. 3789. 2019
[5] C. Ciancico, K. Schädler, S. Pazzagli, M. Colautti, P. Lombardi, J. Osmond, C. Dore, A. Mihi, A. Ovvyan, W. Pernice, E. Berretti, A. Lavacchi, C. Toninelli, F. Koppens and A. Reserbat- Plantey. Narrow Line Width Quantum Emitters in an Electron-Beam-Shaped Polymer. ACS Photonics, 6, 12, 3120. 2019

The study of quantum field theory in de Sitter spacetime is notably motivated by the success of inflation, and the possibility to test fundamental physics through cosmological experiments. Going beyond the usual linearized description, one has to use nonperturbative methods to circumvent the infrared and secular divergences arising from the gravitational amplification of superhorizon modes.

We will first focus on the backreaction of an $O(N)$ quantum scalar theory on the background geometry, related to the cosmological constant problem, as such divergences may signal an instability of de Sitter space towards a Minkowski spacetime. Our study uses the functional renormalization group (FRG) and shows a finite backreaction in the presence of interactions, which saturates as a result of the nonperturbative generation of a mass.

The second topic of the talk is the effect of selfinteractions on scalar field correlators at separate spacetime points, in the infrared limit. The spacetime dependence gives access to several interesting physical quantities such as power spectra, relaxation time, or decoherence time. The stochastic formalism is considered, and we formulate the $1/N$ expansion of its Fokker-Planck formulation. This can be phrased as an equivalent, Schrödinger-like, eigenvalue problem. The corresponding spectrum is entirely computed at NLO, giving access to the spacetime dependence of any correlator.