• Colloque CPTGA
• Personne à contacter : Julia Meyer ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : We will discuss interesting properties of open quantum graphs and microwave networks [1]. We will demonstrate that there exist graphs which do not obey the Weyl’s law N(R) = LR/π + O(1), where O(1) is a function which for R going to infinity is bounded by a constant. The Weyl’s law directly links the counting function N(R) of the number of resonances with the square root of energy k, 0(k), and the total length L of the internal edges of a graph. Graphs which do not obey the Weyl’s law are called non-Weyl graphs. We show that for standard coupling conditions the transition from a Weyl graph to a non-Weyl graph occurs if we introduce a balanced vertex. A vertex of a graph is called balanced if the numbers of infinite leads and internal edges meeting at a vertex are the same. The experimental results demonstrating the existence of non-Weyl networks are in excellent agreement with the theoretical predictions [2].

Acknowledgements

References

• [1]. O. Hul, S. Bauch, P. Pakoński, N. Savytskyy, K. Życzkowski, and L. Sirko, Phys. Rev. E 69, 056205 (2004).
• [2]. M. Ławniczak, J. Lipovsky, and L. Sirko, Phys. Rev. Lett. 122, 140503 (2019).

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

In cavity quantum electrodynamics (QED), the interaction between an atomic transition and the cavity field is measured by the vacuum Rabi frequency omega_0. The regime with omega_0 comparable to the two-level transition frequency is called the ultrastrong coupling regime. In such a regime, and for a large number of atoms coupled to the same cavity mode, a superradiant quantum phase transitions (SQPT) has been predicted, e.g. within the Dicke model.

In this theoretical seminar, I will briefly describe the SQPT at equilibrium, discuss the experimental context, and present our recent proposal where a 2DEG with Rashba spin-orbit coupling placed inside an optical cavity can exhibit the SQPT.

Ref: P. Nataf, T. Champel, G. Blatter, and D. M. Basko, Rashba cavity qed: a route towards the superradiant quantum phase transition, arXiv:1907.02938 (2019)

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Salle de lecture 2, maison des Magistères

Résumé : We study the ground state of a large number N of 2D anyons in an external magnetic field. We consider a scaling limit where the statistics parameter α is proportional to N^-1 when N → ∞. The model is that of bosons in a magnetic field and interacting through long-range magnetic potential generated by magnetic charges carried by each particle, smeared over discs of radius R. Our method allows to take R → 0 not too fast at the same time as N → ∞. We use the information theoretic version of the de Finetti theorem of Brandão and Harrow to justify the so-called "average field approximation": the particles behave like independent, identically distributed bosons interacting via a self-consistent magnetic field.

Liens :[LPMMC]

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : After briefly summarizing my long-term interest in quantum physics of low-dimensional spinless fermions that attract each other, I will present our recent study of a quantum many-body lattice system of one-dimensional fermions interacting with a dynamical Z2 gauge field. The gauge field mediates long-range attraction between fermions resulting in their confinement into bosonic dimers. At strong coupling we developed an exactly solvable effective theory of such dimers with emergent constraints. I will show that even at a generic coupling and fermion density, the model can be rewritten as a local spin 1/2 chain and forms a Luttinger liquid. In a finite chain we observed the doubling of the period of Friedel oscillations which paves the way towards experimental detection of confinement in this system. Finally, I will also discuss the possibility of a Mott phase at the commensurate filling 2/3, connection to quantum scars and our plans to extend this study to two spatial dimensions in pursuit of exotic p-wave superfluidity.

Liens :[Sergej Moroz]

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : A Maxwell demon is a creature (or machine) that reduces the entropy of a system without performing any work on it. It performs this apparent violation the second-law of thermodynamics through the intricate action of measuring individual particles and subsequently performing feedback. Bennett (building on work of Landauer) argued that the second-law is restored once one takes into account the fact that information recorded by the demon is a physical resource like heat or work. Here we show that much simpler setups can also act as demons: we demonstrate that it is sufficient to exploit a non-equilibrium distribution to seemingly break the second law of thermodynamics. No particle-by-particle measurement or feedback is necessary. We call this an N-demon (with the "N" for non-equilibrium), and show that it can reduce the entropy of a system without doing work or exchanging heat with that system. We then show that the second-law is restored by treating ``non-equilibrium'' as a physical resource like heat, work or information. We propose both an electronic and an optical implementation of this phenomenon, realizable with current technology. These examples make it clear that the non-equilibrium distribution can be classical or quantum in nature. Ref: Rafael Sánchez, Janine Splettstoesser, Robert S. Whitney, to appear in Phys. Rev. Lett. Eprint - arXiv:1811.02453

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Amphithéâtre, maison des Magistères

Liens :[Natasha Perkins]

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Liens :[Philippe Jacquod]

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Amphithéâtre, maison des Magistères

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères