Research Michiel Wouters

Abstract

The study of excitons in two-dimensional materials is rapidly evolving with promising applications in various fields such as optoelectronics and quantum information. Of particular interest are the excitons in van der Waals bilayers where the electrons and holes are confined in different layers giving rise to dipolar interactions and long life times. Their properties can be tuned by applying pressure, electric fields or by inducing a moiré modulation by twisting the layers. When a two-dimensional material is embedded in an optical microcavity, it can give rise to the formation of hybrid light-matter quasi-particles, the so-called exciton-polaritons. In this project, we wish to explore theoretically the properties of such exciton-polaritons and exploit their tunability in order to give them desirable properties. In particular, a long standing goal in the field of polariton physics is the realisation of polariton-polariton interactions that are larger than their linewidth and allow to reach exotic phases of strongly correlated photons. A large part of this project will be devoted to the study of the interactions between the polaritons and the exploration of how to enhance them by controlling experimental parameters, in particular the moiré-induced potential. In the second part of the project, we will investigate the many body polariton phases that can be realised with this system, taking into account the particularities of the system unveiled in the first part of our study.

Researcher(s)

• Promoter: Wouters Michiel
• Co-promoter: Conti Sara
• Fellow: Van Hoorebeke Manon

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Universality means that very different physical systems have in some respects identical properties. One of the most celebrated manifestations of universality is for second order phase transitions, resulting in the same behavior of fluctuations in systems as diverse as magnets, superfluids , liquids at the critical point. These examples all share that they occur at thermal equilibrium. Systems that lack thermal equilibrium however are also ubiquitous, including living organisms. Universality turns out to be present in certain nonequilibrium systems as well, with the Kardar-Parisi-Zhang equation capturing the behavior of a surprisingly large set of them, comprising the interfaces of growing crystals, the shape of fire fronts and the fluctuation properties of lasers. The present project is devoted to the latter type of systems. We will address lasers that contain dye molecules that drive the system toward thermal equilibrium. Their nonequilibrium nature on the other hand stems from photon losses that are compensated by an excitation laser. Their deviation from thermal equilibrium being experimentally tunable, these systems are expected to be well suited for the investigation of the role of thermalization on the scaling of the fluctuations in optical systems.

Researcher(s)

• Promoter: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

When a gas of atoms is cooled close to absolute zero, it undergoes a transition to a Bose-Einstein condensate, a quantum mechanical state of matter characterized by frictionless flow or "superfluidity". In this project, we investigate what happens to such a superfluid when a parameter such as the interatomic interaction strength is suddenly changed or "quenched". In particular, the project focuses on how the Bose-Einstein condensate evolves towards the new equilibrium state. Several experimental observations, such as the existence of a prethermal steady state and universal dynamics, pose theoretical challenges that we plan to resolve by taking into account correlations between more than two atoms in our model. The behavior of strongly interacting ultracold atom gases is furthermore archetypical of a broad range of quantum many body systems ranging from neutron stars to superconductors. The research topic thus has many applications, and moreover touches on fundamental questions regarding the role of thermal equilibrium in quantum systems.

Researcher(s)

• Promoter: Wouters Michiel
• Co-promoter: Tempere Jacques
• Fellow: Fernandes Lennart

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Superfluids form a phase of matter, distinct from the gaseous, fluid and solid states, whose most remarkable characteristic is a vanishing viscosity. This absence of friction is a consequence of the fact that all particles move together, in analogy to the photons that come out of a laser. Lasers and superfluids share the coherence of the particles (atoms and photons respectively), but an important difference between them is that the former are driven (e.g. by an electrical current), where the latter are in thermal equilibrium. The need for the driving of the laser is a direct consequence of its usefulness as a source of coherent photons. Recently, several research activities have developed to bridge the differences between the various forms of coherent matter. From the photonic side, experiments have been performed where the photons come very close to thermal equilibrium by working with efficient thermalization mechanisms and long photon life times. From the superfluid atomic side, experiments have been performed where atom losses were induced by an electron beam, that are replenished by a nearby atomic cloud. The aim of this project is to construct theoretical descriptions of atomic superfluids that are driven away from thermal equilibrium by particle losses. Based on previous studies of photonic systems, we expect that the phase transition between the normal and coherent phases as well as their vortex properties will be modified by the atom losses.

Researcher(s)

• Promoter: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Variational principles are fundamental in our theoretical understanding of closed quantum systems at thermal equilibrium. For open, driven-dissipative systems, variational techniques are much less established. Classical examples of driven dissipative systems range from convection cells in hydrodynamics to electrical patterns in the heart. In recent years, progress in fabrication of electromagnetic resonators coupled to matter degrees of freedom, has spurred the theoretical interest in driven-dissipative quantum systems. An important motivation for this research is the possibility of realizing correlated quantum states with potential applications in quantum computing and quantum simulation. For the theoretical simulation of driven-dissipative quantum systems, two equivalent approaches exist: a master equation for the density matrix and a quantum trajectory equation for wave functions. These two techniques relate to each other as the diffusion equation to the Langevin equation in the theory of Brownian motion. A practical advantage of the quantum trajectory method for numerical purposes is that it works on the Hilbert space of wave functions instead of the quadratically larger Hilbert space of density matrices. A conceptual bonus is that it 'unravels' distinct macroscopic superpositions of the Schrödinger cat type and gives insight in the emergence of a classical configurations out of an entangled quantum state. In the present project, we will investigate variational approximations to the quantum trajectory dynamics. The advantage of applying the variational principle to the trajectory dynamics instead of the density matrix itself is that the unraveled states are expected to be more amenable to such a description. This expectation is borne out by a preliminary study with the Gutzwiller approximation to a photonic dimer. Encouraged by this success, we will set out to investigate various variational approximations to the quantum trajectory description of driven-dissipative quantum systems. One of the advantages of such a description is that it can be carried out even for large systems and in more than one dimension, where other numerical techniques become impractical. Access to large systems is in particular important for the descriptions of phase transitions, that only become sharp in the thermodynamic limit. The most important goal of this research project is to provide a new theoretical tool for the simulation of driven-dissipative quantum systems. We envisage to apply this technique to further our understanding of phase transitions, which can lead to new fundamental insights regarding the differences and similarities of driven-dissipative systems with respect to closed systems in thermal equilibrium.

Researcher(s)

• Promoter: Wouters Michiel
• Fellow: Huybrechts Dolf

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The main objective of this project, and moreover the central idea, is to bring complex and difficult to comprehend systems within the reach of realistic quantum fluid experiments. Since phenomena related to cosmology are often extremely difficult, if not impossible, to study experimentally, it can be very stimulating to translate some of the problems to realistic analog experiments. The specific quantum fluid that we will consider is a quantum degenerate Bosonic gas of exciton-polaritons in semiconductor microcavities.

Researcher(s)

• Promoter: Wouters Michiel
• Fellow: Van Regemortel Mathias

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The main objective of this project, and moreover the central idea, is to bring complex and difficult to comprehend systems within the reach of realistic quantum fluid experiments. Since phenomena related to cosmology are often extremely difficult, if not impossible, to study experimentally, it can be very stimulating to translate some of the problems to realistic analog experiments. The specific quantum fluid that we will consider is a quantum degenerate Bosonic gas of exciton-polaritons in semiconductor microcavities.

Researcher(s)

• Promoter: Wouters Michiel
• Fellow: Van Regemortel Mathias

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

In the present project, we will pursue the analogy between the physics of exciton-polaritons in microcavities and cosmological phenomena. Exciton-polaritons are hybrid particles that are half matter (exciton) and half light (photon). Thanks to their hybrid nature, they combine interactions with easy optical manipulation. In particular, the microcavity polariton physics allows studying analogs of the Einstein field equations with a cosmological constant. A second topic of cosmological interest that we plan to address is the creation of particles due to changing gravitational fields.

Researcher(s)

• Promoter: Wouters Michiel
• Fellow: Van Regemortel Mathias

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

This project aims at a theoretical analysis of these quantum fluids in the turbulent regime. Theories for turbulence in superfluid helium will be adapted to account for a larger vortex core size. Interestingly, additional key observables, such as the spatial and temporal coherence, can be measured. We will develop theoretical descriptions for these quantities in order to characterize the turbulent state in these novel quantum fluids.

Researcher(s)

• Promoter: Wouters Michiel
• Co-promoter: Tempere Jacques

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Our main objective is to investigate the transport properties of the Wigner solid in the quantum wire' regime, i.e., in the quasi-one-dimensional (quasi-1D) case when a typical width of the channel is comparable to the inter-electron separation. This new regime, not yet reached in experiments or studied theoretically, is expected to demonstrate new interesting physics. In this study, we will also explore similarities to other quasi-1D systems, e.g., colloids in quasi-1D channels or superconducting vortices in low-pinning narrow channels.

Researcher(s)

• Promoter: Wouters Michiel
• Co-promoter: Misko Vyacheslav

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The subject of the research project is the theory of semiconductor nanostructures and ultracold atomic gases in the regime of strong light-matter coupling, with the goal to advance the theoretical models, to elucidate conceptual issues and to devise technological applications.

Researcher(s)

• Promoter: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

The subject of the research project is the theory of semiconductor nanostructures in the regime of strong light-matter coupling, with the goal to develop the theoretical models, to elucidate conceptual questions and to propose technological applications. In semiconductor microcavities, strong light-matter coupling results in the so-called polariton quasi-particles that are a coherent superposition of quantum well exciton and microcavity photon. Thanks to their composite nature, these bosonic quasi-particles combine significant interactions with good quantum coherence. These favorable properties have led to the first observation of Bose-Einstein condensation (BEC) in the solid state. The research on polariton BEC has developed in a lively subject of fundamental research, on the crossroad between of semiconductor physics, quantum optics and quantum gases and enjoys fruitful collaboration between theorists and experimentalists. Besides their interest from the fundamental physics side, microcavities in the strong coupling regime have high potential for technological applications, such as ultralow threshold lasing, generation of entangled photon pairs, miniaturized nonlinear optical devices and ultrafast optical memories. The two elements that make polaritons different from other realizations of quantum degenerate bose gases are the finite polariton life time and their interactions with the solid state environment. These pose great challenges for their theoretical description. Due to the finite polariton life time, the polariton gas does not reach thermodynamic equilibrium. As a consequence, the steady state cannot be found by minimizing a free energy. Instead, the kinetics has to be modeled. The nonequilibrium character also raises conceptual questions related to the meaning of superfluidity, because standard treatments rely on thermodynamic arguments. We plan to attack the polariton quantum kinetics with methods based on quasi-probability distributions developed in quantum optics. These distributions can be sampled with stochastic classical fields, using Monte Carlo techniques. I have developed effective models of this type before, but these exploratory studies contained quite drastic approximations. It is the aim of the present research project to go beyond these simplifications and to develop a full model for the kinetics of a quantum degenerate polariton gas that interacts with its solid state environment. Applications of the theoretical model will include among others a quantitative study of the long range spatial coherence, density fluctuations, the dynamics of the formation of coherence, the shape of the condensate state and its coherence in the presence of periodic or disordered potentials and the polarization state of polariton condensates. In addition, we will consider the application of the developed formalisms to study different physical systems. One promising example is a nanocavity with an embedded quantum dot, where recently the controversial observation of lasing in the strong coupling regime was reported. In addition, we will seek to make conceptual progress in the domain of polariton superfluidity. Our model will contain all the ingredients to make a microscopic calculation of the superfluid fraction. Such a calculation is important, because of the two dimensional nature of the polariton fluid: from the analogy with the two-dimensional bose gas at thermodynamic equilibrium, it is expected that the transition from the incoherent to coherent state is of the Berezinskii-Kosterlitz-Thouless type, characterized by a jump in the superfluid fraction. Finally, the directions of technological applications that we think of are based on the polarization dynamics, that was recently exploited to construct an ultrafast all optical spin memory.

Researcher(s)

• Promoter: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Researcher(s)

• Promoter: Brosens Fons
• Fellow: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Within the TFVS a model was developed to describe analytically a system of N harmonically interacting identical particles (both fermions and bosons), confined in a parabolical potential, subject or not to an external magnetical field. In this project we will study with this theory the effect of the interaction between particles with different spin. This study will provide us with the zeroth order system that will be extended by variational and perturbative methods to describe more realistic systems like quantum dots, mesoscopic stuctures and superconducting clusters. In a first stage of the project we will study the thermodynamics and statistical correlation functions of a system consisting of unpolarised fermions. The second stage will aim at a further investigation of the interaction of the unpolarised fermions with a phonon bath. To arrive at a description of more realistic systems in a third fase we will take into acount the influence of non-parabolic confining potentials and of the Coulomb interaction between the particles which cannot be described in an analytical way. For this purpose we will use variational methods, in particular, the Jensen-Feynman inequality.

Researcher(s)

• Promoter: Brosens Fons
• Co-promoter: Devreese Jozef
• Fellow: Wouters Michiel

Research team(s)

• Theory of quantum systems and complex systems

Project type(s)

• Research Project

Abstract

Within the TFVS a model was developed to describe analytically a system of N harmonically interacting identical particles (both fermions and bosons), confined in a parabolical potential, subject or not to an external magnetical field. In this project we will study with this theory the effect of the interaction between particles with different spin. This study will provide us with the zeroth order system that will be extended by variational and perturbative methods to describe more realistic systems like quantum dots, mesoscopic stuctures and superconducting clusters. In a first stage of the project we will study the thermodynamics and statistical correlation functions of a system consisting of unpolarised fermions. The second stage will aim at a further investigation of the interaction of the unpolarised fermions with a phonon bath. To arrive at a description of more realistic systems in a third fase we will take into acount the influence of non-parabolic confining potentials and of the Coulomb interaction between the particles which cannot be described in an analytical way. For this purpose we will use variational methods, in particular, the Jensen-Feynman inequality.

Researcher(s)

• Promoter: Devreese Jozef
• Fellow: Wouters Michiel

Research team(s)

Project type(s)

• Research Project