Espace de Fock | EPFL Graph Search

L'espace de Fock est une construction algébrique utilisée en mécanique quantique pour construire l'espace des états quantiques d'un nombre variable ou inconnu de particules identiques à partir d'une seule particule de l'espace de Hilbert H. Il porte le nom de Vladimir A. Fock qui l'a présenté pour la première fois dans son article de 1932 "Konfigurationsraum und zweite Quantelung", traduisible par "espace de configuration et deuxième quantification." De manière informelle, un espace de Fock est la somme d'un ensemble d'espaces de Hilbert représentant zéro état de particule, un état de particule, deux états de particule, et ainsi de suite. Si les particules identiques sont des bosons, les n états de particules sont des vecteurs dans un produit tensoriel symétrisé de n espaces de Hilbert à particule unique H. Si les particules identiques sont des fermions, les n états de particules sont des vecteurs dans un produit tens

Dynamics and coherent effects on high density plasmas in semiconductor nanostructures

Lars Kappei

This thesis work contains an experimental study of many-body and cooperative effects in quantum wells. Many-body effects prove important for an understanding of the physical and specifically optical properties of semiconductors. The Coulomb interaction between electrons profoundly modifies the electronic states and the light-matter interaction, leading to a number of density-dependent effects that can not be accounted for in the one-electron picture which is the starting point of most textbooks on the subject. The simplest of these modifications is the exciton, the bound state of one electron and one hole which leads to the emergence of two-particle states energetically below the continuous band electron-hole states. Optical excitation in the band continuum is possibly followed by the formation of excitons upon release of the excess energy to the lattice vibrations. Chapter 5 contains an experimental analysis of the dynamics of this process. A high quality single In0.05Ga0.95As/GaAs quantum well sample allows us to separately study the temporal evolution of the continuum photoluminescence on the one hand and the excitonic photoluminescence on the other. The results indicate a rather fast formation process, leading to the establishment of a species interconversion equilibrium between free pairs and bound pairs at higher density. However, at the lowest excitation density, we found evidence for a non-equilibrium species repartition. At high electron-hole pair density, the two-particle bound exciton state disappears due to the screening of the Coulomb interaction by the surrounding carriers as well as the blocking of the constituting states according to the Pauli exclusion principle. The transition from an excitonic population to a free carrier plasma is referred to as excitonic Mott transition. Chapter 6 reports an experimental investigation of the repercussions of this transition on the optical spectra, performed on the In0.05Ga0.95As/GaAs quantum well of chapter 5. The time-resolved spectra do not show an abrupt change of photoluminescence intensity or spectral form when transforming gradually from an exciton dominated spectrum to free plasma photoluminescence, indicating a rather gradual transition. The separate visibility of continuum and excitonic PL contributions below the transition density reveals the total absence of exciton binding energy variations, indicating a very low exciton ionization degree in contrast to theoretical work on the subject. Another high-density effect arising from the particle interactions is the energetic lowering of the electronic states due to screening and correlation, denoted as band gap renormalization (BGR). Chapter 7 presents measurements of BGR using a single GaAs/Al0.25Ga0.75As quantum well of 60 Å width. We obtain good agreement with calculations of the carrier self energy within the framework of the random phase approximation. The last part of this thesis (chapter 8) reports an experimental test of the possibility of cooperative spontaneous emission, often denoted as superradiance, from a high-density electron-hole plasma. This effect has been extensively studied in optically excited atomic gases, where it leads under certain conditions to the reemission of the stored energy in a single coherent radiation burst instead of the slow exponential decay according to the spontaneous radiation of a single atom. We find no evidence for the occurrence of this effect in our samples, probably due to the fast incoherent polarization decay in the carrier plasma.

EPFL2005

Preparation and Decay of a Single Quantum of Vibration at Ambient Conditions

Mitchell David Anderson, Christophe Marcel Georges Galland, Nils Kipfer, Kilian Robert Seibold, Vivishek Sudhir

A single quantum of excitation of a mechanical oscillator is a textbook example of the principles of quantum physics. But mechanical oscillators, despite their pervasive presence in nature and modem technology, do not genetically exist in an excited Fock state. In the past few years, careful isolation of gigahertz-frequency nanoscale oscillators has allowed experimenters to prepare such states at millikelvin temperatures. These developments illustrate the tension between the basic predictions of quantum mechanics-which should apply to all mechanical oscillators even at ambient conditions-and the extreme conditions required to observe those predictions. We resolve the tension by creating a single Fock state of a 40-THz vibrational mode in a crystal at room temperature and atmospheric pressure. After exciting a bulk diamond with a femtosecond laser pulse and detecting a Stokes-shifted photon, the Raman-active vibrational mode is prepared in the Fock state vertical bar 1 > with 98.5% probability. The vibrational state is then mapped onto the anti-Stokes sideband of a subsequent pulse, which when subjected to a Hanbury Brown-Twiss intensity correlation measurement reveals the sub-Poisson number statistics of the vibrational mode. By controlling the delay between the two pulses, we are able to witness the decay of the vibrational Fock state over its 3.9-ps lifetime at ambient conditions. Our technique is agnostic to specific selection rules, and should thus be applicable to any Raman-active medium, opening a new general approach to the experimental study of quantum effects related to vibrational degrees of freedom in molecules and solid-state systems.

2019

Dynamics and coherent effects on high density plasmas in semiconductor nanostructures

Lars Kappei

EPFL2005