Electronic, heat and ultra cold atoms transport in quantum dot systems

Résumé

The topic of the present thesis is: transport through mesoscopic systems. The platforms considered for the development of the present thesis are quantum dots, where particles with a defined spin are confined in discrete energy levels. Different quantum dots are coupled by tunnel junctions, overlapping the wave functions of their higher energy states. The particles in these states are then coherently delocalized along the whole quantum dot structure, which behaves as an artificial molecule. A strong particle-particle interaction is present in these systems due to the short distant between them. These systems can be externally controlled by tunning gate voltages capacitively coupled to the dots; therefore, the energy, entropy, and the coherent dynamics of the systems change in a supervised manner. Most of the thesis is based in triple quantum dots, which constitute a perfect device to investigate multilevel quantum interference. One of the main problems that is studied in the triple quantum dot are the coherences of the electronic states that lead to the long-range transition. It indirectly couples distant states, which don¿t have a direct tunnel coupling, by means of a virtual transition through an intermediate and energetically forbidden state. The long-range transition purports charge and spin transfer between distant dots, which is an initial step to the exchange of quantum information between distant qubits with low decoherence. Different virtual trajectories are studied between the two ends of the system which leads to interference phenomena. To manipulate the states of the system one has to change in time the gates coupled to the dots. To study the time dependence properties and get more control over the system, oscillatory fields with a defined frequency, amplitude, and phase are coupled to the dots. The driving field renormalizes the couplings among the different dots and permits resonant transitions between detuned dots by the absorption or emission of n photons. The parameters of the field are easily tunable externally, warranting a direct control on the quantum superpositions by changing the renomalization of the couplings. We extend the study of photon-assisted transitions with a single field to long-range transitions, where the absorption and emission of photons is conserved between distant quantum states. When multiple fields are applied to the system it is investigated the interaction between long-range and direct photon-assisted transitions in transport experiments, where multiple Landau-Zener passages are present. Quantum interferences that depend in a nontrivial way on the phase difference of the locally applied drivings are predicted. These destructive interferences can be experimentally detected as they are of the same nature as long-range current resonances. The transport of particles between the quantum system and the reservoirs imply an energy exchange between them, which is essential to understand the thermodynamics of the quantum dot system. It is studied the equilibration process between two isolated reservoirs of ultracold atoms, which have different temperatures and different chemical potentials, and can be either fermions or bosons. The two reservoirs are weakly coupled through a single quantum dot with a determined number of energy levels, through which particles and energy are exchanged. Additionally, the proposed device is investigated as a particle transistor or particle capacitor. In driven systems the ac field is an additional source of energy with whom the quantum system exclusively exchanges energy, not particles. The purpose of this study is to extract the energy from the ac-field and transport non-locally through a long-range transition to a distant reservoir. In this thermodynamic driven system the energy and heat transport between the reservoirs is studied to implement long-range heat and cooling engines.