Coherent transfer of spin and entanglement in optoelectronics

In the last few years there has been a growing interest in the possible applications of quantum properties of different physical systems in well established technologies. This was motivated by the miniaturization of electronic devices. For very small systems, the laws of quantum mechanics start to govern their behavior and it is then to expect that for instance electronic components reach their "quantum limit". A good example of the introduction of quantum properties into conventional technologies is the case of the electron spin, which offers an alternative to electron-charge for the storage and transport of information in electronics. The idea of using electron spins in electronics gave rise to the new and exciting field of spintronics and was encouraged by experiments that showed coherent spin transport over long distances (over hundred micrometers) in semiconductors and long electron spin dephasing times. In addition to its applications in conventional technologies, the use of the electron spin as information-carrier has been suggested for the new field of quantum information processing. An important advantage of the electron spin with respect to other quantum information-carriers proposed is that it can profit from the well developed semiconductor industry and its related technologies. For the long-range transport of information, however, the highly interacting environment in solid-state devices represents a difficulty to the use of electron spins. In contrast, photons are very weekly coupled to their environment, and therefore more suitable for the transfer of information over long distances (on the order of kilometers), required, for instance, in quantum communication. Furthermore, the technology needed for the transport of photons over such long distances is well developed. Thus, an efficient transfer of quantum information between electron spins and photons is highly desirable.
In this dissertation, we show that the transfer of entanglement, a quantum correlation, from electron spins to the polarization of photons is not only theoretically possible, but also feasible under realistic experimental conditions. We study the transport of an entangled pair of electron spins through Fermi leads which split them spatially until they reach spin-LEDs, semiconductor structures containing quantum dots. In these structures, the electrons optically recombine with holes, producing then photons. We analyze the polarization state of the out-coming photons and give the conditions for which the entanglement of the electron spins is fully recovered in the photon-polarizations. We further show that it is possible to produce four-photon entanglement.
We also include in this work the study of coherent spin-transfer between quantum dots layers bridged by molecules. This study was motivated by an experiment showing that the molecules act not only as binders to construct the multilayer structures, but also as "wires" for the information through electron spins in the quantum dots. We show that a two-site Hamiltonian captures some of the essential features of the experiment. We calculate the dependence of the experimentally observed Faraday rotation (FR) signal as a function of probe energy on microscopic parameters such as spin transfer probabilities. The Faraday angle is related to the difference in the dielectric response for different polarizations of the light. We calculate the dielectric response functions of coupled quantum dots and derive an analytical expression for the FR angle in terms of electron transfer probabilities and Heisenberg exchange splittings.