Contrôle Optique d'un Spin Individuel – CoSin

I- Description of the project and expected results: The coherent control of a quantum object such as semiconductor quantum dot represent an actual topic of interest due to both fundamental interest and possible application for the implementation of quantum information processing in solid state. The confinement of charge carriers in a quantum dot produces a significant increase of their spin relaxation time: the absence of translation degree of freedom decrease the efficiency of the spin relaxation mechanism based on the spin orbit interaction. This property of spin coherence, which is in the centre of this project, made this system very attractive for the development of quantum logic gates using a spin in a solid state environment as a quantum-bit . This increase of the spin coherence time is much more pronounced in well confined systems. For this reason, we chose to investigate the spin properties of strongly confined II-VI (CdTe/ZnTe) self-assembled quantum dots obtained by the Stransky-Krastanov growth method. The possibility to dope this quantum dots with magnetic atoms even adds a new degree of freedom with fascinating aspects due to the exchange interaction of the charge carriers spins with the lattice magnetic moments. Since these two types of spins (magnetic atom and charge carrier spins) can be distinguished by their different g-factors, they form an ideal system to study the coherent interaction of spins in a zero dimensional quantum object. Another interest of this model system is the fact that the density of magnetic moments can be easily tuned by changing the Mn concentration. Quantum dots containing a single magnetic atom and a tuneable density of carriers have been recently realized [1]. The aim of this project is to study by optical methods (spin orientation under circularly polarised light) and microwave methods (optically detected magnetic resonance), the fundamental mechanism controlling the decoherence of a single spin in a quantum dot, like the spin-orbit interaction and the hyperfine interaction with nuclear spins. We will then investigate the different possibilities of spin manipulation to realize the initialization, the coherent control and the optical readout of a single spin in an individual quantum dot. Scientific background: The quantum dots we chose for this study are based on II-VI semiconductor (CdTe/ZnTe) containing, eventually, magnetic atoms (Mn). For practical reasons, the most commonly studied quantum dots are based on III-V compounds. These are indeed currently used in optoelectronic devices and benefit of large technological developments. Their growth is well controlled, they can be easily doped and their electrical connection is ease to implement. However, recently important progress has been done in the growth of II-VI structures. Ferromagnetic two dimensional gaz and quantum dots containing a controlled density of magnetic atoms (single Mn) and a tuneable number of charge carriers have been recently realized [2]. In the field of spintronics, II-VI compounds are model systems which offer some advantages compared to the commonly investigated III-V. First, the growth of II-VI magnetic semiconductors is very well controlled and they have very good structural properties. In these compounds, the incorporation of Mn can be done at the normal growth temperature of the non-magnetic material. This avoids the formation of structural defects and, unlike GaMnAs, permit to conserve very good optical properties. In these magnetic structure, the use of II-VI compounds permits to independently tune the incorporation of charge carriers and of isoelectronic magnetic atoms (Mn) into the crystal matrix. In III-V like GaMnAs, the Mn atoms introduce both a localized spin and an additional hole. This independence of electrical and magnetic doping, permits to realized magnetic nanostructures not perturbed by the ionized acceptors. These structures provide a way to study precisely the interaction betw.