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Single Spin Manipulation towards quantum computation

Final Report Summary - SESAM (Single spin manipulation towards quantum computation)

Following the path of using electronic spins to implement quantum computing, the general theme of this project was to broaden our knowledge on spins in semiconductor systems. The leading objective was to develop new experimental tools and use them to study single Mn+2 ions in individual CdTe quantum dots with high field, magneto-spectroscopy methods. The ultimate goal was to explore the possibility to manipulate localised spins with microwave radiation when reading their polarisation via analysing the emission from coupled (to these spins) excitonic states.

The significant, technical achievement of this project was the design and successful fabrication of the appropriate experimental probe which was suitable for non-trivial, polarisation-resolved, single object photo-luminescence measurements in high magnetic fields. This probe was further updated, and consecutively modified to include the functionality of simultaneous delivering the microwave radiation to the sample (via coaxial cables).

Observation of brightening of dark excitons in a single CdTe quantum dot containing a single Mn2+ ion is one of the relevant scientific results of this project. It has been shown that the exciton-Mn2+ exchange interaction, when combined with a mixing of the heavy-light hole states induced by the in-plane anisotropy, allows Xd (dark exciton) recombination accompanied by a simultaneous Mn2+ spin flip. Thus the Xd recombination was concluded to act as an effective Mn2+ spin orientation mechanism for highly anisotropic quantum dots. High magnetic fields and polarisation resolved measurements have been used to spectrally separate the emission lines related to X (bright) and Xd transitions and to extract the relevant system parameters (e.g. the anisotropic exchange splitting) via comparison with the solution of the appropriate Hamiltonian.

In spite of numerous efforts (different samples, consecutive modifications of the experimental probe) no influence of microwave radiation on single Mn2+ spins in single CdTe quantum dots have been observed so far. Nevertheless the analogous effects have been successfully observed in two-dimensional, quantum well structures and optional method to study the single Mn2+ dynamics, based on time resolved photoluminescence experiments has been work-out. Those two achievements are now further developed within one of PhD projects carried out at the host institution (co-authored publication of the results is foreseen).

Studies of spin-related phenomena have been successfully carried out in quantum Hall effect systems.
The important finding followed optical-absorption investigations of quantum Hall ferromagnet at filling factor n = 1. It has been shown that the spin polarisation of this system is very fragile and the spins depolarise rapidly as either the filling factor or the temperature changes even slightly. (please see http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.102.126806 online).

Other experiments were focused on the n=5/2 fractional quantum Hall effect state. Non-Abelian particles, possibly characteristic of this very peculiar state (Moore-Read proposal) could be optional candidates to constructing qubits for fault tolerant topological quantum computation. Spin depolarisation of the n = 5/2 state revealed in the present experiments points towards more complex nature of this state. (Please see http://physics.aps.org/viewpoint-for/10.1103/PhysRevLett.105.096801 online).

Optical investigations of the quantum Hall effect in CdTe quantum wells uncovered the characteristic, driven by spin polarisation, many body effects in fully populated Landau levels.

Observation of magnetic-field induced suppression of Auger-type scattering in graphene and reporting the unexpected Landau level structure in graphite at very high magnetic fields are additional results of this project, beyond its initial objectives.