Brian D. Gerardot
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh

Photons interact minimally with the environment, making them robust carriers of quantum information. However, processing this information remains very challenging. Perhaps the most feasible approach is to transfer the coherence of a photon to an atomic state. Hence quantum optics, the study of light-matter interaction at the quantum level, occupies a central role in the emerging field of quantum information processing (QIP). A new paradigm stimulated by QIP is the application of quantum optics to solid-state media. Due to strong quantum confinement and optical efficiency, self-assembled quantum dots (QDs) are at the forefront of this domain. However, complete coherent optical manipulation of QD states has not yet been realized. In this talk I will present two experiments which take steps towards this premise and show that the quantum coherence is in fact maintained for particles in a QD, both for two and three level systems, even under intense optical excitation. In the first example, a single valence-band hole is trapped in the QD. Using a resonant laser, optical pumping of the hole-spin is achieved. Due to a very long spin-relaxation time, high fidelity (~99%) hole-spin preparation is realized. Furthermore, the spin preparation is achieved even when the spin states are degenerate at zero external magnetic field. This verifies that hole spins not only have reduced interactions with phonons but also negligible hyperfine interaction with the nuclear spins of the QDs constituent atoms. Secondly, I will discuss a two-color pump-probe experiment on the exciton and bi-exciton states in a QD. In the strong-field excitation regime, the dressed-transition states (Autler-Townes splitting) and quantum interference effects are observed. These results confirm that solid-state mesoscopic systems are suitable for quantum optical techniques and promising for applications in quantum information processing.

References:

B. D. Gerardot et al, Nature 451, 441 (2008).
B. D. Gerardot et al, arXiv:0803.0432 (2008).