Coherence and Coupling of Single Quantum Dots
The coherence and coupling of semiconductor quantum dots (QDs) is receiving increasing attention due to possible solid-state implementations in the emerging field of quantum information processing.
Beside its fundamental interest, the knowledge of the dephasing time, inversely proportional to the homogeneous broadening, of an excitonic transition in a QD is of crucial importance for these applications. The dephasing time sets the time scale during which the coherence of the excitonic transition is preserved and therefore operations based on coherent-light matter interaction can occur. Moreover, it is strictly related to intrinsic mechanisms such as radiative processes, carrier-phonon scattering and carrier-carrier scattering.
The measurement of coherence in QD is typically performed on large ensembles to provide a measurable signal strength. We have developed a detection technique for coherent non-linear optical microscopy which can measure on individual QD's, called heterodyne spectral interferometry . It allows to measure the four-wave mixing of individual excitons, and make images of them (see Figure).
The four-wave mixing in large ensembles creates an echo due to the phase-conjugation. In small ensembles of only a few QD's, the creation of this echo can be observed, deriving from constructive interference of the individual signals at the echo time, being random otherwise.
Coherent coupling mediated by a resonant optical cavity is specifically interesting, as it provides a tuneable coupling. We have shown this in .
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