Universal fine-structure eraser for quantum dots

We analyze the degree of entanglement measurable from a quantum dot via the biexciton-exciton cascade as a function of the exciton fine-structure splitting and the detection time resolution. We show that the time-energy uncertainty relation provides means to measure a high entanglement even in presence of a finite fine-structure splitting when a detection system with high temporal resolution is employed. Still, in many applications it would be beneficial if the fine-structure splitting could be compensated to zero. To solve this problem, we propose an all-optical approach with rotating waveplates to erase this fine-structure splitting completely which should allow obtaining a high degree of entanglement with near-unity efficiency. Our optical approach is possible with current technology and is also compatible with any quantum dot showing fine-structure splitting. This bears the advantage that for example the fine-structure splitting of quantum dots in nanowires and micropillars can be directly compensated without the need for further sample processing.