Electric-field-induced coherent coupling of the exciton states in a single quantum dot

The ability to generate entangled photon pairs from a quantum dot critically depends on the size of the fine-structure splitting of its exciton states. A demonstration of the ability to tune this splitting with an electric field represents a promising step in the use of quantum dots to generate entangled photon pairs on demand. The signature of coherent coupling between two quantum states is an anticrossing in their energies as one is swept through the other. In single semiconductor quantum dots containing an electron–hole pair the eigenstates form a two-level system that can be used to demonstrate quantum effects in the solid state, but in all previous work these states were independent 1 , 2 , 3 , 4 , 5 . Here we describe a technique to control the energetic splitting of these states using a vertical electric field, facilitating the observation of coherent coupling between them. Near the minimum splitting the eigenstates rotate in the plane of the sample, being orientated at 45° when the splitting is smallest. Using this system we show direct control over the exciton states in one quantum dot, leading to the generation of entangled photon pairs.