Site-controlled QDs: A route for dense arrays of integrated entangled photon emitters

Quantum computing is one of future challenges [1]. One of the technologies baring significant potentialities is based on quantum dots (QD), usually referred to as 'artificial atoms'. The analogy refers to their discrete energy levels, which allows atomic concepts to be exported to a system which is, on the other hand, embedded in a semiconductor matrix and can be technologically exploited. A good example of this is the capability to emit single and polarization-entangled photons [2], which are attractive sources of qubits. However, many challenges still have to be overcome. The main problem of most of QD systems is the asymmetry induced fine-structure splitting (FSS) - the degeneracy lifting of the exciton (e+h) level. It compromises entanglement detection which, in general, resides in the polarization of photons emitted in biexciton (2e+2h)-exciton recombination cascade (note: the entanglement resides in the electronic levels, not in the cascaded process). Particular tuning strategies (magnetic field, electric field, strain), indeed, can rectify this issue, however they complicate the set-up and typically can be applied to a single QD at a time, while an array of symmetrical QDs is needed for complicated quantum computational tasks.