Scalable sources of single and entangled photons are fundamental building blocks of quantum information science. They can be used for quantum communication, optical quantum computing, quantum networks and, more in general, are needed to develop future quantum technologies. It is widely accepted that these technologies have the potential to change our society significantly. In order to unlock their full potential, it is essential to go beyond proof of principle experiments performed in research laboratories. This, however, turns out to be an extremely challenging task. The main reason is that any prototype of quantum device has to meet a set of rigorous criteria to be considered for the envisioned application. The ideal source of quantum light, for example, should deliver single and entangled photons on-demand, with high purity, efficiency, indistinguishability and degree of entanglement. Moreover, it should be compatible with current photonic integration technologies and, at the same time, with other quantum systems. While a large number of non-classical light sources have been developed over the years, there is at present no source that can fulfill all these requirements simultaneously. As a consequence, optical quantum technologies have kick-start difficulties.
The SPQRel project focuses on the fabrication and study of near-ideal sources of non-classical light, which enable the construction of a quantum network for the distribution of quantum entanglement among distant parties. The sources are based on epitaxial quantum dots integrated onto innovative semiconductor-piezoelectric devices which allow for full control over the quantum-dot in-plane strain tensor. The sources can deterministically generate highly indistinguishable and strongly entangled photons with high efficiency and their emission wavelength can be finely adjusted via the application of voltages to the piezo-actuators without degrading the quality of the emitted photons. This unique feature enables the distribution of entanglement between distant parties and allows building up artificial-natural atomic interfaces where entangled photons are interfaced to absorption resonances in atomic vapors.
The findings of SPQRel have proven that it is possible to build up a quantum network in which photons from quantum dots are used to distribute entanglement over distant nodes. This research is extremely relevant for society, as it will find applications in the field of secure communication and long-distance quantum networking. Moreover, the hybrid-semiconductor piezoelectric technology developed during the SPQRel project has the potential to impact research fields beyond quantum communication, and in particular the field of strain-engineering of two-dimensional materials.
