The beneficiaries of 4PHOTON strongly believe that Quantum Information Technology (QIT) will be the major disruptive technology in the near future. 4PHOTON outcomes directly impact on the viability of QIT by providing fundamental know-how and trained researcher to this expanding and disruptive field. The researcher profile required by QIT is extremely multi-specialized, being required to tackle problems in fundamental quantum physics, optics, photonics and device fabrication (growth and processing).
Inter‐disciplinary and intersectorial training of ESRs is needed for the future development of the technology, with strong interlink between academia and business. Three main targets are considered fundamental1 (see, e.g. ibm.com/ibmcai/quantumcomputing) to pave the way to a successful impact of QIT on industry and society:
(1) Nurture new skills and roles;
(2) Expand quantum computing education programs;
(3) Increase open innovation between academia and business.
With this goals in mind, 4PHOTON will forge a research and training network between the leading European groups working on the next generation of solid state QIT devices based on novel materials, focusing on the application of quantum dots (QDs) fabricated by Droplet Epitaxy. Future practical nano-photonics and QIT applications demand semiconductor QDs with full freedom to be grown on various substrates with different lattice parameters (Si, Ge, GaAs), substrate orientations and tunable structural, optical, electrical and spin properties. Nano-photonics for solid state QIT is vast and novel research area that lies at the crossroads of photonics, material science, quantum physics and nano-scale device fabrication. It combines challenges in the fabrication and understanding of efficient quantum emitters with tailored properties, in the development of new scientific equipment enabling advanced experiments and devices at the single/multiple quantum level.
Very recent technological, experimental and theoretical breakthroughs, many by members of the consortium, have demonstrated potential for rapid progress in this field. There is therefore a fast growing demand for innovative applications in this strongly multi-disciplinary field3, but, besides mutual collaborations on sub-fields, there is no network covering the entire area. Our aim is to bring together for the first time the growth, microscopy, spectroscopy, theory and device groups that work on the physics and applications of QDs grown by Droplet Epitaxy covering the full chain of research in the field of QIT from basic materials science to practical devices. In this way 4PHOTON will permit to develop further QD flexibility, to deeply understand the physics involved and to devise innovative QIT. 4PHOTON academics and industries will work together to match ESR skills development with actual opportunities. In addition to leading European participants from Industry and Academia, the group that invented the Droplet Epitaxy technique, the National Institute of Material Science, Tsukuba, Japan is a partner of 4PHOTON. The 4PHOTON research at the beneficiaries groups will be carried out by the 15 ESR hired by the network.
The project will address the following four scientific and technological objectives:
KST1: Quantum cryptography and optical computation schemes are based on the manipulation of indistinguishable and entangled photons, for which QDs with vanishing fine structure splitting are needed. In principle this can be achieved through growth along the  crystal axis providing inherently high interface symmetry . Conventional strain driven self-assembly of QDs fails for this orientation, so the recent reports of both the UWu and the NIMS growth groups on high-quality InAs and GaAs QDs grown on GaAs (111) substrates with a very small exciton fine structure splitting between 0 and 10 µeV represent a major breakthrough.
KST2: A quantum network consists of a coherent quantum system at each node, with each node coupled together via single photons, which must be created on demand, stored and later recreated. Realistic progress is most likely to be made with a hybrid quantum system, an integration of two disparate elements: one to create single photons, the other to store them: a semiconductor QD for single photon generation and atoms for single photon storage. Droplet Epitaxy QDs are ideal candidates for hybrid quantum systems with Rb atoms as they can emit at 780 nm.
KST3: The optical and electrical properties of quantum emitters based on semiconductor QDs have to be widely tunable for realistic device applications. Understanding the interplay between QD size, shape, strain (presence or absence of) and charge state and how these parameters control the optical emission wavelength, the polarization eigenstates of the optical transitions, and the photon statistics is of major importance for increasing device design degrees of freedom.
KST4: An important step towards merging quantum photonics and electronics is to insert quantum emitters in optical circuits and on Si-based photonics and electronics. Here a major breakthrough is the development of scalable dot-based quantum technologies and linear optics quantum integrated photoniccircuits on-chip. One of the major advantages of Droplet Epitaxy is that it allows growth of QDs on a variety of substrates (such as Si and Ge on Si), as there are no limitations for the exact lattice match. An alternative approach to avoid lethal stacking faults is to optimize the GaAs nucleation layer, to use nanowires or deep patterned Si substrates