Emerging quantum technologies demands for the development of near-ideal sources of non-classical light [1]. For real-life applications, these sources should fulfill a series of requirements such as true on-demand generation of single and entangled photons, high brightness, and high indistinguishability of emitted photons. Among the many sources available to date, quantum dots are gaining attention as they can fulfill all the requirement of the wish list of single photon sources and, moreover, can generate highly entangled photon pairs through the two-photon cascade from the biexciton state to the ground state.
In this project we present a novel approach to retrieve a cascade of four quantum correlated photons from a single GaAs quantum dot. To achieve this objective a great effort will be devoted to understand and exploit the electronic band structure of these quantum nanostructures. This project will combine well-established spectroscopic techniques (such as photoluminescence and photoluminescence excitation) with photon correlation spectroscopy. The expected results of this experiment would shine light on the multiparticle energy structure of quantum dots and moreover, would provide an interesting tool to implement more complicated quantum protocols involving a higher number of correlated photons. The project will be part of the research line of the Nanophotonics group of the Physics Department of Sapienza University of Rome. The research will be performed thanks to the facilities already present in the laboratory and by implementing a part of the experimental setup with a new laser pulse tailoring device.
[1] Senellart, P., et al. Nature Nanotechnology 12, 1026¿1039 (2017).
Multi-excitonic states in QDs is far from being a completely explored field. The full study of multiparticle states in QDs presents a series of great challenges which makes this topic an interesting area for research and a potential ground for new discovery and new insights on the intrinsic properties of these nanostructures. The energy structure and spin states of multi-excitonic complexes are essential to understand the recombination mechanism and the correlation of photons emitted by QDs [1].
Extensive efforts have been made to explore the electronic structure of multiparticle states in semiconductor QDs. The coherent excitation of the XX state in QDs is now a well-established experimental technique which produced great results in the emission of entangled photon pairs [2- 5]. Several works on the characterization of the 3X and 4X states have been published [1,6- 8] and a successful coherent population of the 3X state was achieved with the combination of three different laser pulses [9]. Despite all the attempts on the study and characterization of the multiparticle states in QD there is no record in literature of a coherently population of the 4X state up to date. We therefore believe that the realization of this challenging research would be a clear advance in the scientific knowledge with respect to the state of the art in this specific topic.
To accomplish this task a detailed knowledge of the energetic structure of the QDs is essential. The new insights brought by this experiment would provide the scientific community new information on few particle state in in semiconductor QDs. For what concerns the generation of entangled couples of photons a considerable obstacle is the inherent anisotropy of most of QDs which ruins dramatically the degree of entanglement. The use of highly symmetric QDs, such as droplet etched GaAs QDs [3], combined with strain tuning of the band structure [10] could be an effective way to overcome this problem. These experimental techniques rely on a good knowledge of the energy structure of QDs and on an accurate prediction of the strain effect on the optical properties. For the reason mentioned above it is clear that an extension of the knowledge of the multiparticle electronic structure would be undoubtedly profitable to the QD community.
Deterministic generation of higher-order multiexcitons is a conceptual way for increasing the number of quantum correlated photons that a single QD emits. Several quantum information protocols require high-order quantum correlation between carrier confined in these QDS and the photons that they emit [11]. The possibility to create a higher number of mutually correlated photons would be of great interest for the quantum information community. This is true if the source of this photons is a reliable source of single and entangled photons with high purity, high efficiency and high indistinguishability, as QDs are proving to be in the last few years [3,5].
We believe that a research on the deterministic generation of the quadexciton state in QDs would give us interesting insights on the fundamentals properties of the energy states and structural composition of widely used nanostructure. This specific topic is also of great interest for quantum communication protocols and quantum computing as widely stated in the literature. Finally, our group research line, i.e. the study of light quantum properties and the manipulation of electronic band structure, represents the right environment for this kind of project and would give us the right tools and know-how to achieve the anticipated results.
[1] Arashida, Y., et al. Physical Review B 84, (2011).
[2] Müller, M., et al. Nature Photonics 8, 224¿228 (2004).
[3] Huber, D., et al. Nature Communications 8, 15506 (2017).
[4] Schweickert, L., et al. Applied Physics Letters 112, 093106 (2018).
[5] Reindl, M. et al. Nano Letters 17, 4090¿4095 (2017).
[6] Arashida, Y., et al. Physical Review B 85, (2012).
[7] Molas, M. R., et al. EPL (Europhysics Letters) 113, 17004 (2016).
[8] Persson, J., et al. Physical Review B 69, (2004).
[9] Schmidgall, E. R. et al. Physical Review B 90, (2014).
[4] Bounouar, S., et al. Applied Physics Letters 112, 153107 (2018).
[10] Trotta, R., et al., Nano Letters 14, 3439¿3444 (2014).
[11] Gisin, N., Thew, R., Nature Photonics 1, 165¿171 (2007).