Future quantum technologies will require non-classical light sources emitting indistinguishable photons on-demand with high efficiency and purity. These sources will have to be fabricated via simple and cost-effective methods and, at the same time, be compatible with current photonic integration technologies. Epitaxial quantum dots are currently emerging as near-ideal single-photon sources but, after almost 20 years of extensive efforts, bringing them out of research laboratories remains a grand challenge, mainly due to the difficulty of fulfilling the scalability requirement set by quantum technologies. The STRATEGINO (STRAin shaping of Two-dimensional crystals for singlE photon GeneratIoN and manipulatiOn) project aims at developing a cheap, scalable approach to the fabrication of near-ideal single-photon sources, by inducing nanoscale deformations in atomically thin membranes made of transition metal dichalcogenides (TMDs). In order to induce these deformations -which have shown the ability to act as efficient single photon emitters- we will rely on our recent discovery that the H irradiation of bulk TMD flakes results in the formation of monolayer-thick, highly strained domes. These domes are stable and robust, and lithographic techniques allow engineering their formation process so that they can be produced with well-ordered positions, controllable shape, and sizes tunable from the nanometer to the micrometer scale. Our nanodome-based photon sources will be created spatially and spectrally resonant with the electromagnetic field of a circular Bragg-grating microcavity, to ensure efficient light extraction. Moreover, they will be integrated onto micro-machined piezo-electric devices, thus enabling a fine tuning of their emission properties. We envisage that our scalable platform will open the way towards the real exploitation of solid-state sources of non-classical light in quantum technologies.
At present, two of the main funding initiatives undertaken by the European Union place a strong focus on condensed matter physics and materials science. The first program, the Future and Emerging Technologies (FET) Flagship on Graphene (see https://graphene-flagship.eu/project/Pages/About-Graphene-Flagship.aspx) is currently in the second half of its planned ten-year run (2013-2023), and it devotes a ~€1 billion budget to the goal of "bringing together academic and industrial researchers to take graphene [and related 2D materials] from the realm of academic laboratories into European society [...], thus generating economic growth, new jobs and new opportunities". The second initiative, the FET Flagship on Quantum Technologies (http://ec.europa.eu/research/participants/portal/desktop/en/opportunitie...) was launched last year with the aim of moving "advanced quantum technologies from the laboratory to industry with concrete prototype applications and marketable products while advancing at the same time the fundamental science basis".
By aiming at the realization of advanced single-photon sources based on the integration of site-controlled TMD nanodomes with bullseye cavities, relying on strain engineering to dynamically control the device properties, the present project sits squarely at the intersection between these two Flagship initiatives. On the one hand, as reported in §2.8 and 2.9 of the "Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems" [Fer15], an important goal of the Graphene Flagship is indeed represented by the identification, characterization and practical exploitation of 2D materials alternative to graphene. As noted in the previous sections, transition metal dichalcogenides (and hBN) hold a preeminent place within this context, due to their huge potential for light-emitting applications. The development of the ability to harness the effects of strain -and of quantum confinement, for domes smaller than 10-20 nm- on the electronic and optical properties of 2D TMD micro- and nano-domes will represent a crucial breakthrough in the field, paving the way to the exploitation of these effects for the fine tuning of the light-emitting properties of the fabricated nanodevices.
This latter goal is also what establishes a strong link between the present project and the newly launched Flagship on Quantum Technologies. As noted in the Final Report of the High-Level Steering Committee of the Quantum Technologies Flagship (https://tinyurl.com/QT-HLSC-report), the development of bright, site-controlled single-photon sources with tuneable properties -possibly working at room temperature, and compatible with current, Si-based, optoelectronic devices- would be beneficial to many of the overarching goals of the Flagship, such as the development of Quantum Simulators with a certified quantum advantage (wherein ordered arrays of identical single-photon sources will likely be required) and the establishment of commercially viable protocols for Quantum Communications.
Moreover, today's commercially available single-photon sources are exclusively based on the InGaAs material platform, and they carry all the related drawbacks: stochastic growth, manual selection of the devices and, last but not the least, the high fabrication costs typical for this platform. While the price for those devices commonly exceeds various tens of thousands € per single photon source, our proposed approach would result in a significantly more economical device: the material costs of high quality TMD crystals as base material are modest (500 € for hundreds of exfoliation processes), and all fabrication steps that will be developed in the project are scalable.
Due to the growing commercial interest in Quantum Technologies, devices such as the one we propose here have left the domain of purely scientific endeavour. A realization of the proposed device will greatly strengthen Europe's innovation capacity and its position in the global quantum communication market. The intellectual property rights (IPR) coming from this project and its derivatives could lead to further products for the future Digital Single Market. The EU-generated IP would help address issues with strong socio-economical weight, such as the future dependence on network components that come from non-EU providers.