ICFO

Nonlinear optical processes are at the foundation of many applications in modern science and engineering. The emerging field of Quantum Technologies is now demanding that we push these processes into the realm of Quantum Nonlinear Optics (QNLO) where nonlinear effects occur at the level of individual photons. Achieving such a regime would allow the generation and manipulation of non-classical states of light and would open exciting new scenarios involving quantum many-body physics of light. Despite the great efforts that have been invested along this line of research, significant improvements are still necessary to fully achieve the QNLO regime. QUANLUX aims to tackle this challenge by proposing a novel light-matter interface consisting of ordered atomic arrays as an ideal platform to implement QNLO processes. The ultimate objectives consist in identifying new strategies for QNLO protocols that can possibly surpass previously established performance bounds as well as investigating the complex emergent behaviour of strongly interacting photons. To tackle and solve these demanding problems the fellow will make use of advanced numerical and theoretical techniques developed in condensed matter and many-body physics (e.g. tensor networks and diagrammatic approaches) that will be acquired through dedicated training visits to experts in the field. The proposed dissemination and outreach program will progressively spread the outcome of the action to the scientific community and to the general public reinforcing the impact of the research’s results. The originality and multidisciplinary nature of the proposal have the potential to revolutionize the major paradigms currently used to implement QNLO processes and drive a technological innovation in the construction of light-matter interfaces. The action will be conducted by Giuseppe Calajò who will join the Theoretical Quantum Nanophotonics group lead by Prof. Darrick Chang at ICFO, Spain.

A central goal of quantum optics is to realize efficient, controlled quantum interfaces between atoms and photons. Such interfaces enable broad applications from quantum information processing to quantum nonlinear optics to metrology, and also open a route toward creating exotic quantum states of light and matter. Today, our major paradigm for realizing an efficient interface is based upon the concept of collective enhancement, where using a large number of atoms creates an enhanced coupling to a preferred optical mode over undesired emission into other directions. However, our known error bounds for applications decrease very slowly as a function of system resources, such as the optical depth, thus posing a great challenge for future technologies. In NEWSPIN, we propose a remarkable new way forward, based upon the realization that these conventional error bounds are derived without accounting for multiple scattering and wave interference between emitting atoms. We aim to establish that interference in light emission is in fact a much more powerful resource than the level that we currently exploit it. In particular, beyond the usual collective enhancement, it can simultaneously enable a much stronger collective suppression of emission into undesired directions, and which can yield exponentially better error bounds than was previously known. Underlying this powerful paradigm shift will be the development of a quantum many-body theory of multiple scattering involving photons and atoms, which takes advantage of state-of-the-art tools from condensed matter physics. Beyond robust new routes toward applications, our theory will also reveal exotic new quantum phenomena and lead to new insights into fundamental questions in optics, such as the physical limits to how large the refractive index of an optical material can be. In total, we anticipate that NEWSPIN could greatly enrich our understanding of atom-light interactions and their realm of possibilities.

The main goal of GTRACK is to demonstrate a semi-transparent eye-tracking system that is disposed in the line of sight of the user, for portable applications. To this end, we will use hybrid Quantum Dot – Graphene photodetectors. Eye-tracking existed since the 1800’s, but is expected to appear abundantly in our daily lives with the advent of virtual and augmented reality. In the existing systems, the camera has to be placed sufficiently close to the eye to capture enough IR light at sufficiently high resolution, while not blocking the user’s vision. By placing the camera directly on the lens, all these disadvantages are circumvented. Moreover, larger detectors can increase the sensitivity of the detectors and hence decrease the power consumption of the active illumination, which would allow for portable applications.

Polaritons are joint excitations of light and matter and constitute an important field of study in optics. Historically, many new types of polaritons have been discovered by inspecting novel and interesting material systems, with graphene plasmons being a prominent example. Project TOPLASMON aims to study and harness the polaritons in an even newer material category - topological materials - which have recently been discovered and are intensively studied in condensed matter physics. These materials include topological insulators, which have conducting edges but insulating bulks, and Weyl Semimetals, which support unique Fermi-arc states. At the heart of project TOPLASMON is a novel measurement system, which combines a recently invented cryogenic scanning near field microscope with a THz laser and detector. This setup will allow, for the first time, the observation of topological polaritons of several varieties: (1) Chiral polaritons in topological insulators which exhibit reduced backscattering from defects. Specifically, I will working with the recently realized 2D topological insulators. (2) Fermi-arc Polaritons in Weyl Semimetals, whose dispersion is tied in with the properties of the underyling crystal, thereby probing the properties of these new materials. These polaritons are expected to have an in-plane hyperbolic dispersion and may even lead to realization of miniaturized optical isolators, leading to an important technological breakthrough. (3) Strong plasmonic resonances. I will study plasmon-polariton excitations in topological material, at frequencies near the plasmonic resonance. Empowered by the exceedingly long electron scattering times measured in several recent experiments, highly confined plasmons with unprecedentedly long propagation distances are exoected, a dramatic result for both science and technology. This proposal is therefore set to open a new study area at the forefront of research both in condensed matter and nanophotonics.