Periodic Reporting for period 1 - QUANLUX (Quantum Nonlinear Optics in Atomic Arrays)
Reporting period: 2020-09-01 to 2022-08-31
Optical and electromagnetic signals in general are the most common carriers to communicate information over long distances, because their elementary constituents (the photons) do not interact with each other in free space, allowing a linear and undistorted propagation. On the other hand, to process the carried information, some form of interaction between the signals is required. This can be achieved when light propagates inside a material, where a nonlinear optical response can allow to achieve such effective optical interaction leading to important applications such as optical modulation and switching, nonlinear spectroscopy, and frequency conversion, commonly used in modern science and engineering. All the mentioned processes usually occur at high light intensities, due to the extremely weak nonlinear optical response of the most common materials, and over the years, many different approaches have been de-veloped among the years to increase the efficiency of such schemes, the most common in-volving cavities, nanophotonics structures and ensembles of Rydberg atoms. Despite the great efforts that have been invested along this research line, significant improvements are still necessary to make a definitive breakthrough and to fully achieve the realm of Quantum Non-linear Optics (QNLO), where nonlinear effects occur at the level of individual photons. This not only enables the realization of nonlinear classical devices operating at the lowest possible intensity levels but also allows the generation and manipulation of non-classical states of light, a task that has become even more timely in light of the emerging field of Quantum Technologies. Furthermore, the perspective to generate and control photon-photon interac-tions brings entirely new possibilities, such as potentially realizing quantum many-body physics with light. QUANLUX aimed to tackle this challenge by identifying novel promising light-matter inter-faces for QNLO protocols and by investigating the complex emergent behavior of strongly interacting photons. These objectives have been achieved via the development of theoretical frameworks and advanced numerical methods capable to solve the many-body problem of multi-photons propagation.
To accomplish the first objective we proposed two novel platforms where QNLO protocols can be implemented with high efficiency. The first one consisted of a graphene layer integrat-ed within a photonics waveguide. Here injected photons are converted to collective charge excitations, the plasmons, and, under certain conditions, they can experience a strong effec-tive interaction. We proposed to make use of such interaction to implement a high-fidelity gate between the two incoming photons, an elusive task in conventional schemes. The second considered platform consisted of an array of compact superconducting resonators coupled to two frequency-tunable artificial atoms. This platform has been implemented ex-perimentally and can seed long-lived atom-photon bound states where photons remain trapped in the structure for a long time. Also in this case, we showed how this feature can be exploited to achieve full control of the interactions among several atoms allowing the imple-mentation of basic gates.
To achieve the second objective we develop a theoretical framework and advanced numerical methods to solve the many-body problem of photons interacting in quantum nonlinear media. Such methods have been applied to ordered arrays of atoms and extended to an ensemble of Rydberg atoms. With these tools, we identified the emergence of bound states of photons (something like molecules made of photons) with a strong photon number-dependent veloci-ty, which can cause a classical incoming pulse to separate into different photon number com-ponents at the output. Such behavior might serve as the basis for photon number-resolving detection. Interestingly, we also demonstrated that, for large photon numbers, such quantum states can undergo a transition into a classical optical solitonic, a particular wave that pre-serves its shape during propagation. All the results of the project were published in peer-reviewed journals and some of them in-cluded a popular summary for the general public. Besides the scientific achievements, an important goal of QUANLUX was the dissemination of the results and of the knowledge acquired to young researchers and the general public. This objective has been pursued via many activities, including meeting with high school stu-dents, supervision of master students, talks at science divulgation events and an introductory course on Modern Platforms for Quantum Optics organized for the PhD school of the Uni-versity of Palermo. The communication of the project activities was promoted via a dedicat-ed project website and a social media Facebook page.
To achieve the second objective we develop a theoretical framework and advanced numerical methods to solve the many-body problem of photons interacting in quantum nonlinear media. Such methods have been applied to ordered arrays of atoms and extended to an ensemble of Rydberg atoms. With these tools, we identified the emergence of bound states of photons (something like molecules made of photons) with a strong photon number-dependent veloci-ty, which can cause a classical incoming pulse to separate into different photon number com-ponents at the output. Such behavior might serve as the basis for photon number-resolving detection. Interestingly, we also demonstrated that, for large photon numbers, such quantum states can undergo a transition into a classical optical solitonic, a particular wave that pre-serves its shape during propagation. All the results of the project were published in peer-reviewed journals and some of them in-cluded a popular summary for the general public. Besides the scientific achievements, an important goal of QUANLUX was the dissemination of the results and of the knowledge acquired to young researchers and the general public. This objective has been pursued via many activities, including meeting with high school stu-dents, supervision of master students, talks at science divulgation events and an introductory course on Modern Platforms for Quantum Optics organized for the PhD school of the Uni-versity of Palermo. The communication of the project activities was promoted via a dedicat-ed project website and a social media Facebook page.
The main innovation and contribution to the state of the art have consisted in the developing of novel theoretical frameworks and advanced numerical techniques to treat multi-photon propagation in quantum nonlinear media. In addition, we identified novel integrated plat-forms, such as graphene nanoribbons and artificial atoms coupled to an array of supercon-ducting resonators, where successful QNLO protocols can be implemented. Indeed, in both cases, we proposed successful schemes to implement qubit-qubit gates between photons, a necessary ingredient to perform photon-based quantum computation. The results have demonstrated important theoretical, numerical, and even applicative advances to the state of the art of QNLO platforms, as demonstrated by the collaborations with experimental groups.