#	Pagina
attuale pagina	/open-h2020/projects/200760/results.html

Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SIRPOL (Strongly interacting Rydberg slow light polaritons)

Teaser

Summary

A fundamental property of optical photons is their extremely weak interactions, which can be ignored for all practical purposes and applications. This phenomena forms the basis for our understanding of light and is at the heart for the rich variety of tools available to manipulate and control optical beams. On the other hand, a controlled and strong interaction between individual photons would be ideal to generate non-classical states of light, prepare correlated quantum states of photons, and harvest quantum mechanics as a new resource for future technology. Rydberg slow light polaritons have recently emerged as a promising candidate towards this goal, and first experiments have demonstrated a strong interaction between individual photons. The aim of this project is to develop and advance the research field of Rydberg slow light polaritons with the ultimate goal to generate strongly interacting quantum many-body states with photons. The theoretical analysis is based on a microscopic description of the Rydberg polaritons in an atomic ensemble, and combines well established tools from condensed matter physics for solving quantum many-body systems, as well as the inclusion of dissipation in this non-equilibrium problem. The goals of the present project addresses questions on the optimal generation of non-classical states of light such as deterministic single photon sources and SchrÃ¶dinger cat states of photons, as well as assess their potential for application in quantum information and quantum technology. In addition, we will shed light on the role of dissipation in this quantum many-body system, and analyze potential problems and fundamental limitations of Rydberg polaritons, as well as address questions on equilibration and non-equilibrium dynamics. A special focus will be on the generation of quantum many-body states of photons with topological properties, and explore novel applications of photonic states with topological properties.

Work performed

\"
A main objective of the proposal is to understand the effective interaction between Rydberg polaritons, and especially, the behavior of higher body interactions. During this project, we studie a system of three photons in an atomic medium coupled to Rydberg states near the conditions of electromagnetically induced transparency. Based on the analytical analysis of the microscopic set of equations in the far-detuned regime, the effective three-body interaction for these Rydberg polaritons is derived. For slow light polaritons, we find a strong three-body repulsion with the remarkable property that three polaritons can become essentially non-interacting at short distances. This analysis allows us to derive the influence of the three-body repulsion on bound states and correlation functions of photons propagating through a one-dimensional atomic cloud. [Phys. Rev. Lett. 117, 053601 (2016)]

In a second step, we focused on the behavior of photons interacting with a cold gas of atoms, where the blockade radius is larger than the size of the atomic cloud. Such a setup establishes a Rydberg \"\"superatom\"\" as the light couples only to a single excited states. While the interaction of a single photon with an individual two-level system is the textbook example of quantum electrodynamics, achieving strong coupling in this system so far required confinement of the light field inside resonators or waveguides. In a collaboration with the experimental group of Prof. S. Hofferberth in Odense, we demonstrated the strong coupling of propagating photons in free space with such a Rydberg \"\"superatom\"\". The strong light-matter coupling in combination with the direct access to the outgoing field allows us to observe for the first time the effect of the interactions on the driving field at the single photon level. We demonstrated that all our results are in quantitative agreement with the predictions of the theory of a single two-level system strongly coupled to a single quantized propagating light mode. The demonstrated coupling strength opens the way towards interfacing photonic and atomic qubits and preparation of propagating non-classical states of light, two crucial building blocks in future quantum networks. Furthermore, we demonstrated theoretically that an important mechanism of dephasing in such a Rydberg \"\"superatom\"\" is the coupling to dark states by virtual exchange of photons. Especially, in a one-dimenionsal wave guide we find a remarkable universal dynamics for increasing atom numbers exhibiting several revivals under the coherent part. While this phenomenon provides a limit on the intrinsic dephasing for such a collective excitation, a setup is presented, where this remarkable universal dynamics can be explored. [Phys. Rev. X 7, 041010 (2017);Phys. Rev. Lett. 121, 103601 (2018);Phys. Rev. Lett. 121, 013601 (2018)]

An additional important research goal of the project is the study of topological properties. Especially, Efficient communication between qubits relies on robust networks which allow for fast and coherent transfer of quantum information. It seems natural to harvest the remarkable properties of systems characterized by topological invariants to perform this task. We showed that a linear network of coupled bosonic degrees of freedom, characterized by topological bands, can be employed for the efficient exchange of quantum information over large distances. Important features of our setup are that it is robust against quenched disorder, all relevant operations can be performed by global variations of parameters, and the time required for communication between distant qubits approaches linear scaling with their distance. We demonstrated that our concept can be extended to an ensemble of qubits embedded in a two-dimensional network to allow for communication between all of them. In addition, we study potential experimental realization of such topological edge states using Rydberg atoms in a honeycomb lattice. The proposal is based on t\"

Final results

The project has substantially contributed to the understanding of Rydberg slow light polaritons and their application for quantum information processing and generation of novel quantum states of matter. The main approach combines well established analytical and numerical tools, which provides the novelty and success in the project. A major aspect are the successful collaborations with world leading experimental groups in the field of photons interacting with Rydberg atoms, as well as to establish novel applications of states of matter with topological properties.