BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:69d4b11f64a5d DTSTART:20240119T090000Z SEQUENCE:0 TRANSP:OPAQUE LOCATION:Auditorium and Online (Teams) SUMMARY:ICFO | JAN LOWINSKI CLASS:PUBLIC DESCRIPTION:Photons have emerged as the main candidates for carrying quantu m information due to their weak interaction with the environment. Unfortun ately\, their limited interaction with one another poses challenges for ph otonic quantum information processing. One of the possible solutions lies in the unique behavior of interacting Rydberg excitations in cold atomic e nsembles\, where strong nonlinearities enable engineering interactions amo ng individual photons. This phenomenon makes Rydberg ensembles a promising platform for quantum information applications\, notably in long-distance quantum communication. This thesis presents a series of experiments that e xplore and exploit Rydberg-mediated interactions\, all with the long-term objective of building an efficient quantum repeater.\nThe thesis begins wi th a concise theory overview of Rydberg and ensemble physics. This is foll owed by an explanation of the experimental setup. I discuss how\, building upon a previously existing setup\, we improved the stability and spectral properties of our laser system\, along with enhancing the quality of the atomic ensemble. The introductory section of the thesis concludes with a d escription of two different single-photon generation methods and an in-dep th review of various decoherence mechanisms impacting Rydberg ensemble exc itations. The single-photon generation performance has been improved by th e modifications implemented in the setup\, resulting in higher generation rates and better single-photon purity. Supported by experimental data and a careful analysis of experimental parameters\, we identify the most proba ble sources of significant decoherence and suggest potential strategies fo r mitigation.\nIn our initial experiment\, we achieve the storage and subs equent retrieval of an on-demand single photon. This photon is generated t hrough the collective excitation of Rydberg states in one cold atomic ense mble\, and it is stored in a low-noise Raman quantum memory situated in an other cold atomic ensemble. Our results show the capability to store and r etrieve these single photons while maintaining a high signal-to-noise rati o of up to 26 and preserving strong antibunching characteristics. We also explore the built-in temporal beam splitting capabilities of the Raman mem ory and successfully use the memory to control the single photon waveshape .\nIn the second experiment\, we demonstrate for the first time an interac tion and storage of single photons in a highly non-linear medium based on cold Rydberg atoms. We employ the DLCZ protocol in a cold atomic ensemble to create single photons\, guiding them to another ensemble for storage in a highly excited Rydberg state under conditions of electromagnetically in duced transparency. By studying the statistics of the light retrieved from the Rydberg atoms\, we show for the first time single-photon filtering wi th non-classical input light. Moreover\, through Monte Carlo simulation\, we get an intuitive understanding of the effect of the (partial) Rydberg b lockade upon the Fock state distribution of arbitrary input light pulses. This allows us to conclude that the response of the medium is determined b y the input Fock state distribution\, what confirms the established unders tanding of Rydberg ensemble nonlinearity. This demonstration can be seen a s a step towards realization of deterministic photon-photon gates based on Rydberg ensembles with single photon inputs. \;\nThe results presente d in this thesis affirm the potential of Rydberg ensembles to become centr al elements of future quantum networks\, both as single photon sources and processing nodes. Furthermore\, auxiliary outcomes provide an additional understanding of the Rydberg ensemble physics and offer insight into limit ations that we need to overcome to improve further our setup.\n \;\nTh esis Director: Prof Dr. Hugues de Riedmatten DTSTAMP:20260407T072415Z END:VEVENT END:VCALENDAR