title: Generation of Narrowband Photon Pairs at High Brightness within a Hollow-Core Fiber
Creators: name: Bruns, Alexander
Publisher: name: UNSPECIFIED
Year: 2023

description: This research project dealt with the implementation of a non-classical photon-pair source based on spontaneous four-wave mixing (SFWM) in an ensemble of cold Rubidium atoms interfaced with a hollow-core photonic bandgap fiber (HCPBGF). Due to the single-mode guidance of the HCPBGF, we can achieve intrinsically optimal spatial mode matching of all four light fields in the SFWM process as well as optimal overlap between the guided light and the atomic ensemble within the fiber. The strong confinement of atoms and light fields within the HCPBGF leads to enhanced optical nonlinearities as compared to free-space setups. In this thesis, we summarized the setup for the preparation of cold atoms inside the HCPBGF and presented a thorough characterization of the confined atomic ensemble in terms of the number of atoms, optical depth, coherence time, and EIT conditions. Subsequently, we introduced the extended setup for the detection and analysis of non-classical photons. Because all SFWM fields propagate collinear within the HCPBGF we could not use angular separation to filter the background of the strong pump fields in the photonic fields. Instead, we implemented a three stage filtering system based on polarization, tunable etalons and optical bandpasses. Furthermore, we implemented the detection and data acquisition system based on two HBT setups to measure auto-and cross-correlations of the generated photon pairs. In the analysis, we had to take the pulsed nature of our experiment into account when determining the normalization factors for the various correlation measurements. In the main part, we presented a detailed analysis of the generated photon pairs. We verified their non-classical nature by measuring the cross-correlation and compare it to the measured thermal auto-correlations of the individual fields. We observed a violation of the Cauchy-Schwarz inequality by a factor of 97(24), i.e., we exceeded the classical limit by about four standard deviations. The use of cold Rubidium atoms as our medium permitted the generation of intrinsically narrow biphotons. We determined the biphoton line width as 2π ⨉ 6.5 MHz which is compatible with atomic quantum memories and similar to the line width achievable with free-space setups. We can also view the photon pair source as a heralded source of single photons. Conditioned on the detection of a heralding event, we observed anti-bunched single photons manifested by a conditional auto-correlation of 0.16 ≪ 0.5. A high heralding efficiency of about 50 % was facilitated by the optimal mode matching and collection efficiency within the HCPBGF. Furthermore, a systematic comparison of the heralded auto-correlation with the cross correlation yielded good agreement with the theoretical expectation for a two-mode squeezed vacuum state. Furthermore, by optimizing the experimental parameters towards a high generation rate we achieved a generated spectral brightness per pump power (GSBP) of up to 2 ⨉ 10⁹ pairs / (s MHz mW) at pump powers below 100 nW and for a cross-correlation ≥ 3. Our results represent a 10-fold increase of the GSBP compared to the previous highest value achieved using a micro-ring cavity. Our photon pairs exhibit a 10-fold reduced bandwidth and due to the pump beams guided within the HCPBGF we require a 100-fold reduced pump power. Moreover, by increasing the generated spectral brightness even further, we reached the regime where different photon pairs start to overlap in time. Here, we demonstrated that the cross-correlation approaches a limit corresponding to thermal statistics. Finally, in an additional experiment, we investigated superfluorescent (SF) scattering in the cold atomic ensemble within the HCPBGF. Such medium facilitates cooperative effects already at comparably low numbers of emitters due to the strong coupling between the guided light and the confined atoms. We developed a model to carefully determine the effective number of emitters involved in the collective scattering and observed the expected scaling for both the ratio between SF and single-atom scattering rate and the SF intensity versus the effective number of atoms.