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Cavity-field induced transparency and dipolar-exchange induced transparency with Rydberg atoms

Subject Area Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 394243350
 
Final Report Year 2023

Final Report Abstract

The scientific objective of the project has been the study of coherent and dissipative interactions of quantum light fields with atomic media, which we named cavity-field induced transparency (CIT). In contrast to electromagnetically induced transparency (EIT) it is based on a quantized control field, which in our case is the field mode of a superconducting microwave cavity. The quantized excitations of the control field shall then make the transparency of the atomic medium for the input probe field photon-number selective, allowing to realize number-state filters and transistors for optical photons, coherent conversion and dissipative state mapping between microwave and optical photons, and studies of atom-photon bound states. Within the funding period we achieved important advances towards the demonstration of CIT by elaborating concepts and developing techniques towards an experimental realization with Rydberg atoms near a superconducting coplanar microwave resonator. First results could be gained from simulations into CIT and the preparation of experiments with Rydberg atoms interacting with microwave fields in a room-temperature vapor cell. Using this system we successfully demonstrated the effects of microwave-induced transparency (MWIT). Experimental steps towards CIT we made with a coherent MW control field. For the envisioned scheme, the probe field was chosen as two-photon field, giving rise to EIT. Unexpectedly, upon separating the EIT and MWIT signals from each other, we found a strong resonance connected to the coupling scheme with a two-photon control field and a single photon probe field. These findings together with theoretical studies and numerical simulations helped to understand the effects of finite temperature onto the CIT and MWIT signals. Alongside the vapor cell experiments, we have prepared for CIT experiments with ultracold Rydberg atoms trapped near the field mode of a superconducting coplanar microwave waveguide (CPW) resonator, operated at 4K temperature. Unfortunately, unexpected failure of components of the existing experimental systems (in-vacuo microwave system) hindered efficient progress. Nevertheless, we have taken important steps by finding stable experimental conditions for preparing, manipulating and detecting Rydberg atoms near a superconducting microwave cavity. Based on extensive Stark map calculations and taking into account magnetic fields, electric fields and the fixed CPW mode frequency, we successfully identified and experimentally prepared a suitable Rydberg state pair. This task was additionally complicated due to surface adsorbates causing inhomogeneous Rydberg Stark shifts, effectively localizing the volume for resonant coupling of the probe and control field within the cloud. In a further step we worked out a viable single atom detection and analysis procedure based on selective field ionization (SFI) and proper post-processing to distinguish the Rydberg state pair in the corresponding ionization signal. Therefore, we simulated the time evolution of the atomic population through the Stark map, extracted the expected ionization signal and derived a procedure to determine the initial state populations. In a final step, we combined these achievements to demonstrate a coherent Rydberg-cavity coupling for trapped cold atoms inside a CPW resonator structure by monitoring corresponding Rabi oscillations. Thereby, we found a linear scaling of the dephasing time with the Rabi frequency, which we denoted to the inhomogeneity of the CPW mode field. Thus, the adsorbate fields have shown to be essential for the coherent coupling, as they limit the size of the excitation and coupling region to a small volume. Especially in the near-field of a resonator structure, this is essential to minimize dephasing effects due to the spatial MW mode profile. The observation of coherent coupling via the CPW resonator mode is thus an important step towards CIT experiments. However, the strong coupling required for CIT has not yet been achieved, and thus the realization of CIT remains an experimental task for the future.

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