Periodic Reporting for period 1 - InterPol (Interfacing interacting Rydberg polaritons: From few- to many-body interactions) Reporting period: 2019-04-01 to 2021-03-31 Summary of the context and overall objectives of the project Rydberg atoms are atoms in which the valence electron is excited to loosely bound state, which makes their properties highly sensitive to external electric fields, but also the presence of other Rydberg atoms nearby. This results in strong interactions between them, for example a blockade effect that prevents the excitation of more than one Rydberg atom over several micrometers. Thanks to their strong interaction, Rydberg atoms have found many applications, for example in quantum simulation and information processing. Rydberg atoms also find applications in nonlinear quantum optics, that is to make individual photons, which would normally pass each other unnoticed, effectively interact with each other. To this end, photons are mapped into Rydberg polaritons, quasi-particles that carry the strong interactions of Rydberg atoms, as they travel through a gas of ultracold atoms. Alternatively, one can also use Rydberg superatoms, to effectively mediate interactions between photons. Rydberg superatoms are based on ensembles of many atoms that are saturated following the absorption of a single photon due to the blockade mechanism. They act like a single quantum emitter, but photons couple much more strongly to them compared to a single atom.The objective of InterPol has been to study Rydberg polaritons and Rydberg superatoms in systems of increasing size in terms of both fundamental and application aspects and analyse the performance of such systems for future applications in optical quantum technology. For example, we show that a chain of Rydberg superatoms can remove photons from a light pulse one by one, but also identified effects that need to be mitigated to successfully scale systems to larger numbers of superatoms and polaritons. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far We have implemented a system that allows us to remove individual photons from a pulse of probe light one-by-one, for example for applications in optical quantum metrology or number-resolved photon detection. The system uses a chain of multiple Rydberg superatoms placed along the mode of the pulse from which the photons are to be subtracted. Due to their saturation following absorption of a single photon a single superatoms cannot extract more than one photon as long as it does not decay during the duration of the pulse. To remove multiple photons from a light pulse, we trap multiple atom ensembles along the probe beam path in optical tweezers that can be moved and switched individually to control the number of photons that is to be subtracted. Besides a proof-of-principle demonstration with up to three superatoms, we have also analysed the potential to subtract larger photon numbers and identified Raman decay of the superatoms as the main source of deviations in the subtracted photon number. The results are currently under review for publication.Besides using of the effect of multiple Rydberg superatoms to manipulate light pulses, we have also investigated the effect of the consecutive interaction with the same light pulse has on the superatoms, i.e. whether we can observe any signatures of a coherent interaction mediated this way that would lead to an entanglement of the superatom’s quantum states.During this investigation, we have focused on the superatoms’ decay and the re-emission of probe photons into their original mode and found that a similar coherent exchange interaction already occurs between the individual atoms that contribute to a single Rydberg superatom. The interaction puts the superatom into a superposition of multiple quantum states which are characterized by different decay rates (super- and subradiant states). Interference between those states during re-emission of an absorbed probe photon leads to a deviation of the decay from the typical exponential behaviour and the decay is no longer independent of e. g. the duration and intensity of the driving pulse. We have analysed these decay dynamics both experimentally and theoretically in collaboration with the Büchler group at the University of Stuttgart and extended the theoretical analysis more generally to quantum emitters coupled to a directional 1D-waveguide. Similar effects to our observations also occur in other collectively excited systems of quantum emitters which are for example used for photon storage schemes.Finally, we have observed nonreciprocal transmission of photons through an atomic vapour in a room-temperature glass cell. We exploit electromagnetically induced transparency (EIT) to render the atoms transparent to probe light resonant with an atomic transition to a low-lying excited state using a control laser that couples them to a Rydberg state. To observe EIT, both the probe and control lasers have to be on resonance with the respective atomic transitions. Due to the optical Doppler effect, the moving atoms see the probe and control light at a shifted frequency beams and the frequency conditions to observe transparency depend on the direction of the probe and control beams with respect to each other. This allows us to choose parameters such that a probe beam is transmitted if its direction is opposite to the control beam but absorbed if the directions are the same. Such nonreciprocal optical transmitters have long been sought after for example for integration into optical quantum networks. A publication of the results is planned. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) A core result of the project is the first implementation of a system of multiple Rydberg superatoms cascaded along the optical mode of a probe beam. This not only enables controlled subtraction of specific photon numbers from a light pulse to manipulate its quantum state photon-by-photon, but also represents a novel setting to study the behaviour of individual cascaded quantum emitters that interact via the directional exchange of individual photons. The system is equivalent individual emitters coupled to a 1D chair waveguide for which plethora of interesting experiments have been proposed. Beyond proof-of-principle results, we have also initiated upgrades to our experiment to reduce dephasing of the superatoms' quantum states and increase the number of superatoms in future work. In addition, we have also observed evidence of a related photon-exchange interaction between the atoms that contribute to an individual collective Rydberg excitation. This was previously neglected, but can become more relevant in the future in other contexts such as quantum metrology, simulation, or computation with collective Rydberg excitations. Very recently, applications of Rydberg atoms in quantum technology have also attracted commercial interest and Rydberg atoms interfaced with optical photons could potentially be used for information transfer in this context. Illustration of controlled single photon subtraction using Rydberg superatoms.
