Using electromagnetically induced transparency and photon storage, the strong dipolar interactions between Rydberg atoms and the resulting dipole blockade can be mapped onto light fields to realise optical non-linearities and interactions at the single photon level. We report on the realisation of an experimental apparatus designed to study interactions between single photons stored as Rydberg excitations in optically trapped microscopic ensembles of ultracold 87Rb atoms. A pair of in-vacuum high numerical aperture lenses focus excitation and trapping beams down to 1 μm, well below the Rydberg blockade. Thanks to efficient magneto-optical trap (MOT) loading from an atomic beam generated by a 2D MOT and the ability to recycle the microscopic ensembles more than 20000 times without significant atom loss, we achieve effective repetition rates exceeding 110 kHz to obtain good photon counting statistics on reasonable time scales. To demonstrate the functionality of the setup, we present evidence of strong photon interactions including saturation of photon storage and the retrieval of non-classical light. Using in-vacuum antennae operating at up to 40 GHz, we perform microwave spectroscopy on photons stored as Rydberg excitations and observe an interaction induced change in lineshape depending on the number of stored photons.
Using electromagnetically induced transparency and photon storage, the strong dipolar interactions between Rydberg atoms and the resulting dipole blockade can be mapped onto light fields to realise optical non-linearities and interactions at the single photon level. We report on the realisation of an experimental apparatus designed to study interactions between single photons stored as Rydberg excitations in optically trapped microscopic ensembles of ultracold 87Rb atoms. A pair of in-vacuum high numerical aperture lenses focus excitation and trapping beams down to 1 μm, well below the Rydberg blockade. Thanks to efficient magneto-optical trap (MOT) loading from an atomic beam generated by a 2D MOT and the ability to recycle the microscopic ensembles more than 20000 times without significant atom loss, we achieve effective repetition rates exceeding 110 kHz to obtain good photon counting statistics on reasonable time scales. To demonstrate the functionality of the setup, we present evidence of strong photon interactions including saturation of photon storage and the retrieval of non-classical light. Using in-vacuum antennae operating at up to 40 GHz, we perform microwave spectroscopy on photons stored as Rydberg excitations and observe an interaction induced change in lineshape depending on the number of stored photons.
Rydberg quantum optics achieves optical non-linearities at the single-photon level by mapping the strong dipolar interactions between Rydberg atoms in cold atomic gases onto light fields using electromagnetically-induced transparency and photon storage. The non-linearities are a direct consequence of the long-range character of the interaction which allows a single photon to modify the optical response in a volume containing many atoms. In this thesis, the long-range character of the resulting effective photon-photon interaction is directly observed as photons propagating in non-overlapping optical modes are stored as collective Rydberg excitations in adjacent and non-overlapping microscopic clouds of 87Rb atoms. While stored, van-der-Waals interactions imprint spatially non-uniform phase shifts in the collective excitations. These distort the photons' retrieval modes resulting in anti-correlated retrieval between the original modes.In this first demonstration of contactless effective interactions between photons, these effects are observed between photons separated by more than 15 times their wavelength, well above the optical diffraction limit.This represents a promising step towards the implementation of scalable, multichannel quantum optical devices such as quantum gates. The experiments are enabled by a new, specialised experimental setup centred around a pair of in-vacuo aspheric lenses. These provide optical resolution of order 1 µm to optically trap and address the ensembles separated by distances well below the range of Rydberg interactions. The ensembles are prepared in approximately 100 ms thanks to efficient loading of a magneto-optical trap (MOT) from an atomic beam produced by a 2D MOT. Combined with the ability to recycle the ensembles > 20000 times, effective cycle times exceeding 100 kHz enable the acquisition of large datasets for the analysis of photon statistics within a matter of minutes.
High optical nonlinearities can be achieved at the single photon level by coupling the photon states to strongly interacting Rydberg excitations under the conditions of electromagnetically induced transparency. The nonlinear response in Rydberg quantum optics comes as a direct result of the long range dipole-dipole interactions via Rydberg blockade and interaction induced dephasing. In this thesis, Rydberg quantum optics experiments are performed with two spatially separated mediums with a nonlinear response at the single photon level. Two ultracold rubidium atomic clouds are tightly confined by a pair of in-vacuo aspheric lenses such that only a few Rydberg excitations can exist in each one of the clouds simultaneously. The long range character of the dipole-dipole interactions leads to the generation of quantum states of light. The effective photon-photon interactions are directly observed as an anti-correlation in the simultaneous photon retrieval as a result of the non resonant van der Waals interactions. The collective Rydberg excitations, stored in the non-overlapping mediums, experience an additional spatial non-uniform phase. In addition, we experimentally characterize the cooperative optical response of a cold atomic medium at a single photon level. This aims to exploit the enhancement of the spontaneous emissions decay rate (superradiance) as a dependence of the number of atoms involved in the ensemble. The observed atomic response in two different regimes gives a further understanding of the system dynamics and unlocks a promising potential route towards the implementation in scalable, multichannel quantum optical devices.
This thesis proposes a means of implementing quantum information processing using photonic qubits as information carriers. Electromagnetically-induced transparency (\textsmaller{EIT}) is used to store information encoded in photons into Rydberg excitations in a cloud of cold atoms, where strong dipole-dipole interactions induce interactions between qubits. After a storage time, information is mapped back into photons collectively emitted from the cloud again via \textsmaller{EIT}. A new experimental apparatus is built to implement non-linear Rydberg quantum optics. A high repetition rate is achieved owing to a \textsmaller{2D-MOT} atom source, and high optical resolution for trapping and probing microscopic atomic ensembles is achieved by the use of aspheric lenses inside the vacuum chamber. A new, high resolution computer control scheme is implemented. This thesis demonstrates that, during the holding time, multiple collective Rydberg excitations at a controlled separation interact with each other to imprint a non-uniform phase gradient resulting in anti-correlation of photon emission. Interactions are observed at up to 15 times the wavelength of the photonic qubits. These long range interactions offer a promising approach to scaling all-optical quantum computing. Applying resonant microwave fields during the storage time is demonstrated to offer a competitive method of performing sensitive microwave electrometry. A sensitivity of $12 \pm 7 \uV \percm \perRHz$ is found at a frequency of $7.7\GHz$. The high sensitivity is shown to arise from remnant, Rydberg excitations providing an additional source of atom loss from the atomic ensemble, leading to a suppression of photon storage and retrieval efficiency. An additional stage of microwave driving to sanitise the atomic ensemble for recycling is shown to successfully suppress the atom-loss mechanism. The use of successive microwave pulses is shown to provide a feasible approach to interfacing microwave and optical quantum information processing systems. Information encoded as the presence or absence of a microwave field is translated into information encoded as early or late retrieval of single photons, demonstrating proof of principle for an approach to implementing a proposal for an all-optical controlled-z gate. Externally driven microwave fields are used to provide rapid, low-loss modulation of the signal retrieved from an atomic ensemble, demonstrating the proof of principle of implementing a probabilistic single photon source that can intensity modulate with low-loss at frequencies of at least $27\MHz$, with evidence that modulation may be achievable at rates in excess of $500\MHz$.
The effect of strong interatomic interactions on an optical field stored in Rydberg states of a cold atomic gas is investigated. Due to their large dipole moments Rydberg atoms interact very strongly with each other. The strength of these interactions can be significant over length scales of several microns. An effect known as dipole blockade leads to a suppression in the number of Rydberg excitations supported in a medium of finite size. The experiments described in this thesis aim to exploit this effect to create a medium with a non-linear response which occurs at the level of single-photons. A cold atomic cloud is tightly confined such that only a few Rydberg excitations are supported. It is shown that the dipole blockade phenomenon leads to strong photon-photon interactions, resulting in the generation of quantum states of light. A microwave field is used to control the strength and range over which the interactions between the stored optical photons occur. In addition, it is shown that the propagation of the optical field through the medium is non-trivial. Preliminary evidence is presented suggesting that the slow-light group delay in the medium is dependent on the number of propagating photons.
We demonstrate a coherent microwave manipulation of a single optical photon based on a single Rydberg excitation in an atomic ensemble. Due to the strong nonlinearities in a Rydberg blockade region, a single photon can be stored in the formation of Rydberg polariton using electromagnetically induced transparency (EIT). The manipulation of the stored single photon is performed by applying a microwave field that resonantly couples the nS 1/2 and nP 3/2 , while the coherent readout is performed by mapping the excitation into a single photon. We achieve a single photon source with g (2) (0) = 0.29 ± 0.08 at 80 S 1/2 without applying microwave fields. By implementing the microwave field during the storage time and retrieval process, we show the Rabi oscillation and modulation of stored photons that can be controlled to retrieve early or late. Rapid modulation frequencies up to 50 MHz can be obtained. Our experimental observations can be well explained via numerical simulations based on an improved superatom model accounting for the dipole-dipole interactions in a Rydberg EIT medium. Our work provides a way to manipulate the stored photons by employing the microwave field, which is significant for developing quantum technologies.
Photons are good candidates for carrying quantum information because they are very stable particles: they interact weakly with the medium and barely with each other. However, this has drawbacks when you want to process the information because, in this case, it is preferable to have photon-photon interactions. For example, for applications in quantum repeaters, such interactions would allow deterministic Bell state measurements, increasing the entanglement distribution rate between two remote nodes. Getting two photons to interact with each other efficiently requires mapping them into a nonlinear medium at the single-photon level, that is, a medium that reacts differently when it interacts with a single photon than when it does with two. Such strong nonlinearity has been demonstrated with Rydberg atoms, which are atoms excited to a state with a high principal quantum number.
In this thesis we have performed nonlinear quantum optics experiments using an ensemble of cold Rydberg atoms, where we have studied the properties of the quantum light emitted by these atoms. First, we demonstrated nonlinearities at the single-photon level. To reach this stage, we made several improvements to the previous experimental setup available in the group, of which the implementation of a dipole trap was especially relevant. We evidence quantum nonlinearity by measuring photon antibunching for the transmitted light after interacting with the Rydberg state under electromagnetically induced transparency (EIT). We also showed the generation of single photons on-demand after storing weak coherent states of light pulses as collective Rydberg excitations.
Then, we studied the variation of the light statistic throughout the output pulse after propagating through the medium as Rydberg polaritons, which are superposition states of light and Rydberg excitations. We showed that the properties at the beginning and the end of the pulse were different from those of the steady state. In particular, the light detected after the input pulse was abruptly turned off gave much stronger suppression of two-photon events. Then, we investigated how to exploit this effect to generate single photons on demand. To do this, we analyzed the quality of the single photons detected at the end of the pulse as a function of the detection probability and compared the results with those obtained by storing the input pulse as collective Rydberg excitations. We showed that the photons were generated more efficiently when increasing the detection window at the cost of deteriorating the single photons statistics.
Finally, we investigated the indistinguishability of the photons emitted by our Rydberg atomic ensemble, a crucial property for using Rydberg atoms as nodes in quantum networks. We also compared the single photons generated after storage under EIT conditions with those obtained using a two-photon Raman excitation to the Rydberg state. We measured the indistinguishability by making them interfere with weak coherent states of light in a Hong-Ou-Mandel experiment. And we showed that, although we obtained better photon statistics under EIT conditions, the indistinguishability from those obtained with Raman excitation was significantly higher.