The combination of electromagnetically induced transparency with the nonlinear interaction between Rydberg atoms provides an effective interaction between photons. In this Letter, we investigate the storage of optical pulses as collective Rydberg atomic excitations in a cold atomic ensemble. By measuring the dynamics of the stored Rydberg polaritons, we experimentally demonstrate that storing a probe pulse as Rydberg polaritons strongly enhances the Rydberg mediated interaction compared to the slow propagation case. We show that the process is characterized by two time scales. At short storage times, we observe a strong enhancement of the interaction due to the reduction of the Rydberg polariton group velocity down to 0. For longer storage times, we observe a further, weaker enhancement dominated by Rydberg induced dephasing of the multiparticle components of the state. In this regime, we observe a nonlinear dependence of the Rydberg polariton coherence time with the input photon number. Our results have direct consequences in Rydberg quantum optics and may enable the test of new theories of strongly interacting Rydberg systems.
The combination of electromagnetically induced transparency with the nonlinear interaction between Rydberg atoms provides an effective interaction between photons. In this Letter, we investigate the storage of optical pulses as collective Rydberg atomic excitations in a cold atomic ensemble. By measuring the dynamics of the stored Rydberg polaritons, we experimentally demonstrate that storing a probe pulse as Rydberg polaritons strongly enhances the Rydberg mediated interaction compared to the slow propagation case. We show that the process is characterized by two time scales. At short storage times, we observe a strong enhancement of the interaction due to the reduction of the Rydberg polariton group velocity down to 0. For longer storage times, we observe a further, weaker enhancement dominated by Rydberg induced dephasing of the multiparticle components of the state. In this regime, we observe a nonlinear dependence of the Rydberg polariton coherence time with the input photon number. Our results have direct consequences in Rydberg quantum optics and may enable the test of new theories of strongly interacting Rydberg systems.
The combination of electromagnetically induced transparency with the nonlinear interaction between Rydberg atoms provides an effective interaction between photons. In this Letter, we investigate the storage of optical pulses as collective Rydberg atomic excitations in a cold atomic ensemble. By measuring the dynamics of the stored Rydberg polaritons, we experimentally demonstrate that storing a probe pulse as Rydberg polaritons strongly enhances the Rydberg mediated interaction compared to the slow propagation case. We show that the process is characterized by two time scales. At short storage times, we observe a strong enhancement of the interaction due to the reduction of the Rydberg polariton group velocity down to 0. For longer storage times, we observe a further, weaker enhancement dominated by Rydberg induced dephasing of the multiparticle components of the state. In this regime, we observe a nonlinear dependence of the Rydberg polariton coherence time with the input photon number. Our results have direct consequences in Rydberg quantum optics and may enable the test of new theories of strongly interacting Rydberg systems.
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.
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.
Mapping the strong interaction between Rydberg excitations in ultra-cold atomic ensembles onto single photons enables the realization of optical nonlinearities which can modify light on the level of individual photons.This novel approach forms the basis of a growing Rydberg quantum optics toolbox, which already contains photonic logic building-blocks such as single-photon sources, switches, transistors, and conditional π-phase shifts.This thesis reports on two experiments investigating strong photon-photon interactions mediated by Rydberg interactions.First, applying a ladder-type EIT scheme to Rydberg atoms in D-states with high principal quantum number results in the Rydberg interaction mediated nonlinearity accompanied by a time-dependent decay of transmission of probe light through the medium.In a joint experimental and theoretical analysis, this effect is attributed to the dephasing of propagating polaritons into stationary Rydberg excitations caused by the state mixing interaction occurring with Rydberg D-state atoms.Second, via a two-photon Raman excitation scheme Rydberg atoms are efficiently excited in a small atomic cloud.As the size of the Rydberg blockade exceeds the dimension of the medium, only a single Rydberg excitation can be present at a time.By fast engineered dephasing this Rydberg excitation is decoupled from the light field.Measurement of the transmitted light and the amount of excited Rydberg atoms gives evidence for the realization of a deterministic single-photon absorber.
Interacting Rydberg excitations in cold atomic ensembles can exhibit large quantum nonlinearities that enable engineering of strong interactions between individual photons. Consequently, Rydberg ensembles are a promising platform for quantum information applications and the study of more fundamental physics of few- and many-body phenomena with interacting photons. This thesis presents a series of experiments that study and exploit different regimes of Rydberg-mediated interactions. We report the realization of an efficient on-demand single-photon source. The strong long-range Rydberg interactions allow the excitation of only a single collective Rydberg state within the entire atomic medium. The collective excitation can be subsequently retrieved as a single-photon. We use this scheme to generate highly pure and highly indistinguishable photons, which are suitable for scalable quantum information applications. These photons can be compatible with other atomic systems due to their narrow bandwidth, which makes building practical hybrid quantum systems feasible. Here, we demonstrate high visibility two-photon quantum interference between our Rydberg-produced photons, and photons emitted by a remote single-trapped ion. We also study Rydberg atoms under electromagnetically induced transparency conditions, where coherent superpostions of photons and Rydberg excitations propagate through the atomic medium as lossless dark-state polaritons. In the experiment, we use the external control fields to tune the interactions to a many-body regime where we can observe resonant scattering of dark-state polaritons to lossy channels. We show that the enhanced scattering process arises as a pure three-body effect.
Nonlinear optics at the single-photon level enables deterministic photonphoton interaction, a long-standing goal in quantum photonics science. Besides its implication in fundamental aspects of physics, this would unlock several applications in quantum information science. A relevant example are deterministic Bell state measurements, which find a prominent application in long-distance quantum communications using quantum repeater architecture. Toward this goal, we built a highly nonlinear system based on Rydberg excited states of a cold atomic ensemble and electromagnetically induced transparency (EIT). Our work first focuses on the storage of weak coherent light pulses in the atomic ensemble analysing the resulting nonlinear response of the medium. Then we demonstrate storage and retrieval of a paired single photon by coupling the Rydberg ensemble with a second remote cold atomic based quantum memory. Our set-up is based on magneto-optically trapped 87Rb atoms cooled down to temperature of 50 to 100 μK, with a cloud density of 10 ^(10) cm^(-3). Using EIT, light pulses are slowed down, stored as Rydberg collective atomic excitation and retrieved. We characterize EIT, slow-light and light-storage on a variety of Rydberg states, from the 26S up to the 80S. Depending on the state, the typical storage efficiency observed is of the order of few percents at a storage time of ~200 ns while the typical 1/e coherence time is of ~2us. By studying the optical response at different mean probe photon number, we have examined the Rydberg induced nonlinearity in EIT, slow-light and stored-light case for different Rydberg states. In particular, we have measured the nonlinear Rydberg induced dephasing of a stored collective Rydiberg state. For few microseconds storage time we have measured nonlinear response at the order of tens of photons. Our results show that light-storage enhances the nonlinear response of the atomic ensemble when compared to the slow light case, this possibly facilitating photonic quantum information processing using Rydberg excited atoms. Finally, we have used a separated cold atomic quantum memory to generate pair of correlated single photons. One photon of the pair is stored as single collective Rydberg atomic excitation and we show that non-classical correlations between the two photons and the single-photon statistic persist after the retrieval from the Rydberg ensemble. This result marks an important step towards deterministic photon-photon interactions. At the same time, linking a quantum memory with a highly nonlinear system may enable deterministic distribution of entanglement over long-distance using quantum repeaters. Un dels objectius que la fotònica quàntica persegueix des de temps ençà és la interacció determinista entre fotons, fenomen que podria assolir-se gràcies als avanços en l'òptica no-lineal a nivell de fotó únic. Aquests avanços, a més de les seves implicacions en la física fonamental, possibilitarien diferents aplicacions de la informació quàntica. A mode d'exemple, es podrien portar a terme mesures d'estat de Bell deterministes que facilitessin un funcionament eficient de l'arquitectura dels repetidors quàntics. En aquest projecte, i seguint aquesta línia, hem construït un sistema altament no-lineal basat en estats excitats de Rydberg d'un gas d'àtoms freds i hem aplicat el fenomen de la transparència induïda electromagnèticament (EIT). En un primer moment, ens hem focalitzat en l'emmagatzematge de polsos febles de llum coherent dins el núvol atòmic i en l'anàlisi de la resposta no-lineal del medi. Després, hem demostrat el emmagatzematge i la posterior recuperació d'un fotó emparellat (que forma part d'una parella de fotons correlacionat) gràcies a l'acoblament del nostre sistema d'àtoms Rydberg amb una altra memòria quàntica basada en conjunt diferenciat d'àtoms freds. El nostre dispositiu es compon d'àtoms de 87Rb dins una trampa magneto-òptica que han estat refredats a una temperatura de 50uK amb una densitat de 10^(10) cm^(-3). Utilitzant EIT, alentim els polsos de llum, els emmagatzemem en forma d'excitacions col·lectives a nivells de Rydberg i, posteriorment, els recuperem. Després, caracteritzem la llum lenta i l'emmagatzematge de fotons. Per fer-ho, utilitzem diferents estats de Rydberg: des de el 26S fins al 80S. En funció de l'estat, l'eficiència d'emmagatzematge observada és típicament de l'ordre de 2-8% en un temps ~200 ns, mentre que el temps de coherència 1/e típic és de 2us. A través de l'estudi de la resposta òptica a diferents escales de fotons, hem examinat la no-linealitat induïda per les interaccions dipolo-dipolo entre els estats de Rydberg a l'EIT i quins són els seus efectes, tant en la llum lenta, com en l'emmagatzematge. En concret, hem mesurat el desfasament quàntic induït per les interaccions dipolo-dipolo en els estats col·lectius de Rydberg. Al emmagatzemar-los durant uns microsegons, hem pogut mesurar les respostes no-lineals al nivell de desenes de fotons. Els nostres resultats demostren que l'emmagatzematge de la llum augmenta la resposta no-lineal del sistema atòmic respecte a la mera propagació a través del núvol d'àtoms facilitant, d'aquesta forma, el processament de la informació quàntica amb l'ús d'àtoms de Rydberg excitats. Per últim,hem acoblat el nostre sistema amb una altra memòria quàntica basada en àtoms freds que és capaç de generar parells correlacionats de fotons únics. Així, hem pogut demostrar que tant les correlacions no-clàssiques entre els fotons de la parella, com l'estadística que caracteritza el fotó únic, persisteixen després de l'emmagatzematge i la posterior recuperació d'un dels fotons de la parella dins del conjunt d'àtoms en forma d'excitació col·lectiva de Rydberg. Aquest resultat és un pas important cap a la demostració de les interaccions deterministes entre fotons. A la vegada, el fet d'acoblar una memòria quàntica amb un sistema altament no-lineal podria facilitar la distribució determinista d'entrellaçament a llargues distàncies amb l'ús de repetidors quàntics.