We demonstrate the storage and retrieval of an on-demand single photon generated by a collective Rydberg excitation in a low-noise Raman quantum memory located in a different cold atomic ensemble. We generate single photons on demand by exciting a cold cloud of rubidium atoms off resonantly to a Rydberg state, with a generation probability up to 15% per trial. We then show that the single photons can be stored and retrieved with a storage-and-retrieval efficiency of 21% and a low noise floor in the Raman quantum memory. This leads to a signal-to-noise ratio ranging from 11 to 26 for the retrieved single photon, depending on the input-photon-generation probability, which allows us to observe significant antibunching. We also evaluate the performance of the Raman memory as a built-in unbalanced temporal beam splitter, tunable by varying the write-in control-pulse intensity. In addition, we demonstrate that the Raman memory can be used to control the single-photon wave shape. These results are a step forward in the implementation of efficient quantum repeater links using single-photon sources.
We demonstrate the storage and retrieval of an on-demand single photon generated by a collective Rydberg excitation in a low-noise Raman quantum memory located in a different cold atomic ensemble. We generate single photons on demand by exciting a cold cloud of rubidium atoms off resonantly to a Rydberg state, with a generation probability up to 15% per trial. We then show that the single photons can be stored and retrieved with a storage-and-retrieval efficiency of 21% and a low noise floor in the Raman quantum memory. This leads to a signal-to-noise ratio ranging from 11 to 26 for the retrieved single photon, depending on the input-photon-generation probability, which allows us to observe significant antibunching. We also evaluate the performance of the Raman memory as a built-in unbalanced temporal beam splitter, tunable by varying the write-in control-pulse intensity. In addition, we demonstrate that the Raman memory can be used to control the single-photon wave shape. These results are a step forward in the implementation of efficient quantum repeater links using single-photon sources.
We demonstrate the storage and retrieval of an on-demand single photon generated by a collective Rydberg excitation in an ultra-low noise Raman quantum memory located in a different cold atomic ensemble. We generate single photons on demand by exciting a cold cloud of Rubidium atoms off resonantly to a Rydberg state, with a generation probability up to 15 % per trial. We then show that the single photons can be stored and retrieved with an efficiency of 21 % and a noise floor of pn=2.3(3)×10−4 per trial in the Raman quantum memory. This leads to a signal-to-noise ratio ranging from 11 to 26 for the retrieved single photon depending on the input photon generation probability, which allows us to observe significant antibunching. We also evaluate the performances of the Raman memory as built-in unbalanced temporal beam splitter, tunable by varying the write-in control pulse intensity. In addition, we demonstrate that the Raman memory can be used to control the single-photon waveshape. These results are a step forward in the implementation of efficient quantum-repeater links using single-photon sources.
Quantum memories are key components in quantum information networks. Their ability to store and retrieve information on demand makes repeat-until-success strategies scalable. Warm alkali-metal vapours are interesting candidates for the implementation of such memories, thanks to their long storage times and experimental simplicity. Operation with the Raman protocol enables high time-bandwidth products, which allows for multiple synchronisation trials of probabilistically operating quantum gates via memory-based temporal multiplexing. This makes the Raman memory a promising tool, whose broad spectral bandwidth facilitates direct interfacing with other photonic primitives, such as single photon sources. Here, such a light-matter interface is implemented in a warm caesium vapour. Firstly, we study the storage of polarisation-encoded information in the memory. High quality polarisation preservation for bright coherent state input signals can be achieved, when operating the Raman memory in a dual-rail configuration inside a polarisation interferometer. Secondly, heralded single photons are stored in the memory. To this end, the memory is operated on-demand by feed-forward of source heralding events, which is a key technological capability. Prior to storage, single photons are produced in a spontaneous parametric down conversion source, whose bespoke design spectrally tailors the photons to the memory acceptance line. The faithful retrieval of stored single photons is found to be currently limited by noise in the memory, with a signal-to-noise ratio of 0.3 in the memory output. Nevertheless, a clear influence of the input's quantum nature is observed in the retrieved light by measuring signal's photon statistics. Finally, the memory noise processes are examined in detail. Four-wave-mixing noise is determined as the sole important noise source for the Raman memory.
Summary form only given. Strong interaction between two single photons [1], is a long standing and important goal in quantum photonics. This would enable a new regime of nonlinear optics and unlock several applications in quantum information science, including photon-photon gates and deterministic Bell-state measurements for quantum networking. In the context of quantum networks [2], a particularly important case is to achieve strong interaction between single photons from independent photon pairs that can be stored in quantum memories. So far, most of the efforts towards nonlinear effects at the single-photon level have been realized using weak classical input light.In this contribution, we demonstrate the storage and retrieval of a paired single photon emitted by an ensemble quantum memory in a strongly nonlinear medium based on highly excited Rydberg atomic excitations [3,4]. We demonstrate that non-classical correlations between the two photons persist after the retrieval from the Rydberg ensemble. We show that our Rydberg cold atomic ensemble displays optical nonlinearity with input pulses containing few tens of photons. Our results [5], mark an important step towards deterministic photonphoton interactions, and may enable deterministic Bell-state measurement with multimode quantum memories.
Single photons are the best carriers of quantum information for long-distance transmission. Nevertheless, maximal achievable distance is limited by the exponential decay of photons as a function of link length. The protocol of quantum repeater provides a promising solution by replacing direction transmission with segmented entanglement distribution and entanglement connection via swapping. The quantum repeater necessitates a key element of quantum memory for making efficient interconnections. An atomic ensemble is very suitable for this purpose due to the collective enhanced interaction. Single photons are stored as collective excitations in an atomic ensemble. Thus a comprehensive study of the physics relating to collective excitations is crucially important for improving the quantum memory performance and its reachable applications in quantum repeater and quantum network. In this article, we review our experimental work on cold atomic ensembles in recent years, focusing on the coherent manipulation of collective excitations. We first briefly introduce the general concept of collective excitations and the preparation process through spontaneous Raman scattering, and we review our experimental work on extending the coherence time, such as suppressing motional dephasing by increasing the spin-wave wavelength, by confining atoms with a three-dimensional optical lattice. Afterwards, we discuss about the retrieval process of collective excitations and review our experiments on using a ring-cavity enhanced setup to improve the retrieval efficiency. The coherent qubit operation in a quantum memory is very useful for enabling new functionalities for a quantum network, in a subsequent section, we thus review our work on developing Raman-based coherent operations for single excitations. Afterwards, we mention our experiments on creating a pair of atom-photon entanglement by interfering two modes of a collective excitation. Improving the entanglement preparation efficiency is crucially important, and Rydberg-based interaction provides a promising solution. Our experimental work in this direction is also reviewed. Additionally, as an application in coherent manipulation with collective excitations, we show several experiments on using excitations in remote atomic memories and demonstrating basic functionality of quantum repeater and quantum network. In short, significant progress has been made in the coherent manipulation of single collective excitations in cold atomic ensembles, and further improvement will be accelerated by the Rydberg-enabled interactions; practical applications in quantum repeater and quantum network is foreseeable in the near future.
Room-temperature, easy-to-operate quantum memories are essential building blocks for future long distance quantum information networks operating on an intercontinental scale, because devices like quantum repeaters, based on quantum memories, will have to be deployed in potentially remote, inaccessible locations. Here we demonstrate controllable, broadband and efficient storage and retrieval of weak coherent light pulses at the single-photon level in warm atomic cesium vapor using the robust far off-resonant Raman memory scheme. We show that the unconditional noise floor of this technically simple quantum memory is low enough to operate in the quantum regime, even in a room-temperature environment.
(English) Photons have emerged as the main candidates for carrying quantum information due to their weak interaction with the environment. Unfortunately, their limited interaction with one another poses challenges for photonic quantum information processing. One of the possible solutions lies in the unique behavior of interacting Rydberg excitations in cold atomic ensembles, where strong nonlinearities enable engineering interactions among individual photons. This phenomenon makes Rydberg ensembles a promising platform for quantum information applications, notably in long-distance quantum communication. This thesis presents a series of experiments that explore and exploit Rydberg-mediated interactions, all with the long-term objective of building an efficient quantum repeater. The thesis begins with a concise theory overview of Rydberg and ensemble physics. This is followed by an explanation of the experimental setup. I discuss how, building upon a previously existing setup, we improved the stability and spectral properties of our laser system, along with enhancing the quality of the atomic ensemble. The introductory section of the thesis concludes with a description of two different single-photon generation methods and an in-depth review of various decoherence mechanisms impacting Rydberg ensemble excitations. The single-photon generation performance has been improved by the modifications implemented in the setup, resulting in higher generation rates and better single-photon purity. Supported by experimental data and a careful analysis of experimental parameters, we identify the most probable sources of significant decoherence and suggest potential strategies for mitigation. In our initial experiment, we achieve the storage and subsequent retrieval of an on-demand single photon. This photon is generated through the collective excitation of Rydberg states in one cold atomic ensemble, and it is stored in a low-noise Raman quantum memory situated in another cold atomic ensemble. Our results show the capability to store and retrieve these single photons while maintaining a high signal-to-noise ratio of up to 26 and preserving strong antibunching characteristics. We also explore the built-in temporal beam splitting capabilities of the Raman memory and successfully use the memory to control the single photon waveshape. In the second experiment, we demonstrate for the first time an interaction and storage of single photons in a highly non-linear medium based on cold Rydberg atoms. We employ the DLCZ protocol in a cold atomic ensemble to create single photons, guiding them to another ensemble for storage in a highly excited Rydberg state under conditions of electromagnetically induced transparency. By studying the statistics of the light retrieved from the Rydberg atoms, we show for the first time single-photon filtering with non-classical input light. Moreover, through Monte Carlo simulation, we get an intuitive understanding of the effect of the (partial) Rydberg blockade upon the Fock state distribution of arbitrary input light pulses. This allows us to conclude that the response of the medium is determined by the input Fock state distribution, what confirms the established understanding of Rydberg ensemble nonlinearity. This demonstration can be seen as a step towards realization of deterministic photon-photon gates based on Rydberg ensembles with single photon inputs. The results presented in this thesis affirm the potential of Rydberg ensembles to become central elements of future quantum networks, both as single photon sources and processing nodes. Furthermore, auxiliary outcomes provide an additional understanding of the Rydberg ensemble physics and offer insight into limitations that we need to overcome to improve further our setup. (Català) Els fotons han sorgit com els principals candidats per transportar informació quàntica a causa de la seva feble interacció amb el medi ambient. Malauradament, la seva interacció limitada entre ells planteja reptes per al processament de la informació quàntica fotònica. Una de les possibles solucions rau en el comportament únic de les excitacions de Rydberg interactuants en conjunts atòmics freds, on les fortes no linealitats permeten interaccions d'enginyeria entre fotons individuals. Aquest fenomen fa que els conjunts de Rydberg siguin una plataforma prometedora per a aplicacions d'informació quàntica, especialment en la comunicació quàntica de llarga distància. Aquesta tesi presenta una sèrie d'experiments que exploren i exploten les interaccions mediades per Rydberg, tots amb l'objectiu a llarg termini de construir un repetidor quàntic eficient. La tesi comença amb una visió general concisa de la teoria de Rydberg i la física de conjunts atòmics. Tot seguit s'explica la configuració experimental. Parlo de com, a partir d'una configuració existent anteriorment, hem millorat l'estabilitat i les propietats espectrals del nostre sistema làser, juntament amb la millora de la qualitat del conjunt atòmic. La secció introductòria de la tesi conclou amb una descripció de dos mètodes diferents de generació de fotons únics i una revisió en profunditat de diversos mecanismes de decoherència que afecten les excitacions del conjunt de Rydberg. El rendiment de la generació de fotons únics s'ha millorat amb les modificacions implementades a la configuració, donant lloc a taxes de generació més altes i una millor puresa dels fotons únics. Amb el suport de dades experimentals i una anàlisi acurada dels paràmetres experimentals, identifiquem les fonts més probables de decoherència significativa i proposem estratègies potencials per a la seua mitigació. En el nostre experiment inicial, aconseguim l'emmagatzematge i la posterior recuperació dels fotons únics sota demanda. Aquest fotó es genera mitjançant l'excitació col·lectiva dels estats de Rydberg en un conjunt atòmic fred i s'emmagatzema en una memòria quàntica de Raman de baix soroll situada en un altre conjunt atòmic fred. Els nostres resultats mostren la capacitat d'emmagatzemar i recuperar aquests fotons únics mantenint una alta relació senyal-soroll de fins a 26 i conservant fortes característiques antiagrupament. En el segon experiment, demostrem per primera vegada una interacció i emmagatzematge de fotons únics en un medi altament no lineal basat en àtoms freds de Rydberg. Utilitzem el protocol DLCZ en un conjunt atòmic fred per crear fotons únics, guiant-los a un altre conjunt per emmagatzemar-los en un estat de Rydberg molt excitat en condicions de transparència induïda electromagnèticament. En estudiar les estadístiques de la llum recuperada dels àtoms de Rydberg, mostrem per primera vegada el filtratge del fotó únic amb llum no clàssica d'entrada. A més, mitjançant la simulació de Monte Carlo, obtenim una comprensió intuïtiva de l'efecte del bloqueig (parcial) de Rydberg sobre la distribució d'estat de Fock dels polsos arbitraris de llum d'entrada. Això ens permet concloure que la resposta del medi està determinada per la distribució d'estat de Fock d'entrada, la qual cosa confirma la comprensió establerta de la no linealitat del conjunt de Rydberg. Els resultats presentats en aquesta tesi afirmen el potencial dels conjunts de Rydberg per convertir-se en elements centrals de futures xarxes quàntiques, tant com a fonts de fotons individuals com nodes de processament. A més, els resultats auxiliars proporcionen una comprensió addicional de la física del conjunt de Rydberg i ofereixen una visió de les limitacions que hem de superar per millorar encara més la nostra configuració.