Quantum memories for light are important devices in quantum information science, in particular for applications such as quantum networks and quantum repeaters. It has been predicted that multimode quantum memories able to store independently multiple modes would greatly help the scaling of quantum networks by decreasing the entanglement generation time between remote quantum nodes. Current research focuses mostly on time multiplexing in rare-earth doped crystals and in spatial multiplexing in atomic gases. Beyond these demonstrations, rare-earth doped crystals, thanks to their large inhomogeneous broadening, represent a unique quantum system which could also add another degree of freedom for multiplexing, the storage of multiple frequency modes. In this contribution, we report on the first demonstration of quantum storage of a frequency multiplexed single photon into a laser-written waveguide integrated in a Pr3+:Y2SiO5crystal. The great advantage of using a confined waveguide for this task is that the power required to prepare the quantum memory is strongly reduced due to the increased light-matter interaction in the waveguide. This enables simultaneous preparation of several memories at different frequencies, with a moderate laser power.
Quantum memories for light are important devices in quantum information science, in particular for applications such as quantum networks and quantum repeaters. It has been predicted that multimode quantum memories able to store independently multiple modes would greatly help the scaling of quantum networks by decreasing the entanglement generation time between remote quantum nodes. Current research focuses mostly on time multiplexing in rare-earth doped crystals and in spatial multiplexing in atomic gases. Beyond these demonstrations, rare-earth doped crystals, thanks to their large inhomogeneous broadening, represent a unique quantum system which could also add another degree of freedom for multiplexing, the storage of multiple frequency modes. In this contribution, we report on the first demonstration of quantum storage of a frequency multiplexed single photon into a laser-written waveguide integrated in a Pr3+:Y2SiO5crystal. The great advantage of using a confined waveguide for this task is that the power required to prepare the quantum memory is strongly reduced due to the increased light-matter interaction in the waveguide. This enables simultaneous preparation of several memories at different frequencies, with a moderate laser power.
Efficient and long-lived multimode quantum memories are crucial devices in the development of quantum technolgies. The reversible mapping of quantum states of light in rare earth doped crystals represents one of the most promising routes towards the realization of this goal. Such systems are also compatible with the miniaturization of quantum memories in integrated optics platforms, which offer unique features in terms of experimental scalability and enhanced light-matter interaction. Here, we fabricate single mode channel waveguides for 606 nm light in a praseodymium-doped yttrium orthosilicate crystal (Pr3+:Y2SiO5), that, thanks to its excellent coherence properties, is a widely studied material for light storage experiments. Waveguides are inscribed by femtosecond laser writing, adopting the so-called Type I configuration, where the core is directly obtained at the irradiated area. Remarkably, fabricating this kind of waveguides in crystals is a difficult task, as it requires to operate in a very narrow processing parameters window, if existing. We then use these waveguides for performing the storage and retrieval of single photons, implementing the atomic frequency comb protocol. We achieve a storage time of 5,5 µs, which is almost 2 orders of magnitude longer than previous realizations of quantum light storage in a waveguide. In addition, we investigate the potential information multiplexing capabilities of our system by performing the quantum storage of single photons delocalized over 14 different spectral modes. Our results show that laser written waveguides in rare earth-doped solid state systems are very promising for the development of efficient and long-lived multimode quantum memories.
We report on the quantum storage of a heralded frequency-multiplexed single photon in an integrated laser-written rare-earth doped waveguide. The single photon contains 15 discrete frequency modes separated by 261 MHz and spanning across 4 GHz. It is obtained from a nondegenerate photon pair created via cavity-enhanced spontaneous down-conversion, where the heralding photon is at telecom wavelength and the heralded photon is at 606 nm. The frequency-multimode photon is stored in a praseodymium-doped waveguide using the atomic frequency comb (AFC) scheme, by creating multiple combs within the inhomogeneous broadening of the crystal. Thanks to the intrinsic temporal multimodality of the AFC scheme, each spectral bin includes 9 temporal modes, such that the total number of stored modes is about 130. We demonstrate that the storage preserves the nonclassical properties of the single photon, and its normalized frequency spectrum.
Quantum memories (QMs) for light represent a fundamental ingredient for the development of a quantum internet. Among other applications, they are a building block for the distribution of entanglement on large scale, i.e. for the realization of a quantum repeater architecture. Rare earth doped crystals (REDCs) are a promising candidate towards this goal. In my thesis I use a Pr3+:Y2SiO5 crystal. The longest storage time and the highest retrieval efficiency for a solid-state memory measured so far, were demonstrated with this system (in the classical regime). However, the main advantages of solid-state platforms are their suitability for miniaturization and integration as well as their inhomogeneous broadening, which enables broadband storage and spectral multiplexing. In this thesis we demonstrate an on-demand solid-state QM for real single photons. Moreover we study new platforms for integrated QM based on the same material. We employ the atomic frequency comb (AFC) technique, which is the most promising storage protocol in terms of temporal multiplexing up to now. Until the start of my PhD there was still no demonstration of storage of a real quantum state of light with an on-demand readout in REDCs. We achieved this in the course of this thesis, measuring also for the first time (and only, at the time of writing) non-classical correlation between a single spin wave in a solid-state QM and a telecom photon. After proving the suitability of Pr3+:Y2SiO5 crystals for on-demand QMs, we demonstrated novel types of integrated optical memories based on the same system. We studied the spectroscopic and coherence properties of the ions in laser-written waveguides fabricated by fs-laser micromachining. These projects were developed in collaboration with Dr. R. Osellame and Dr. G. Corrielli at Politecnico di Milano, who fabricated the waveguides and analysed their guiding properties. In a first kind of waveguide, called type II, we performed the first storage with on-demand retrieval ever done in solid-state integrated optical memories (with classical light). We continued analysing a so-called type I waveguide, in which the mode-size is comparable with the mode guided in a single-mode fiber at the same wavelength. Here we showed storage of heralded single-photons for a pre-programmed time. The demonstrated storage time, 5.5 µs, is the longest quantum storage demonstrated in any integrated waveguide up to now. Finally, we performed in the same waveguide storage of the whole spectrum of a frequency-multiplexed heralded photon, spanning a range of frequencies of ˜ 4 GHz. The photon is naturally multiplexed due to the generation method used, namely cavity-enhanced SPDC. The possibility of storing such a broad spectrum comes from the intrinsic inhomogeneous broadening present in REDCs. Together with the 15 frequency modes constituting the multiplexed photon, 9 temporal modes were stored thanks to the intrinsic temporal multimodality of the AFC protocol. The method used to fabricate our waveguides, fs-laser micromachining, is the only one to our knowledge that allows for direct 3D fabrication in the substrate. In the future, this will yield matrices of fiber-pigtailed waveguide-based QMs, thus enabling a high degree of spatial multiplexing, which nowadays is mostly exploited in atomic clouds, where temporal and spectral multiplexing are more difficult to achieve. The crystal, the protocol and the waveguide fabrication technique employed in this thesis, represent all together a very promising system, opening the way for a future quantum repeater architecture based on scalable highly multiplexed QMs. Les memòries quàntiques (MQs) per a la llum constitueixen un ingredient fonamental per al desenvolupament d’un Internet quàntic. Entre altres aplicacions, són un element bàsic per a la distribució de l’entrellaçament a llargues distancies, és a dir, per a la realització d’un repetidor quàntic. Els cristalls dopats amb terres rares (REDC) són candidats prometedors cap a aquest objectiu. En la meva tesi uso el cristall Pr3+:Y2SiO5. Amb aquest sistema (en el règim clàssic) es va demostrar el temps d’emmagatzematge més llarg i la major eficiència d’una memòria d’estat sòlid. No obstant això, els principals avantatges de les plataformes en estat sòlid són la possibilitat de miniaturització i integració, així com la ampliació inhomogènia dels seus perfils d’absorció, que permet emmagatzemar fotons amb banda ampla o multiplexats en freqüència. En aquesta tesi demostrem una MQ d’estat sòlid amb lectura on-demand per a fotons únics reals. A més, estudiem noves plataformes per a MQs integrades basades en el mateix material. Utilitzem la tècnica de pinta de freqüència atòmica (AFC), que és el protocol d’emmagatzematge més prometedor per a multiplexació temporal fins ara. Al començament del meu doctorat no hi havia cap demostració d’emmagatzematge d’un real estat quàntic amb una lectura on-demand del fotó en REDC. Ho hem aconseguit en el curs d’aquesta tesi, mesurant també per primera vegada (i única, en el moment d’escriure), una correlació no-clàssica entre una única ona de spin en una MQ d’estat sòlid i un fotó de telecomunicacions. Després de demostrar la idoneïtat dels cristalls Pr3+:Y2SiO5 com MQs, vam demostrar nous tipus de memòries òptiques integrades basades en el mateix sistema. Vam estudiar les propietats espectroscòpiques i de coherència dels ions en guies d'ones escrites amb làser fabricades amb la tècnica del fs-làser micromachining. Aquests projectes van ser desenvolupats en col·laboració amb el Dr. R. Osellame i el Dr. G. Corrielli del Politècnic de Milà, que van fabricar les guies d'ones i van analitzar les seves propietats orientadores. En un primer tipus de guia d'ona, anomenada tipus II, vam realitzar el primer emmagatzematge amb lectura on-demand mai realitzada en memòries òptiques integrades en estat sòlid (amb llum clàssica). Després vam analitzar un altre tipus de guia d’ondes anomenada tipus I, en la qual la mida del mode és comparable amb el mode guiat en una fibra monomode a la mateixa longitud d’ona. Aquí vam mostrar l’emmagatzematge de fotons simples durant un temps preprogramat. El temps d’emmagatzematge demostrat, de 5.5 µs, és fins ara l'emmagatzematge quàntic més llarg demostrat en qualsevol guia d'ones integrada. Finalment, es va realitzar en la mateixa guia l’emmagatzematge de tot l'espectre d'un bi-fotó multiplexat en freqüència, abastant un rang de freqüències de ˜4 GHz. El fotó és multiplexat de forma natural gracies al mètode de generació utilitzat, és a dir, el SPDC millorat per cavitat. La possibilitat d’emmagatzemar un espectre tan ampli prové de l’ampliació intrínseca de l’absorció inhomogènia present en els REDC. Juntament amb els 15 modes de freqüència que constitueixen el fotó multiplexat, s'han emmagatzemat 9 modes temporals gràcies a la multimodalitat temporal intrínseca del protocol AFC. El mètode utilitzat per fabricar les nostres guies d'ona, fs-làser micromachining, és l'únic que coneixem que permet directament fabricar en 3D en el substrat. En el futur, això donarà matrius de MQs basades en guies d’onades integrades con fibres, que permetran un alt grau de multiplexació espacial, que avui en dia s’explota sobretot en núvols atòmics, on el multiplexatge temporal i espectral és més difícil d’aconseguir. El cristall, el protocol i la tècnica de fabricació de guies d'ona utilitzats en aquesta tesi, representen tots junts un sistema
Efficient and long-lived interfaces between light and matter are crucial for the development of quantum information technologies. Integrated photonic solutions for quantum storage devices offer improved performances due to light confinement and enable more complex and scalable designs. We demonstrate a novel platform for quantum light storage based on laser written waveguides. The new adopted writing regime allows us to attain waveguides with improved confining capabilities compared to previous demonstrations. We report the first demonstration of single-photon storage in laser written waveguides. While we achieve storage efficiencies comparable to those observed in massive samples, the power involved for the memory preparation is strongly reduced, by a factor 100, due to an enhancement of the light–matter interaction of almost one order of magnitude. Moreover, we demonstrate excited-state storage times 100 times longer than previous realizations with single photons in integrated quantum memories. Our system promises to effectively fulfill the requirements for efficient and scalable integrated quantum storage devices.
The faithful storage and coherent manipulation of quantum states with matter-systems would enable the realization of large-scale quantum networks based on quantum repeaters. To achieve useful communication rates, highly multimode quantum memories are required to construct a multiplexed quantum repeater. Here, we present a demonstration of on-demand storage of orbital-angular-momentum states with weak coherent pulses at the single-photon-level in a rare-earth-ion-doped crystal. Through the combination of this spatial degree-of-freedom (DOF) with temporal and spectral degrees of freedom, we create a multiple-DOF memory with high multimode capacity. This device can serve as a quantum mode converter with high fidelity, which is a fundamental requirement for the construction of a multiplexed quantum repeater. This device further enables essentially arbitrary spectral and temporal manipulations of spatial-qutrit-encoded photonic pulses in real time. Therefore, the developed quantum memory can serve as a building block for scalable photonic quantum information processing architectures.
The ability of storing and retrieving quantum states of light is an important experimental challenge in quantum information science. A powerful quantum memory for light is required in a quantum repeater, which would allow long distance (>500km) quantum communications. To be implemented in such application, the quantum memory must allow on-demand readout, with high fidelity and efficiency, and a long storage time. Additionally a multimode capacity (for temporal or spatial modes) would allow multiplexing. Our approaches focus on rare-earth doped crystals, i.e. solid state quantum memory. I present, in this work, our contributions for a solid-state quantum storage, with good performances in every criteria. In particular, I present the preservation of quantum entanglement during the storage, which paves the way for the implementation of quantum memories in quantum repeaters.