Long-distance transfer of quantum information is an essential ability for many applications of quantum science. A natural choice to distribute quantum information is to encode it into photons and transfer it through optical fibers. However, due to the unavoidable transmission losses present in every communication channel, the distances for efficient quantum communication via direct-state transfer are limited to a few hundred kilometers. To overcome this limitation, the use of quantum repeaters has been suggested. A quantum repeater protocol aims to establish entanglement (i.e., quantum correlation) between remote nodes by first generating entanglement over shorter distance pieces, storing it in quantum memories, and finally extending it to the whole distance using entanglement swapping. The main goal of this thesis is to design quantum repeater architectures using single solid-state quantum emitters and to develop the two-qubit gates required for performing entanglement swapping. We first explain the basic ideas of quantum repeaters and introduce potential material candidates, single erbium (168Er) and europium (151Eu) ions doped yttrium orthosilicate photonic crystals. Next, we propose a quantum repeater scheme combing erbium and europium ions to generate and distribute entanglement over long distances. We study the entanglement generation rate of the protocol and compare it with the rate of a well-known ensemble-based quantum repeater. Then, using cavity assisted interactions, we propose three different schemes to perform high fidelity two-qubit gates between single quantum systems. We quantify their expected performance in detail by taking into account many realistic imperfections and compare their strengths and weaknesses. The ability to perform local two-qubits gates is especially crucial in terms of distributing entanglement. Finally, based on our gained knowledge through these projects, we propose our second quantum repeater architecture based on erbium (167Er) ions, which outperforms the first scheme. We study two possibilities for distributing entanglement and calculate the overall fidelity as well as the distribution rate of the protocol.
Long-distance transfer of quantum information is an essential ability for many applications of quantum science. A natural choice to distribute quantum information is to encode it into photons and transfer it through optical fibers. However, due to the unavoidable transmission losses present in every communication channel, the distances for efficient quantum communication via direct-state transfer are limited to a few hundred kilometers. To overcome this limitation, the use of quantum repeaters has been suggested. A quantum repeater protocol aims to establish entanglement (i.e., quantum correlation) between remote nodes by first generating entanglement over shorter distance pieces, storing it in quantum memories, and finally extending it to the whole distance using entanglement swapping. The main goal of this thesis is to design quantum repeater architectures using single solid-state quantum emitters and to develop the two-qubit gates required for performing entanglement swapping. We first explain the basic ideas of quantum repeaters and introduce potential material candidates, single erbium (168Er) and europium (151Eu) ions doped yttrium orthosilicate photonic crystals. Next, we propose a quantum repeater scheme combing erbium and europium ions to generate and distribute entanglement over long distances. We study the entanglement generation rate of the protocol and compare it with the rate of a well-known ensemble-based quantum repeater. Then, using cavity assisted interactions, we propose three different schemes to perform high fidelity two-qubit gates between single quantum systems. We quantify their expected performance in detail by taking into account many realistic imperfections and compare their strengths and weaknesses. The ability to perform local two-qubits gates is especially crucial in terms of distributing entanglement. Finally, based on our gained knowledge through these projects, we propose our second quantum repeater architecture based on erbium (167Er) ions, which outperforms the first scheme. We study two possibilities for distributing entanglement and calculate the overall fidelity as well as the distribution rate of the protocol.
Abstract We design a quantum repeater architecture using individual 167 Er ions doped into Y 2 SiO 5 crystal. This ion is a promising candidate for a repeater protocol because of its long hyperfine coherence time in addition to its ability to emit photons within the telecommunication wavelength range. To distribute entanglement over a long distance, we propose two different swapping gates between nearby ions using the exchange of virtual cavity photons and the electric dipole–dipole interaction. We analyze their expected performance, and discuss their strengths and weaknesses. Then, we show that a post-selection approach can be implemented to improve the gate fidelity of the virtual photon exchange scheme by monitoring cavity emission. Finally, we use our results for the swapping gates to estimate the end-to-end fidelity and distribution rate for the protocol.
"Quantum communication is the art of transferring a quantum state from one place to another". The capability of distributing quantum states allows, for instance, provably secure communication and, in the near future, will enable the interconnection of quantum computers to establish a quantum internet. A convenient way to send quantum states from one place to another is to encode them into photons and distribute them using optical bres. The distance over which this can be achieved is still limited to a few hundred of kilometers. However, it can be extended in principle to arbitrary long distances by means of quantum repeaters. The realization of such quantum repeaters will require the engineering and interfacing of heterogeneous elements such as entangled photon-pair sources, quantum memories and entanglement swapping operations. The main goal of this thesis is to develop and study the compatibility of the main elements required to build a frequency multiplexed quantum repeater. This architecture requires a frequency multimode entangled photon pair source which, for example, could be implemented exploiting the process that involves the interaction of a strong light pulse with a non-linear crystal. Such a process is versatile and allows easy tailoring of the photon-pair spectral properties in order to match the requirements of the frequency multiplexed quantum repeater architecture. First, as a precursor of such a photon pair source, we developed a novel heralded single photon source that involves spectrally resolved single photon detection and on-demand frequency shifting. These capabilities are also essential tools in a frequency multiplexed quantum repeater. Next, we demonstrated the compatibility of such a sources with frequency multimode photonic quantum memories by performing spectrally multiplexed storage of single photons. We also tested the capability of high- delity storage of photons encoding quantum information in their polarization degree of freedom. Finally, we showed that entanglement swapping operations, i.e. Bell-state measurements, can be performed using photon pair sources compatible with quantum memories, quantum repeater architectures, and real-world scenarios. While a lot of e ort remains to be done, in the work presented in this thesis we demonstrated that all the key elements required to build a frequency multiplexed quantum repeater can be built and eventually be interfaced. The main obstacles towards that goal and possible ways to overcome them are also discussed.
Abstract Quantum repeaters are proposed to overcome exponential photon loss over distance in fibers. One-way quantum repeaters eliminate the need for two-way classical communications, which can potentially outperform quantum-memory-based quantum repeaters. I propose that rare-earth ions doped in solids and coupled with nanophotonic cavity can be used to generate photonic cluster state efficiently, which serve as good platforms for one-way quantum repeater nodes. In addition, I propose a multiplexed scheme of photonic tree cluster state generation with multiple quantum emitters. With less than 100 quantum emitters per repeater node, secret key rates can reach the order of 100 kHz for a communication distance of one thousand kilometers, with the assumption of fast and high-fidelity atom–atom and photon–atom gate operations, as well as high photon collection and detection efficiencies. This proposal is especially useful for generating large-scale photonic cluster state, which is essential for cluster state-based quantum repeater schemes and measurement-based quantum computing.
As the reach of quantum technologies extends ever further in communication and information science, a reliable way of transferring quantum information between distant locations becomes ever more crucial. While photons are widely accepted as excellent carriers due to their speed and low decoherence, losses of transmission (in free space or fibre) and the impossibility of cloning quantum information still pose a great challenge. The quantum repeater architecture was suggested as a solution to both problems [1]. In a quantum repeater the information encoded in an input state is transferred to a new one through entanglement swapping, that is then sent on along the channel. In this work we present our advances towards the realisation of a quantum repeater. Our system of choice combines a solid-state quantum memory with a source of photon pairs. The memory is based on a Rare-Earth Doped crystal, where quantum information can be stored in Pr3+ ions as a collective excitation using the Atomic Frequency Comb technique. On demand retrieval of the information is realised by transferring the excitation to a long-lived spin state. Record values of storage times and retrieval efficiencies have been demonstrated in this system [2]. Entangled pairs of single photons are generated by parametric down conversion in a periodically poled crystal placed inside a bow-tie cavity. This allows us to generate narrow band photons pairs, where the signal is spectrally matched to the memory (606nm), while the idler is in the telecom band [3]. Such a configuration allows us to benefit from the high performance of the memory, that also allows for temporal [2] and frequency [4] multimodality, while at the same time overcoming the high optical losses of 606nm photons by pair generation of a telecom photon. The first stepping stone, progress towards which is presented in this work, is the successful demonstration of energy-time entanglement between the telecom idler photon and the signal photon, stored as spin-wave excitation. The entanglement of the original pair is maintained by the memory temporal multimodality. The entanglement analysis will be made through time-bin qubit analysers made of a fibre-based Mach-Zehnder interferometer, for the former, and a solid-state equivalent based on two AFC with different storage times, for the latter [5]. In this direction we have already doubled the efficiency of the AFC storage protocols, that will be beneficial to count rates and signal-to-noise ratio. With respect to [2], we also increased the spectral-matching between the source and the memory [4]. Our experiment will provide an increase in storage time of 3 orders of magnitude with respect to previous demonstrations, as well as introducing for the first time on-demand read-out in a highly multi-mode memory. Demonstration of the successful transfer of quantum information between the signal photon and the long-lived solid-state excitation will open the way to the demonstration of long-distance entanglement between individual nodes in a quantum network.
Future quantum networks will enable fascinating applications such as secure communication, quantum-enhanced distributed sensing, distributed quantum computing, and accurate global timekeeping. Center to these applications is quantum entanglement distribution, whose effective distance is limited by photon loss. Quantum repeaters were proposed to extend the effective distance, but their demonstration has not been reported yet. In this thesis, we study quantum repeaters with both single-emitter based and ensemble-based quantum memories. In particular, we study the feasibility of meaningful proof-of-principle demonstrations of several quantum repeater protocols (with two-links) with photon (single-photon and photon-pair) sources and atomic-ensemble based quantum memories. We take into account non-unit memory efficiencies that decay exponentially with time, which is more realistic and accurate compared to previous treatments. We discuss implementations based on quantum dots, parametric down-conversion, rare-earth-ion doped crystals, and Rydberg atoms. Our results provide guidance for the near-term implementation of long-distance quantum repeater demonstrations, suggesting that such demonstrations are within reach of current technology.
Abstract Efficient long-distance entanglement distribution is a key requirement for a global quantum internet. Quantum repeaters overcome photon loss in optical fibers, offering a route to long-distance entanglement distribution. Here, we study heralded quantum repeater architectures that are based on two-photon interference and integrate local or remote swapping. We derive an analytical expression for the entanglement distribution time as a function of transmission distance for both heralded local-swapping and remote-swapping schemes. This expression provides physical insight into how repeaters overcome channel loss in long-haul fibers. Considering a realistic quantum memory, we present theoretical curves showing the effect of finite memory lifetime on the entanglement distribution time. By comparing optimal distribution times over distances up to thousands of kilometers, we determine the optimal nesting architectures for the single-repeater channel in each scheme and compare the optimal distribution rates and entanglement fidelities of these schemes at various distances. Finally, by examining the limiting cases of zero channel loss, zero device loss, and lossless repeater systems, we find fundamental performance bounds for quantum repeaters. These results provide practical design principles for scalable quantum repeater networks and pave the way toward continent-scale quantum communication.