Due to the inevitable loss in communication channels, the distance of entanglement distribution is limited to approximately 100 km on the ground. Quantum repeater can circumvent this problem by utilizing quantum memory and entanglement swapping. As the elementary functional nodes for quantum repeater, the heralded generation of two-party entanglement between two remote nodes has only been realized with built-in-type quantum memories. These schemes suffer from the trade-off between multiplexing capacity and deterministic property and hence hinder their way to efficient quantum repeaters. Here we present the first experimental demonstration of functional quantum repeater nodes using absorptive quantum memories. We build two nodes separated by 3.5 m, each contains a polarization-entangled photon-pair source and a solid-state quantum memory. A joint Bell-state measurement in the middle station heralds the successful generation of maximally-entangled states between the two quantum memories with a fidelity of (80.4$\pm$2.1)%. The quantum memories used here are compatible with deterministic entanglement sources and can support multiplexing simultaneously, which paves the way to the construction of solid-state quantum repeaters and high-speed quantum networks.
Due to the inevitable loss in communication channels, the distance of entanglement distribution is limited to approximately 100 km on the ground. Quantum repeater can circumvent this problem by utilizing quantum memory and entanglement swapping. As the elementary functional nodes for quantum repeater, the heralded generation of two-party entanglement between two remote nodes has only been realized with built-in-type quantum memories. These schemes suffer from the trade-off between multiplexing capacity and deterministic property and hence hinder their way to efficient quantum repeaters. Here we present the first experimental demonstration of functional quantum repeater nodes using absorptive quantum memories. We build two nodes separated by 3.5 m, each contains a polarization-entangled photon-pair source and a solid-state quantum memory. A joint Bell-state measurement in the middle station heralds the successful generation of maximally-entangled states between the two quantum memories with a fidelity of (80.4±2.1)%. The quantum memories used here are compatible with deterministic entanglement sources and can support multiplexing simultaneously, which paves the way to the construction of solid-state quantum repeaters and high-speed quantum networks.
Due to the inevitable loss of single photon transmission in optical fiber, a quantum repeater scheme is needed to establish large-scale quantum networks. The existing elementary quantum repeater links are all based on emissive quantum memories, with entangled photons emitted by the memory itself. This architecture is difficult to support deterministic photon emission and multiplexing storage simultaneously, which fundamentally limits the rate of entanglement distribution. In this talk, I will present our recent work about the realization of heralded quantum entanglement between two absorptive quantum memories based on rare-earth-ion-doped crystals. This work confirmed the feasibility of constructing quantum repeater based on absorptive quantum memories and demonstrated the acceleration effect of multiplexing in quantum repeater for the first time, which lays a solid foundation for the construction of practical high-speed quantum networks.
Quantum networks, integrating quantum communication, quantum metrology, and distributed quantum computing, could provide secure and efficient information transfer, high-resolution sensing, and an exponential speed-up in information processing. Deterministic entanglement distribution over long distances is a prerequisite for scalable quantum networks, enabling the utilization of device-independent quantum key distribution (DI-QKD) and quantum teleportation to achieve secure and efficient information transfer. However, the exponential photon loss in optical fibres prohibits efficient and deterministic entanglement distribution. Quantum repeaters, incorporating entanglement swapping and entanglement purification with quantum memories, offer the most promising means to overcome this limitation in fibre-based quantum networks. Despite numerous pioneering efforts toward realizing quantum repeaters, a critical bottleneck remains, as remote memory-memory entanglement suffers from decoherence more rapidly than it can be established and purified over long distances. We overcome this by developing long-lived trapped-ion memories, an efficient telecom interface, and a high-visibility single-photon entanglement protocol. This allows us to establish and maintain memory-memory entanglement over a 10 km fibre within the average entanglement establishment time for the same distance. As a direct application, we demonstrate metropolitan-scale DI-QKD, distilling 1,917 secret keys out of 4.05*10^5 Bell pairs over 10 km. We further report a positive key rate over 101 km in the asymptotic limit, extending the achievable distance by more than two orders of magnitude. Our work provides a critical building block for quantum repeaters and marks an important step toward scalable quantum networks.
Quantum communication between two remote locations often involves remote parties sharing an entangled quantum state. At present, entanglement distribution is usually performed using photons transmitted through optical fibers. However, the absorption of light in the fiber limits the communication distances to less than 200 km, even for optimal photon telecom wavelengths. To increase this distance, the quantum repeater idea was proposed. In the quantum repeater architecture, one divides communication distance into segments of the order of the attenuation length of the photons and places quantum memory nodes at the intermediate locations. Since the photon loss between intermediate locations is low, it is possible then to establish entanglement between intermediate quantum memory nodes. Once entanglement between adjacent nodes is established, one can extend it over larger distances using entanglement swapping.\n\nThe long coherence time of a quantum memory is a crucial requirement for the quantum repeater protocol. It is obvious that the coherence time of a quantum memory should be much longer that the time it takes for light to travel between remote locations. For a communication distance l = 1000 km, the corresponding time is t = l/c = 3.3 ms. One can show that for a simple repeater protocol and realistic success probabilities of entanglement generation, the required coherence time should be on the order of many seconds, while for the more complicated protocols that involve multiplexing and several quantum memory cells per intermediate node, the required coherence time is on the order of milliseconds.\n\nIn this thesis, I describe a quantum memory based on an ensemble of rubidium atoms confined in a one-dimensional optical lattice. The use of the magnetically- insensitive clock transition and suppression of atomic motion allows us to increase coherence time of the quantum memory by two-orders of magnitude compared to previous work. I also propose a method for determining the Zeeman content of atomic samples. In addition, I demonstrate the observation of quantum evolution under continuous measurement. The long quantum memory lifetime demonstrated in this work opens the way for scalable processing of quantum information and long distance quantum communication.
We demonstrated entanglement distribution between two remote quantum nodes located 3 meters apart. This distribution involves the asynchronous preparation of two pairs of atomic memories and the coherent mapping of stored atomic states into light fields in an effective state of near-maximum polarization entanglement. Entanglement is verified by way of the measured violation of a Bell inequality, and it can be used for communication protocols such as quantum cryptography. The demonstrated quantum nodes and channels can be used as segments of a quantum repeater, providing an essential tool for robust long-distance quantum communication.
Quantum Key Distribution (QKD) has opened a new avenue for secure communication, by allowing one to distribute a random secret key between two users that are connected through a public channel without revealing information to unauthorized parties. If a message is encrypted with the one time pad (a well-known crytography algorithm) using secret keys created by QKD, then the ciphertext is information-theoretically secure and thus unbreakable for adversaries. Despite its unprecedented security, loss in the physical channel has prevented QKD from being used over distances beyond a few hundred kilometers. Fortunately, quantum repeaters have opened a path for long-distance QKD by providing a means to establish entanglement between distant users. The goal of this thesis has been to develop a source of entangled photon pairs that is suitable for quantum repeaters and then use it to test some of the fundamental building blocks of a quantum repeater : the heralded creation of entangled photons by means of entanglement swapping with properties that allow interfacing with optical quantum memories; the reversible mapping of quantum states from members of entangled photon pairs in and out of solid-state quantum memories; two-photon interference and a Bell state measurement with photons recalled from separate quantum memories. The demonstration of these key ingredients of a quantum repeater constitutes a significant step towards the establishment of entanglement over hundreds of kilometer distance, and hence long-distance QKD.
Over the last decade, the record distance for quantum communications remains in the range of 100 km due to losses and decoherence in the quantum channel. To achieve long distance, H.J. Briegel et al proposed a quantum repeater protocol, which combine quantum memory, entanglement purification and entanglement swapping [1]. Due to the lack of perfect quantum memory, the quantum repeater has not yet been implemented experimentally though much efforts have been made over many years. Here we present a solid-state repeater scheme using a quantum-dot spin in an optical microcavity [2,3]. This repeater has several advantages: (1) deterministic (if optimized); (2) complete and loss-resistant Bell-state analyzer with a built-in spin memory; (3) the storage time for spin memory could be significantly prolonged using proposed spin-echo techniques; (4) compatible with standard semiconductor processing techniques. [1] H. -J. Briegel et al, Phys. Rev. Lett. 81, 5932 (1998). [2] C.Y. Hu et al, Phys. Rev. B, 78, 085307 (2008); 78, 125316 (2008); 80, 205326 (2009). [3] C.Y. Hu and J.G. Rarity, ArXiv: quant-ph 1005.5545 (2010). [4] S. Reitzenstein et al, Appl. Phys. Lett. 90, 251109 (2007).