Quantum teleportation and entanglement swapping
Long-distance teleportation over space : Ground-to-Satellite Quantum Teleportation was demonstrated for distances over 1400 km from the satellite ‘Mencius’. This avoids the issue of photon loss, which limited distances to 100 km in experiments on Earth. This was done by Ren et al. (2017). [8]
Quantum Network : Wehner et al. have defined a functionality-driven sequence of development stages that can be used to achieve a mature Quantum Network, with the end goal being multi-node services . This uses results from the previously mentioned experiments, alongside Azuma 2023. to provide a phased maturity model for quantum internet. [7][9] Issues faced during implementation Exponential decay with distance leads to loss : Directly transmitting quantum states leads to exponential decay with distance. In classical communication, the issue of exponential attenuation can be solved through using repeaters at certain points, which amplify the signal and restore it to its original form. Similarly, by breaking a long route into shorter spans through nested entanglement swapping (via BSMs) in the form of quantum repeaters, we are able to avoid loss. The process of segmentation → storage → purification → entanglement swapping is the core architecture of quantum repeaters, and helps us avoid some exponential decay, but it is still an issue. [5] Each swap increases risk : While entanglement swapping has been experimentally verified, in practice, each swap layer multiplies both the success probabilities and error sources, increasing the risk of failure. This means that performance of quantum repeaters hinges on the quality and stability of BSMs used. [4] BSMs are a limiting factor to performance : Similar to the point above, the success probability of BSMs directly affects entanglement distribution rates. Practical analysers are currently unable to discriminate all four Bell States with a success rate greater than 50 percent: systematic errors (practical analysers, detector imperfections etc.) lower the success rate of BSMs outside an experimental setting. This is important as a low BSM success probability means that many attempts must be repeated, slowing down the overall entanglement rate. Conversely, a higher BSM effectiveness translates into fewer retries and higher overall throughput. [5][7] Near-future outlook Over the next few years, progress will mainly be coming from better BSMs, longer-lived and more efficient quantum memories, and multiplexing (where many parallel attempts are taken) to raise rates. However, even with these gains, the performance will be modest at first and require tight stabilization, alongside a solid control system. [7] For very long distances, networks with satellite links are very likely, since ground-to-satellite teleportation has already been shown for distances over a thousand kilometers. [8] Finally, the initial usages of quantum networks through quantum repeaters would most likely be entanglement-based QKD (Quantum Key Distribution). Currently, the outlook for this is incremental, but solid pace has been made. Through many experiments, it has been shown that the theoretical physics behind the concept works; however, real-world applications, engineering and scaling are the remaining hurdles that need to be overcome. [9][7]
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