Efficient multi-step distillation of quantum states counteracting Gaussian decoherence without the need for quantum memories

量子通信协议的一个重要组成部分是在远距离各方之间的量子态分布。光是最合适的载体，因为它对退相干效应的敏感度很低。然而，当桥接远距离时，特别是高斯退相干效应，如光学损耗，成为关键问题。退相干通常会降低量子通信速度，而在实践中，量子通信速度很快就会降为零。退相干确实是新量子技术诞生的主要障碍之一。多步(迭代)蒸馏协议长期以来一直被提出以克服退相干，但其概率性质使其效率低下，因为成功概率随步骤数呈指数衰减。为了克服这一点并使多步蒸馏高效，人们已经考虑了量子存储器。但到目前为止，合适的量子存储器还没有完全实现。该项目的目的是在不使用量子存储器的情况下，实验演示受高斯退相干影响的高斯态的高效多步蒸馏。我们特别关注了高斯纠缠(双模压缩)态，以及高斯退相干的光学损耗。为了绕过广为人知的关于高斯区域内蒸馏的不去定理，我们触发了我们的协议，成功地减少了光子。我们项目的具体和新特点是不需要量子存储器。取而代之的是，我们遵循最近提出的测量Husimi Q函数的方法，并结合适当的数据后处理。这种方法能够使用不同时间采集的数据来模拟多步精馏。蒸馏后的数据与使用量子存储器的高效蒸馏方案产生的数据难以区分。由于该方法包含了最终的测量，因此在增强现实世界环境中的连续变量量子密钥分发方面特别有希望。

An essential component to quantum communication protocols is the distribution of quantum states between distant parties. Light is the most suitable carrier, due to its low sensitivity to decoherence effects. When bridging long distances, however, in particular Gaussian decoherence effects, such as optical loss, become critical issues. Decoherence generally degrades quantum communication speed, which quickly reaches zero in practice. Decoherence is indeed one of the main obstacles at the dawn of new quantum technologies. Multi-step (iterative) distillation protocols have long been proposed to overcome decoherence, but their probabilistic nature makes them inefficient since the success probability decays exponentially with the number of steps. To overcome this and to make multistep distillation efficient, quantum memories have been contemplated. But suitable quantum memories are not fully realised to date. The aim of this project is the experimental demonstration of efficient multi-step distillation of Gaussian states that suffered from Gaussian decoherence, without using quantum memories. We particularly focus on Gaussian entangled (two-mode-squeezed) states, and Gaussian decoherence in terms of optical loss. To work around the well-known no-go theorem for distillation within the Gaussian regime, we trigger our protocol on successful photon subtraction. The specific and new feature of our project is that no quantum memories are required. Instead, we follow the recently proposed approach of measuring the Husimi Q-function combined with appropriate data post-processing. This approach is able to emulate multi-step distillation using data taken at different times. The distilled data are indistinguishable from those an efficient distillation scheme using quantum memories would produce. Since the approach includes the final measurement it is particularly promising for enhancing continuous-variable quantum key distribution in real-world environments.