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量子态在远端方之间的分布。光是最合适的载体，因为它对退相干效应的敏感性低。然而，当桥接长距离时，特别是高斯退相干效应，例如光学损耗，成为关键问题。退相干通常会降低量子通信速度，在实践中很快达到零。退相干确实是新量子技术出现的主要障碍之一。长期以来，人们一直提出多步（迭代）蒸馏方案来克服退相干，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有效，已经考虑了量子存储器。但合适的量子存储器迄今尚未完全实现。该项目的目的是在不使用量子存储器的情况下，对遭受高斯退相干的高斯态进行有效的多步蒸馏的实验演示。 我们特别关注高斯纠缠（双模压缩）态，和高斯退相干的光损耗。为了解决高斯区域内众所周知的蒸馏不通过定理，我们在成功的光子减法上触发我们的协议。我们项目的特殊和新的特点是不需要量子存储器。相反，我们遵循最近提出的测量Husimi Q函数结合适当的数据后处理的方法。该方法能够使用在不同时间获取的数据来模拟多步蒸馏。提取的数据与使用量子存储器的有效提取方案将产生的数据无法区分。由于该方法包括最终测量，因此特别有希望在现实环境中增强连续变量量子密钥分配。

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.