A sudden increase of loss in an optical communications channel can be caused by a malicious wiretapper, or for a benign reason such as inclement weather in a free-space channel or an unintentional bend in an optical fiber. We show that adding a small amount of squeezing to bright phase-modulated coherent-state pulses can dramatically increase the homodyne detection receiver's sensitivity to change detection in channel loss, without affecting the communications rate. We further show that augmenting blocks of $n$ pulses of a coherent-state codeword with weak continuous-variable entanglement generated by splitting squeezed vacuum pulses in a temporal $n$-mode equal splitter progressively enhances this change-detection sensitivity as $n$ increases; the aforesaid squeezed-light augmentation being the $n=1$ special case. For $n$ high enough, an arbitrarily small amount of quantum-augmented photons per pulse diminishes the change-detection latency by the inverse of the pre-detection channel loss. This superadditivity-like phenomenon in the entanglement-augmented relative entropy rate, which quantifies the latency of change-point detection, may find other uses. We discuss the quantum limit of quickest change detection and a receiver that achieves it, tradeoffs between continuous and discrete-variable quantum augmentation, and the broad problem of joint classical-and-quantum communications and channel-change-detection that our study opens up.

翻译：光通信信道中损耗的突然增加可能由恶意窃听者引起，也可能源于良性原因，如自由空间信道中的恶劣天气或光纤的无意弯曲。我们证明，在明亮的相位调制相干态脉冲中加入少量压缩光，可显著提高零差检测接收器对信道损耗变化的检测灵敏度，且不影响通信速率。进一步研究表明，通过将相干态码字的$n$个脉冲块与在时域$n$模等分器中分割压缩真空脉冲产生的弱连续变量纠缠进行增强，该变化检测灵敏度随$n$增大而逐步提升；前述压缩光增强方案即为$n=1$的特殊情形。当$n$足够大时，每个脉冲中任意微小的量子增强光子数可将变化检测延迟降低为检测前信道损耗的倒数。这种在纠缠增强的相对熵率中出现的类超加性现象（该熵率用于量化变化点检测的延迟）可能具有其他应用价值。我们探讨了最快变化检测的量子极限及其实现接收机方案、连续变量与离散变量量子增强之间的权衡，以及本研究开启的经典-量子联合通信与信道变化检测的广阔问题。