Quantum-enhanced quickest change detection of transmission loss

2025-03-15

AI Summary

To address the challenge of rapidly detecting abrupt transmission losses—such as those induced by eavesdropping, fiber bending, or atmospheric turbulence—in optical communication channels, this paper proposes a quantum-enhanced quickest change-point detection (CPD) method. We introduce a weak continuous-variable (CV) entanglement encoding scheme based on phase-modulated bright coherent states, squeezed vacuum injection, homodyne detection, and a time-domain *n*-mode beam splitter. Crucially, we first reveal a superadditive effect of CV entanglement in CPD. Our approach achieves detection delay inversely proportional to pre-detection channel loss—surpassing the classical Shannon limit. It requires only a minimal number of quantum-enhanced photons per pulse yet yields substantial sensitivity gains. As *n* increases, the detection delay asymptotically approaches the fundamental quantum limit.

Technology Category

• Machine Learning: Quantum Machine Learning
• Computer Vision: Learning & Optimization for CV
• Search and Optimization: Mixed Discrete/Continuous Search

Application Category

• Security and Privacy: Large-scale security measurements
• Search and Retrieval-Augmented AI: Efficiency and scalability of Web search engines
• Semantics and Knowledge: Methods to enhance, augment, integrate or synergize semantic models such as knowledge graphs and LLMs

Abstract

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.

Problem

• Enhancing sensitivity to detect sudden optical channel loss changes
• Using quantum squeezing to improve change-detection without affecting communication rate
• Exploring quantum limits and tradeoffs in joint classical-quantum communication systems

Innovation

• Squeezing enhances homodyne detection sensitivity.
• Entanglement boosts change-detection sensitivity progressively.
• Quantum-augmented photons reduce change-detection latency.