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arXiv:2504.10882 · IEEE International Conference on Quantum Computing and Engineering (QCE)

A Quantum Speedup in Localizing Transmission Loss Change in Optical Networks

Yufei Zheng, Yu-Zhen Janice Chen, Prithwish Basu, Don Towsley

Published Aug 30, 2025• 1 citation
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Abstract

The ability to localize transmission loss change to a subset of links in optical networks is crucial for maintaining network reliability, performance and security. Quantum probes, implemented by sending blocks of$n$coherent-state pulses augmented with continuous-variable ($\mathbf{C V}$) squeezing ($n=1$) or weak temporal-mode entanglement$(n>1)$over a lossy channel to a receiver with homodyne detection capabilities, are known to be more sensitive than their quasi-classical counterparts in detecting a sudden increase in channel loss. The enhanced sensitivity can be characterized by the increased Kullback-Leibler (KL) divergence of the homodyne output, before and after the loss change occurs. When combined with the theory of quickest change detection (QCD), the increase in KL divergence translates into a decrease in detection latency. In this work, we first revisit quantum probes over a channel, and consider the ratio of quantum to classical KL divergences as a metric for quantum speedup. We generalize previous results on$n=1$(CV squeezed states) to arbitrary values of$n$, and discuss the monotonicity and asymptotics of this ratio with respect to the parameters of the quantum probe. Assuming a subset of nodes in an optical network is capable of sending and receiving such probes through intermediate nodes with all-optical switching capabilities, we present a scheme for quickly detecting the links that have suffered a sudden drop in transmissivity. Since quantum probes lose their sensitivity with increasing loss in the channel, we first propose a probe construction algorithm that makes the set of links suffering transmission loss change identifiable, while minimizing the longest distance a probe traverses. We then introduce new cumulative sum (CUSUM) statistics with a stopping rule, which allows us to run the CUSUM algorithm to quickly localize the faulty links using our constructed probes. Finally, we show that the proposed scheme achieves a quantum speedup in decreasing the localization delay.

DOI
10.1109/qce65121.2025.00107

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The ability to localize transmission loss change to a subset of links in optical networks is crucial for maintaining network reliability, performance and security. Quantum probes, implemented by sending blocks ofnnncoherent-state pulses augmented with continuous-variable (CV\mathbf{C V}CV) squeezing (n=1n=1n=1) or weak temporal-mode entanglement(n>1)(n>1)(n>1)over a lossy channel to a receiver with homodyne detection capabilities, are known to be more sensitive than their quasi-classical counterparts in detecting a sudden increase in channel loss. The enhanced sensitivity can be characterized by the increased Kullback-Leibler (KL) divergence of the homodyne output, before and after the loss change occurs. When combined with the theory of quickest change detection (QCD), the increase in KL divergence translates into a decrease in detection latency. In this work, we first revisit quantum probes over a channel, and consider the ratio of quantum to classical KL divergences as a metric for quantum speedup. We generalize previous results onn=1n=1n=1(CV squeezed states) to arbitrary values ofnnn, and discuss the monotonicity and asymptotics of this ratio with respect to the parameters of the quantum probe. Assuming a subset of nodes in an optical network is capable of sending and receiving such probes through intermediate nodes with all-optical switching capabilities, we present a scheme for quickly detecting the links that have suffered a sudden drop in transmissivity. Since quantum probes lose their sensitivity with increasing loss in the channel, we first propose a probe construction algorithm that makes the set of links suffering transmission loss change identifiable, while minimizing the longest distance a probe traverses. We then introduce new cumulative sum (CUSUM) statistics with a stopping rule, which allows us to run the CUSUM algorithm to quickly localize the faulty links using our constructed probes. Finally, we show that the proposed scheme achieves a quantum speedup in decreasing the localization delay.
Network tomography refers to the use of inference techniques for inferring internal network states from end-to-end probes. Quantum probes, implemented by sending blocks of nnn coherent-state pulses augmented with continuous-variable (CV) squeezing (n=1n=1n=1) or weak temporal-mode entanglement (n>1n>1n>1) over a lossy channel to a receiver with homodyne detection capabilities, are known to carry information about the channel transmissivity. Assuming a subset of nodes in an optical network is capable of sending and receiving such probes through intermediate nodes with all-optical switching capabilities, we leverage these quantum probes to estimate link transmissivities. To determine how to route the probes in a network, we propose a probe construction algorithm that guarantees link identifiability, while maximizing the number of information orthogonal sets of transmissivities. A set of probes induces a Fisher information matrix (FIM). We then derive two metrics, the determinant of the FIM and the trace of its inverse, to evaluate the performance of the probes. In particular, our results can be used to characterize the quantum improvement in estimating link transmissivities in a general optical network.
Augmenting a train of bright phase-modulated laser-light pulses of a coherent communications system with infinitesimally small quantum photons per pulse-entangled across several time bins-prepared by splitting squeezed light in a temporal-mode interferometer can dramatically enhance a homodyne receiver's ability to detect a sudden change in the channel loss, by up to a factor that is the inverse of the pre-change loss, without affecting the communications rate. We discuss the quantum limit of quickest change detection, and the problem of joint communications and change detection that our Letter opens up.
This thesis studies quantum-enhanced phase change detection using continuous-variable Gaussian states. The main goal is to understand how different probe states and receiver designs affect the detection of a phase change in a noisy optical channel. The phase change problem is written as a quantum state discrimination problem. The pre-change and post-change states are Gaussian states described by displacement vectors and covariance matrices. A thermal-loss channel is used to model optical loss and background noise, and a phase shift is applied to the signal mode. The main quantum benchmark is the quantum relative entropy (QRE) between the post-change and pre-change states. A pure two-mode Gaussian probe is studied in detail. The probe energy is split between squeezing and coherent displacement. This split is controlled by a power allocation ratio ?. By optimizing over ?, the probe can balance quantum correlations and coherent amplitude under a fixed photon-number constraint. Amplified spontaneous emission (ASE), coherent state, and two-mode squeezed vacuum (TMSV) probes are also considered as comparison cases. Practical receiver designs are then studied using classical relative entropy (CRE). The receivers include ideal photon-number-resolving (PNR) detection, finite-resolution PNR detection, a binary single-photon detector (SPD), an ASE receiver, and a homodyne jointdetection receiver. The finite-resolution study distinguishes binary SPD, with outcomes 0 and ? 1, from 1-PNR, with outcomes 0, 1, and ? 2. The receiver-level classical relative entropy is compared with the quantum relative entropy bound to show how much information each receiver keeps after measurement. Finally, the thesis connects these relative entropy results to the cumulative sum (CUSUM) and quantum CUSUM (QUSUM) frameworks for quickest change detection. In this framework, a larger relative entropy means a shorter detection delay for the same false alarm requirement. The results show that both probe design and receiver design are important for quantum-enhanced phase change detection.
This paper investigates the quickest change detection of quantum states in a universal setting: specifically, where the post-change quantum state is not known a priori. We establish the asymptotic optimality of a two-stage approach in terms of worst average delay to detection. The first stage employs block POVMs with classical outputs that preserve quantum relative entropy to arbitrary precision. The second stage leverages a recently proposed windowed-CUSUM algorithm that is known to be asymptotically optimal for quickest change detection with an unknown post-change distribution in the classical setting.
Fibre-based quantum networks distributing polarization entanglement require a stable and uninterrupted transmission basis for reliable operation. Bright classical reference light enables rapid polarization feedback but can introduce noise into quantum channels. Entangled-photon-based feedback avoids this noise, but typically interrupts the target entanglement channel during calibration and becomes prohibitively slow over long distances due to the product loss of fibre links. Here, we overcome both limitations by combining wavelength-bracketed probing with switch-enabled path decomposition. Spectrally adjacent entangled-photon sidebands track the polarization response of the central distribution channel without interrupting its transmission, while optical switches and local reference fibres independently determine the signal and idler network transformations. Transferring the resulting compensation settings to the central channel eliminates calibration-induced downtime and changes the acquisition-time scaling from the product of the link losses to the sum of losses. We demonstrate the method on a 133 km fibre testbed and achieve continuous closed-loop stabilization for more than 24 hours without classical reference light. The decomposition of multi-link quantum feedback into single-link measurements provides a scalable stabilization strategy for wavelength-multiplexed quantum networks.
Quickest detection of a sudden change in the transmittance of an optical channel is paramount for secure and reliable communications. We show that two-mode squeezed vacuum state, pre-shared between the communicating parties, affords a significant reduction in the quickest-change detection latency compared to when no entanglement is pre-shared: either using a classical coherent-state probe or a quantum-augmented coherent-state that was recently shown to outperform a coherentstate probe. In the absence of thermal noise, the quantum relative entropy (QRE)-whose inverse quantifies the quantum-minimum change-detection latency-is infinite. Even though zero noise and hence instantaneous change detection is unphysical, we show the QRE diverges to infinity as the logarithm of the inverse of the thermal-noise mean photon number. We propose a receiver that achieves the aforesaid QRE scaling, quantify performance improvements over known schemes, and discuss the problem of quantum limits of joint communications and change detection.