Quick detection of transmittance changes in an optical channel is crucial for secure communication. We demonstrate that preshared entanglement using two-mode squeezed vacuum states significantly reduces detection latency compared to classical and entanglement-augmented coherent-state probes. The change-detection latency is inversely proportional to the quantum relative entropy (QRE), which goes to infinity in the absence of thermal noise, suggesting idealized instantaneous detection. However, in realistic scenarios, we show that QRE scales logarithmically with the inverse of the thermal-noise mean photon number. We propose a receiver that achieves this scaling and quantify its performance gains over existing methods. Additionally, we explore the fundamental trade-off between communication capacity and change-detection latency, highlighting how preshared entanglement enhances both.

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• Quantum communication
• Quantum information theory
• Quantum metrology