A new form of eavesdropping technology, using ubiquitous installed FTTH fiber optic cables, against which countermeasures do not yet exist. Of course, that's just the hardware; AI voice recognition and reception sort of amplifies the monitoring capability by a factor of infinity.
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The Distributed Acoustic Sensing technology mentioned is a mature technology and widely available. It may not be all that cheap. A for instance manu:
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Machine learning coupled with AI is just now breaking into translating into words brain magnetometrics using quantum AI, making mind-reading eavesdropping next up on the menu.