The cable's trick is a technology called quantum key distribution, or QKD. Any half-decent intelligence agency can physically tap normal fiber optics and intercept whatever messages the networks are carrying: They bend the cable with a small clamp, then use a specialized piece of hardware to split the beam of light that carries digital ones and zeros through the line. The people communicating have no way of knowing someone is eavesdropping, because they're still getting their messages without any perceptible delay.
QKD solves this problem by taking advantage of the quantum physics notion that light -- normally thought of as a wave -- can also behave like a particle. At each end of the fiber-optic line, QKD systems, which from the outside look like the generic black-box servers you might find in any data center, use lasers to fire data in weak pulses of light, each just a little bigger than a single photon. If any of the pulses' paths are interrupted and they don't arrive at the endpoint at the expected nanosecond, the sender and receiver know their communication has been compromised.
Encryption is worthless if an attacker manages to get the digital keys used to encode and decode messages. Each key is usually extra-encrypted, but documents disclosed by former National Security Agency contractor Edward Snowden in 2013 showed that the U.S. government, which hoovers up most of the world’s internet traffic, can also break those tougher codes.
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