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Commuters inching through rush-hour traffic in the Holland Tunnel between Lower Manhattan and New Jersey don’t know it, but a technology likely to be the future of communication is being tested right outside their car windows. Running through the tunnel is a fiber-optic cable that harnesses the power of quantum mechanics to protect critical banking data from potential spies.
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.