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The Super-Secure Quantum Cable Hiding In the Holland Tunnel (bloombergquint.com)
Zorro shares a report: 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.
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.
You can use classical physics to do quantum stuff.
Quantum Key Distribution uses polarized light, and one interesting property is that unless the polarizes are orthogonal to each other, you're going to have a non-zero probability of light going through. So what you do at the sender end is send pulses of polarized light at random polarizations (say, 0 degrees, 0 degrees, 90 degrees, 45 degrees, 135 degrees, etc). Of course, the pulses are coded to represent your bit pattern, so a pulse could mean a 1, no pulse could mean 0.
At the receiver end, the receiver picks a random polarization and measures the output - either light, or no light. It doesn't matter which.
What happens after sending a copious amount of data is the two ends then compare their polarizer settings and discard the bits where the polarizer setting did not match (e.g., sender used 0 degrees, receiver used 45 degrees). Most of the data will be discarded, but you'll have plenty more where by chance both sender and receiver picked the same polarizer.
You can then do a quick hash to compare the final results - the two hashes should be the same.
Now what happens if someone taps the line? Well, they don't know the polarizer settings, so at best they're going to guess. But the act of inserting the eavesdropping polarizer into the bitstream changes the polarization of the light! If the sender uses 0 degrees, and the eavesdropper uses 45 degrees, light will have a 50% chance of going through the polarizer. But even stranger, at the receiver, if they use a 0 degree polarizer or a 90 degree polarizer, light again will have a 50% chance of getting through. So even though the sender and receiver may both use a 0 degree polarizer, the eavesdropper using a 45 degree polarizer has changed the end result. Maybe the eavesdropper gets lucky, maybe not.
Doing it for a large number of bits and you'll detect the line tap too easily because of it.
If you want to see this in action, you can do the standard two polarizer test, set them orthogonally to each other (so the two polarizers let no light through). Now add a third polarizer AFTER than two polarizers and oddly, you'll get light going through! It doesn't have to be in the middle of the polarizer stack - just the act of the third polarizer interacts with the other two such that some light now goes through where it didn't before makes things extra spooky.