Going for Unbreakable
But the fact remains: Cracking today's toughest encryption may be tough, but it is possible. The extraordinarily paranoid worry that the National Security Agency, the government organization that works at protecting the nation's secrets and decrypting foreign intelligence, has already built a machine that breaks RSA. And those with particularly secret secrets are worried about Eve breaking their codes in the future, when such a machines will most certainly be available.
"There could always be some guy out there who's stockpiling your transmissions," says Bell Labs' Fuchs. "He can just wait 20 years until we have more powerful computers, and at that point, he'll open the files and find out who was sleeping with whom."
The government itself probably has reason to worry about such things. "Sometimes, they want to keep a secret for 20 years, sometimes 50 years," continues Fuchs. "And presumably, the government wants to keep some secrets forever."
With quantum cryptography, forever is not too strong a word. Rather than relying, like RSA and any other electronic encryption on the limited scope of classical physicsthe science and mathematics of the everyday worldquantum cryptography delves down into the world of very small particles, where there are entirely new ways of building ciphers.
In particular, the technology makes use of the Heisenberg uncertainty principle, named for its discoverer, German physicist Werner Heisenberg. According to the principle, when you try to measure a quantum systema system that involves a very small particleyou alter it in such a way that your measurement cannot be completely accurate.
"If you have an unknown quantum state and you try to learn something about it, you'll inevitably disturb it," explains Shor. You can't completely tell what the system was like before you measured it. There's no classical analogy for this principle, but it's been proven to be true.
With their original quantum cryptography system, Brassard, the quantum cryptography pioneer, and his partner, IBM Research Fellow Charles Bennett, employed individual photons of light as their very small particles. When a photon is on the move, it vibrates, and the angle of vibration is called its polarization. Let's say a photon has four possible polarizations. Two of these are rectilinear polarizations: up/down (best represented thus: "|") and left/right (""). The other two are diagonal polarizations: diagonal left ("\") and diagonal right ("/"). Using a so-called Polaroid filter, you can try to measure a photon's polarization, but the uncertainty principle says you can't do so with complete accuracy.
A Polaroid filter is like sieve, and the holes in the sieve are shaped like one of the possible photon polarizations. You can have an up/down Polaroid, a left/right Polaroid, and so on. As you'd expect, if you send a photon with an up/down polarization ("|") towards an up/down Polaroid ("|"), the photon will pass through the filter and continue on its merry way. But if you send a left/right photon ("") towards an up/down Polaroid ("|"), the filter will block it. Imagine putting a quarter into the horizontal coin slot on a Coke machine. If you turn the quarter on its side, you can easily push it through the slot. If you turn it so the edge is perpendicular to the slot, it won't go through, no matter how hard you push.
This makes sense to all of us who live the classical world. But then the uncertainty principle throws a wrench in the works, and the photons start behaving like nothing we've ever seen. If you send a diagonal photon ("\" or "/") towards a rectilinear Polaroid ("|" or ""), the filter will block the photon some of the time, and other times, the photon will pass through. And when it does pass through, it's polarization changes. The up/down filter turns it into an up/down photon. The same sort of thing happens if you send a rectilinear photon towards a diagonal filter.
If a photon with an unknown polarization is coming towards you, there's no way to determine what that polarization is. If you hold up an up/down filter ("|") and it passes through, it could be an up/down photon ("|"), but it may also be one of the diagonal photons ("\" or "/"). If it's blocked, it could be a left/right photon (""), but it may be a diagonal photon in this case as well. And if you hold up another type of filter, you run into all the same problems.
Of course, if you've been told whether the photon is rectilinear or diagonal, you can pick out its exact polarization quite easily. You can't measure the photon unless you already know something about the photon.
Brassard and Bennett built their quantum cryptography system around this basic Catch-22. Under this system, Alice sends Bob a random sequence of photons, and she orients their polarizations so as to represent a binary number. Let's say that both up/down polarizations ("|") and diagonal left polarizations ("\") represent "1," and both left/right ("") and diagonal right ("/") represent "0."
"You're using quantum states to encode information," explains Shor.