Dear Bob,

Whatever anyone says, quantum cryptography is spooky—especially since scientists have actually done it numerous times, exchanging secrets through a fiber-optic cable beneath Lake Geneva. There is even a Swiss company, id Quantique, that is marketing what it touts as a plug-and-play personal quantum encryption system. Its motto: “Quantum security … at last.” Just connect the unit to the USB port of your PC and away you go. (I haven’t tried this intriguing product. It only works on Windows, so we Mac/Unix users will have to wait a while.)

The company also offers a “quantum random number generator” and a “single photon detection device” for use in experiments with quantum teleportation. So far no mention of a quantum cuckoo clock. (Quantum teleportation, by the way, is not quite so *Star Trek*ky as it sounds, but I’ll forgo the details, which are in the book.)

So, yes, given all the absurdities of quantum mechanics rearing up in the everyday world, I sympathize with your annoyance at physicists who deny that this stuff is profoundly bizarre. Say the words “spooky action at a distance” and they cringe. (I can picture one Nobel laureate here in Santa Fe who gets particularly exercised over this point.) They’re right, though, that no information is really passing instantaneously from point A to B (called Alice and Bob in the cryptography trade). It’s more like the key to encrypt the message is *simultaneously created* in two places at once. Weird enough, but for Alice and Bob to actually make use of this data they must confer on an old-fashioned, cumbersomely slow light-speed connection.

Even without superluminal cell phones, harnessing quantum mechanics opens up all kinds of barely believable possibilities. There are certain problems in mathematics (and I’ll get to the practical applications in a paragraph or two) that are essentially impossible to solve—*unless you have a quantum computer*.

Earlier you mentioned factoring—finding the numbers you can multiply together to get a larger number. For 15 it’s easy—3 and 5. But, quick, what are the factors of 14,321? You can plod through the problem with a pocket calculator. But as numbers get longer, the factoring time increases exponentially. A number with thousands of digits would take the fastest supercomputer longer to crack than the universe is likely to exist.

But for a quantum computer, consisting of a long row of calculating atoms, the problem would be trivial. Every time you add another atom to the string, the calculating power increases exponentially. You’re fighting fire with fire, and even the longest numbers could be factored with ease.

Here is the punch line: The codes used to encrypt information on computer networks—everything from credit card numbers for an eBay purchase to military secrets—depend on the “impossibility” of factoring long numbers. A quantum computer is a hacker’s dream.

So is there a mad scientist, maybe a very smart teenager, a quantum geek, hiding in a basement somewhere preparing to unleash one of these beasts? The Defense Department and the National Security Agency are worried enough that they are pouring money into quantum code-breaking. They want to get there first. At the same time, they’re funding some of the Alice and Bob experiments—using quantum mechanics to devise truly unbreakable codes.

The result is a cryptographic arms race. If a large-scale quantum computer seems like a far-off dream, consider how quickly nuclear energy was harnessed once the generals decided they could use it to blow up things, or at least threaten to.

George