01 March 2006


I seem to be stumbling across a lot of weird QM stuff on the web these days. First, this may seem obvious ...
A non-running computer produces fewer errors
... in the sense that it produces no errors, because it handles no data. But not so; that's a quote from a New Scientist article about a quantum computer in a superposition of wave states between on and off, so the one thing it ain't is "obvious." They ran an experiment with a quantum computer. It turned out that the wave state of the computer collapsed to off, but it still solved the problem ... in fact, it turns out to be better if the computer was off.

What do I mean, "superposition of wave states"? That's a little more than I can fully explain today. Mr. Schrödinger's famous cat—half alive and half dead at the same time— is the most famous description of superposition, though Bram Cohen offers a vivid and disturbing metaphor for superposition that attempts to improve on the cat.

I will describe the strange case of the Quantum Duelist. Instead of a cat, the Quantum Duelist is a person, although like the cat he goes inside a black box which is left completely sealed for some period of time, and then when the box is opened the quantum state of the contents of the box are forced into a specific value, rather than the multiple interacting values which it has until it interacts with the outside world.

After entering and locking the black box, the Quantum Duelist flips a coin. If it comes up heads, he turns to the right, if it comes up tails, he turns to the left. Either way, he then takes ten paces forwards, turns around, and fires a gun straight ahead.

According to quantum mechanics, after the black box is opened, there's some chance that the duelist will be lying on the floor ten paces to the left, dead from a gunshot between the eyes fired by himself at a distance of twenty paces.

Unsettlingly, experiments clearly demonstrate that individual particles really do behave this way, so we can't dismiss quantum mechanics on the grounds of absurdity.

Which leads us to a gendankenexperiment in which you want to look at things without actually shining light on them.
The two physicists who brought this topic to light in 1993, Elitzur and Vaidman (EV), considered a "superbomb", which, if it possessed the trigger/detonator element, would explode whenever hit by even a single photon .... The goal now is, given a supply of these bombs in sealed crates, find out which ones are the good bombs.
A detective limited to the realm of classical physics is in trouble. He can go into a completely darkened room, and pry off the lid of the crate. Then what? If there really is no light at all -- if no photons at all hit the trigger element -- then he gets no information. If, on the other hand, a single photon hits the element, well then by definition there is a loud explosion, and the detective knows that this was a good bomb. There seems to be no way to find the good bombs without always exploding them.
There's also a real experment that can be performed for that one, too. The universe is a strange place.

1 comment:

Reya Mellicker said...

This is so cool. What's strange about the universe is us, our need to believe we can actually understand the mysteries. We're the weird ones, I think.