量子理论:随机发生的并非随机

We assume that a lot of things in our world are random.

For example, Michele, pick a number between one and 10.

MICHELE NORRIS, host:

I'm going to say seven.

SIEGEL: Seven, okay. Now, did Michele pick number seven because it randomly popped into her head, or because that's the age of one of her children, or the time she woke up this morning?

In fact, most of the things that we think are random are not. That is, until you enter the strange world of quantum mechanics as NPR's Joe Palca reports.

JOE PALCA: This story starts in NPR's virtual casino.

(Soundbite of slot machines)

PALCA: How about a game of chance, like roulette?

Now, which number the ball falls on may seem like a random event. But Antonio Acin from the Institute of Photonic Science in Barcelona says it's not, because like every device, it follows physical rules that can be worked out.

Professor ANTONIO ACIN (Institute of Photonic Sciences, Spain): If you are able to compute the initial position and the speed of the ball, and you have a perfect model for the roulette, then you can predict where the ball is going to finish with certainty.

PALCA: Acin says everything we call random in our world may just appear random because of lack of knowledge. But there is one place where true randomness does exist. That's the world of atoms and electrons and the laws of quantum mechanics. Or to be more precise...

Prof. ACIN: Nonlocal correlations of entangled quantum particles.

PALCA: The nonlocal correlations of entangled quantum particles. Okay, let me try to explain. The laws of quantum mechanics say some things truly are random. Take the clicks you hear from a Geiger counter; these are bursts of energy coming from decaying atoms.

Professor CHRISTOPHER MONROE (Joint Quantum Institute, University of Maryland): Those are random due to quantum mechanics.

PALCA: Chris Monroe of the University of Maryland is Acin's co-author on this study.

The random clicks of a Geiger counter can be used to generate random numbers by measuring the intervals between the clicks. True random numbers are crucial for devising secret codes or else the codes can be cracked by clever computers.

But Monroe says there's a problem with using a Geiger counter: You never know if it has some electrical flaw throwing off the randomness.

Prof. MONROE: A stronger form of random number generators are those that produce random numbers that are so-called device independent. It doesn't matter what's inside the box.

PALCA: This is where you can take advantage of the weird properties of quantum mechanics. For example, in the quantum world, a magnet can point north and south at the same time, just as long as you don't look at it.

Prof. MONROE: When you do look, it randomly pops into one or the other.

PALCA: And if you can capture that random popping, you have a true random number generator. So Monroe took an atom, it happened to be ytterbium.

Prof. MONROE: It has an outer electron that is very much like a bar magnet. It can be north up or south up.

PALCA: But in order to capture that randomness, you have to entangle two ytterbium atoms, yolk them together in a quantum sense. Why? Because work by Albert Einstein and another physicist named John Bell says you have to, that's why.

It wasn't easy. As reported in Nature, it took three weeks of running their equipment round the clock to get just 42 random events - 42 ups or downs - that could be turned into zeroes or ones that were truly, totally 100 percent random.

If the generator can be speeded up, cryptographers are likely to find it useful. Antonio Acin says a casino operator who wants to prove he's running an honest operation could find it helpful too.

Prof. ACIN: I prepare for him two entangled particles. Now he takes these entangled particles, he goes to his lab. He knows he can generate randomness out of them.

PALCA: Of course, random or not, the house still gets a cut.

Joe Palca, NPR News, Washington.