Spooky action at the Space Station

Contributed by
Jun 9, 2008

Quantum entanglement is weird. And I mean weird even for quantum mechanics, which sets a whole new level for weird.

Why is Dick York's picture here?
Is it entangled with this post? Spooky.
In the world we live in, the macroscopic world, if you flip a coin, it can either land heads or tails (ignore it landing on its side, the results of which have been thoroughly discussed in The Twilight Zone). That's it. But in the quantum world, that coin could be both heads and tails, with an equal probability of both. Until you actually examine the coin and see which way it landed, it exists in that weird in-between state.

I'm not making this stuff up. This is very real, as real as atomic bombs, computer chips, and solar cells. In fact, all three of those depend on quantum effects, so we know, with absolute certainty, that quantum mechanics works.

Now imagine two coins. They are rigged in such a way that if one of them is flipped, the other will invariably land the other way. So if coin A lands heads, then coin B lands tails.

Flip coin A. Don't look yet! What state is coin B? Well, it's either heads or tails, but which one depends on coin A, and we don't know that yet. In fact, since we haven't looked at coin A, it's both heads and tails according to QM. So coin B is as well, but opposite (think of it as tails or heads).

So let's look at coin A. Oh, it's heads! So coin B is tails.

OK, cool. Now repeat the experiment. But this time, before looking at either coin, glue them each to a piece of wood, seal them in separate boxes, and move them hundreds of kilometers apart. Mind you, they are both in their heads-and-tails/tails-and-heads states. OK, after they have been separated, then look at coin A. Heads again! That means suddenly coin B is tails.

But wait a sec. How did coin A tell coin B which state it was in? They couldn't talk to each other; they were too far apart!

If that bugs you, well, it should. It bugged Einstein a lot, and no offense or anything, but he was probably a lot smarter than you. But he never really liked quantum mechanics, and he dubbed this "spooky action at a distance". In principle, this could be done light years apart, where the coins are so far apart there is no way for them to communicate.

This was just a thought experiment, and in the real world coins won't work. They're too big for QM rules to apply. But you could do this with photons, which obey the rules of QM to the letter. There are ways to set this up, and in fact it's been tested on Earth many times.

It works. I'm no Einstein (my hair is better, marginally), but this freaks me out. QM is really really weird.

This effect, called quantum entanglement, isn't completely understood. There are different explanations for it, but they're hard to test, so it's hard to know which ones may be right and which may be wrong. The problem is, using photons limits you to line-of-sight: you can separate the two photons, but you have to be able to see both of them to measure this effect. This has been done with baselines of over 100 kilometers, but that's not far enough to distinguish some of the hypotheses.

So some European scientists came up with the idea of using the International Space Station (I know! Using ISS for science! Wow!) to test this out. They can create a small setup with a laser which can create entangled photons. The entangled photons are then sent simultaneously to two different ground stations, widely separated on the surface of the Earth, so that both have a copy of the entangled photons. In addition, two quantum keys are created based on the photons; this is essentially a code based on the state of the photons -- like winning a bet is based on which way a coin lands. The two keys are different, and one each is sent to the two ground stations. So both stations have a pair of entangled photons (identical to the other station's) and a different key.

Each key is actually a long chain of 1s and 0s. The two keys are then compared on the ISS to create what's called a bitwise XOR -- for example, if two coins both land heads then the XOR operation yields a 0, but if they land differently (one heads and one tails) then it yields a 1 -- it's just telling you whether they are the same or different. So for each place in the key, the two numbers are compared, and if they're the same (both 1s or both 0s) then a 0 is written down. If they are different then a 1 is put there. When this is done, you get a third string of 1s and 0s, representing a comparison of the two keys.

Still with me? Yeah, me neither, but we're almost done. So now the ISS has this long number string which represents whether the keys are alike or different. It then transmits this to one of the two stations on Earth.

So? What does this mean? This means that now the two ground stations can create a code between them based on their keys, a code that is known only to them and no one else. Furthermore, this code cannot be cracked by anyone, anywhere, because it's based on entangled photons that cannot be known to anyone else! Because of entanglement, they know what the other station has because they can look at their key and figure it out. But no one else can.

If I am interpreting this correctly -- and no guarantees, me droogs, because after thinking about this for an hour now my brain cells are starting to turn into oatmeal -- then we have a code that is basically impregnable. No one else can tap into it, making it pretty secure. And this can be done over thousands of kilometers, not hundreds like on Earth.

Now, I have readers who are more familiar than I am with this stuff, so please, if I am misinterpreting this please hash it out in the comments section. But if I understand this correctly it will have all sorts of uses in encryption, as well as helping us understand quantum mechanics. And by "us" of course I mean other people who already do understand these aspects of quantum mechanics. As for me personally, I simultaneously understand and don't understand it.

Which means I understand it.

Hmmm, I may have just collapsed my wave function. I think maybe now it's time for me to stop worrying about this and instead just go play with my cat.

Tip o' the phase-shifted aggregate wave state to Larry Klaes.

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