Quantum of Promise

Cool blog post yesterday from Wired on the future of quantum teleportation — and whether the U.S. is going to be a part of it (I’ll go ahead and link to my own thoughts on the government sponsoring science, but rest assured I think it’s a shame the funding got cut).  The blog’s got a pretty great explanation of the science behind quantum teleportation, but I thought I’d take a crack at explaining it and what it could mean.  And why Scotty won’t be beaming me anywhere anytime soon.

Okay, so.  First of all, the word “teleportation” is a total misnomer for this (admittedly awesome) technology, but we’ll get to that later.  Quantum teleportation involves three subatomic particles, two of which are identical and generated in such a way as to be entangled — that is, they have the exact same intrinsic properties (something like “spin,” a measurement of subatomic particles’ angular momentum, or “polarization,” the direction a light photon* oscillates in) regardless of the distance between them.  Why is this idea of entanglement important?  Well, this syncing happens instantaneously — literally faster than the speed of light.

Sounds like a great way to send information, right?  Wall Street would love it.  NASA would devour it.  Starcraft players would finance the entire sector.

Problem is you can’t quite send information with just those two particles.  Yes, what’s true in one is instantly true in the other — but there’s no real way to change what’s true in that first particle.  It’s either “spin up” (spinning anticlockwise) or “spin down” (clockwise), oscillating horizontally or vertically, red or blue, whatever.  In order to successfully entangle two particles, you cannot physically know which choice the particles have made.  Even as its creator, you have no control over the randomness of nature.**

So game over, right?  Well, not so fast.  Man has never been one to give up when nature seems to say something is fundamentally impossible.

You can’t choose the properties of the two entangled particles.  And you don’t really know what those properties are — observing something’s quantum state collapses its wavefunction, which is a fancy way of saying you make it useless and kill the cat to boot.

(A hopefully illustrative example: You flip a coin five times in California, and your friend in New York has a magic coin that flips spontaneously and identically every single time you flip yours.*** You can either not look at your results and have no idea what message you’re sending to New York, or look at the results of your coin tosses but then send your friend a meaningless string of heads and tails he can’t make heads or tails of.  Either way, there’s no way to send useful information.)

This is where the third particle comes in.  Quantum teleportation uses this third particle to pass along information by way of the entangled pairs, but the information isn’t in the entangled particles themselves.  It’s in this third particle — the one that never leaves your side.

Here’s how it works:

  • Your friend in New York has forgotten how old your daughter is, but wants to get her the appropriate birthday card.
  • Naturally, you make a bunch of entangled pairs of coins.  Mail half of each pair to your friend in New York.  Pay for delivery confirmation.
  • You have a separate coin that’s not entangled with anything else — and therefore, you’re free to choose its properties, over and over again.  You want to send tails-heads-heads-tails-tails (“12” in binary), so you first set this coin tails side up on the table.
  • You then flip the first entangled coin.  Because they’re magic entangled coins and because my metaphor has broken down, you can’t tell what side your entangled coin landed on — but you can tell that it’s not the same as tails.
  • Next, you take the tails-side-up coin on the table and place it down heads up.  You flip the entangled coin, and this time it looks like you got something that’s identical to heads.
  • Repeat with each entangled particle and each bit of your message until you have a string that contains the response of your entangled particle to your binary message (e.g. opposite-identical-opposite-opposite-identical).
  • Call your friend.  Tell him your encrypted string.  He’s been watching his halves of the entangled coins, and so knows exactly what side they landed on in response to the coins you flipped in California: heads-heads-tails-heads-tails.
  • Your friend keeps the second and fifth results as they are, but because of your phone call, knows to switch the first, third, and fourth result, giving him tails-heads-heads-tails-tails.  Or 12.
  • Voila — you’ve just sent an unbreakable coded message across the country in the time it took to make a phone call, and your friend buys your daughter the perfect age-appropriate birthday card.

PRETTY COOL YEAH?  Potential for some real James-Bond-level spy stuff, since the only way to decode the message is to be right there with the guy getting it as he gets it.

It is not, however, science fiction teleportation by any means.  The name arises, I think, because you’re spontaneously transmitting information — albeit useless information without the opposite-identical cipher.

Just remember I told you all this when the devious sneak attack we couldn’t possibly have realized was coming hits in 2041.  And then wait for Michael Bay Model Cinemabot 3000 to make an explosion-filled holopic about it in 2101.

It’s gonna be great.

* But Seth! You said particles — light is a wave!  Well yes, true.  Except when it’s not.  Which it turns out is all the time.  One of the weirdest of all the weird quantum phenomena, wave-particle duality says all particles behave like waves sometimes and all waves behave like particles sometimes — we just don’t notice it (really, can’t possibly observe it) in our daily lives.

** God, as it turns out, may well play dice with the universe — so you damn well better believe your mortal ass is playing some craps when you’re playing god.

*** He has avoided calling the Ghostbusters about this for who knows why.

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