In this week's Ask a Physicist, we answer a question that's on everyone's mind: Can we use quantum entanglement to make a mockery of the speed of light, and create intergalactic communications devices like Le Guin's "ansible"?
This week's Ask a Physicist comes to us from a huge number of you, but was first posed by Michael Glasky who asks:
In sci-fi movies and books and games we consistently hear that quantum entanglement is used to communicate across the galaxy instantaneously. Would this really work? Does action on one particle instantly affect the other no matter what distance? And could the movements of the particle be interpreted for the purpose of communication?
This being io9, you're probably already familiar with the basic argument: you and a friend each have a particle which is "entangled" in some way. You manipulate your particle in such a way that yours acts as a transmitter and your friend's acts as a receiver. Since we're talking about quantum mechanics, this typically involves subatomic particles, and there aren't that many things you can jigger in your particle. In fact, there's only one: spin.
Spin sounds familiar, and for the most part it is. Electron spin is different from the spin of the earth in that you can't get rid of it no matter how hard you try. You can only change its direction. Since you've got a spinning electron, it forms a microscopic electromagnet. If you want to figure out which direction an electron is spinning (and you'll see in a bit that you do), all you need to do is run it through a device consisting of a pair of ordinary magnets and see which direction the electron gets deflected.
But here's the weird part: If the device is oriented vertically, the electron will be measured to be either spin-up or spin-down, never somewhere in between. Again, this is way different from the spin of the earth which is tilted about 23 1/2 degrees compared to the ecliptic (that's why your globe is all tilty). Likewise, if you turn your device to measure the horizontal spin, you'll find that it's either spin-left or spin-right.
Stranger still, is that your measurements depend in a big way on the order in which you make them. I can measure an electron to be spin up, for example, and then turn my measuring device on its side to try to measure the left-right spin. You'd think, if you were a rationally-minded person, "The spin is up. Therefore, the left-ness and right-ness should both be zero."
You'd be wrong.
There's a 50% chance of measuring spin-left and a 50% chance of measuring spin-right. Measurement once again messes up the system.
I'll bet you can already see the appeal of spin to someone building a communicator. You run a particle through a device and get a readout saying up or down. It could just as easily be a zero or a one, and you could send binary messages (Q: Will you go out with me? A: Spin down!). To send the message, though, you need to send a particle, and that decidedly goes slower than light. To build our faster-than-light ansible, we need a pair of them.
Take a spinless particle that decays into a electron and (its anti-particle) a positron, each shooting off in opposite directions. Since we started with no spin, the spin of the positron must be opposite of that spin of the electron — they have to add up to zero. They are, in other words, entangled.
It's very easy to underestimate how weird quantum entanglement is. For instance, if I measure the electron as spin up, I know that the positron is spin down. This is ridiculously trivial, and barely deserves the name entanglement.
However, in the instant before we measured the spin of the electron, nothing in the world could have told you whether it was spin up or spin down. Supposing you measure it as spin-up, then it's not only the wave function of your electron that collapses but the positron as well, even if the positron is halfway across the universe.
Einstein, famously, couldn't believe that spin was really random, and really couldn't believe in a spooky action at a distance. But in the 1960's John Bell came up with a way of testing whether quantum mechanics really was random, and these experiments were first performed in the 1980's. As funny as it's making you feel, it's apparently the way the universe works. We talk a lot about spin in Chapter 3 of my book, if you'd like to learn more through goofy cartoons.
So we can collapse the wavefunctions of two particles at opposite ends of the universe instantaneously. This ends up being surprisingly unhelpful.
From the positron guy's perspective, 50% of the time, he measures spin-up (when you measure spin-down), and 50% of the time he measures spin-down (when you measure spin-up). Let's say you decide to be clever and send him a message by turning your measurement device on its side. Half the time you see spin-left, and the other half, spin-right. But in each case, positron guy has a 50-50 chance of measuring spin-up versus spin-down. In other words, there's absolutely nothing he can observe on his end that tells him anything about what you're doing on yours. What we have here is a failure to communicate.
I'm sure you've heard some things in the schoolyard. Things that suggest that there is a way to cheat and send messages. This brings us to:
Common Misconception 1: Messing with one of the electrons directly alters the other electron.
I put it in bold so you wouldn't miss the point. If io9 allowed me to, I'd make it blink as well. There's this idea that because the pair of particles is entangled, you can decide to simply snatch the electron out of the air and manipulate it until it is spin-up, and then you know that the positron is spin-down. If this were true, then you could send instantaneous messages by simply successively flipping your electron spin and having your friend read the spin of his positron.
This does not work. If you mess with the electron, the pair simply isn't entangled any more. If you want the technical term, it's called decoherence, and it's simply a fancy way of saying that entanglement doesn't last forever. And by "not forever," I mean that the longest decoherence timescales are tiny fractions of seconds. So much for sending messages across the universe.
Common Misconception 2: Quantum Teleportation works at faster than light speeds.
Some of you have likely heard of more sophisticated attempts to send information quantum mechanically. The design specs get a little convoluted, but the simplest procedure is:
1. Take an entangled particle pair and keep one for yourself, and send another to your friend.
2. Wait a while (till your friend has gotten good and far away), and then take a particle you'd like to "teleport," (call it "A") and set it to whatever state you like. Teleportation in this case will involve sending the spin of your particle to your friend.
3. Scatter your entangled particle with particle "A."
4. Your friend's particle will now be related to the original spin state of particle "A."
Voila! You've teleported a particle, and you've done so instantaneously, but don't hurt yourself patting yourself on the back. Things get kind of tricky because of the "related to" that I slipped in. The fact is that your friend's particle can actually be in either the spin-up or spin-down state, depending on the final state of particle "A." In other words, to figure out whether you've send a positive image or a negative image of the original, you need to measure particle "A" and then send that info to your friend using telegraph or smoke signals.
Or to put it another way, you are the speed of light's bitch once again.
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (Like us on facebook or follow me on twitter.) He is an Associate Professor of Physics at Drexel University. Feel free to send him your questions about the universe.
Top image by El Caganer.