My colleague-in-blogging Ethan Siegel does a regular reader-request feature, and this week’s edition drifts into my territory, answering a question about using quantum entanglement for faster-than-light communication. As I jokingly said on Twitter, Ethan’s answer is pretty good for an astronomer, but there are bits of it I don’t like. Not because he gives a wrong answer– he correctly notes that it’s not possible to use entangled particles to send messages faster than the speed of light– but because there’s a lot of subtle and complex stuff at work here. I’ve written about this before, but it’s useful to poke at this a little more because the phrasing I dislike ties into some fairly deep stuff.
Schematic of the third Aspect experiment testing quantum non-locality. Entangled photons from the source are sent to two fast switches, that direct them to polarizing detectors. The switches change settings very rapidly, effectively changing the detector settings for the experiment while the photons are in flight. (Figure by Chad Orzel)
The basic scenario for entanglement-based communication looks like this: two people, traditionally named “Alice” and “Bob” share a pair of particles that can each be measured in one of two quantum states, which we’ll call “0″ and “1.” These particles are prepared in an entangled state in which a measurement of the state of Alice’s particle is correlated with the measured state of Bob’s particle, no matter how far apart they are. That is, if Alice measures her particle in state 1 at precisely noon in Schenectady, she knows that Bob in Portland will also measure his particle to be in state 1, whether he’s in Portland, Maine, Portland, Oregon, or Portland Station on one of the moons of Yavin.
This seems like a perfect mechanism for sending information over vast distances, as Ethan notes: