How Does Faster-Than-Light Quantum Communication Work?

Every so often, I ask readers to submit their sci/tech questions, so that I can go pester people until I have some answers that I can share with the rest of the class. One  recent question was: “How does faster-than-light quantum communication work?” Short answer: it doesn’t. But of course there’s more to it than that.

Basically, there is no such thing as faster-than-light communication, because nothing can move faster than light. (There is a potential qualification here, which I will explain below.)

However, the idea of quantum communication is worth exploring. There is a type of quantum communication that is based on the concept called quantum entanglement, which goes like this:

There are ways to prepare two subatomic particles so that they interact and are then separated. For example, under some circumstances a photon (or discrete packet of light energy) can be split into two photons, each of which has half the energy of the original photon.

Some of these paired photons are “entangled,” meaning that they have orthogonal, or perpendicular, polarizations. That means, for example, that if the electric field of one of the photons is vibrating vertically, its twin is vibrating horizontally. The same will hold true regardless of the polarization – if one is polarized to vibrate in a clockwise motion, the other vibrates in a counterclockwise motion, et cetera.

According to a common interpretation of quantum mechanics, both photons are in indeterminate states until you measure them. It is important to make this distinction: it’s not simply that you don’t know what the polarization of each photon is until you measure it; instead, the polarization does not take on a definite value until you measure it. No matter how far apart they are, when the polarization of one photon is determined by a measurement, somehow the other photon instantaneously “knows” the outcome and will always be found to be vibrating in a perpendicular direction.

Now, that’s pretty cool – but it can’t really be used for faster-than-light communication purposes for the simple reason that we cannot control the polarization of the entangled particles. And while it may seem that the two photons are able to communicate with each other in some way, we have no idea how.

However, quantum communication does have non-faster-than-light value, and it has to do with security.

Because the act of measuring a photon actually changes the photon, any attempts to eavesdrop by intercepting a transmission of photons would be detected – even if the eavesdropper passes the photon on after intercepting it. Quantum mechanics also has utility in terms of encryption, but I am not getting into that here. If you’re curious, try this or this.

Finally, here’s that qualifier about nothing being faster than light. Scientists have recently reported that neutrinos are capable of traveling faster than light. (There’s a nice overview of it here.)

Many people probably want this to be true, because it would be one of the most exciting discoveries, well, ever. By the same token, most people suspect that this will turn out to be untrue – that the meticulous scientists involved with this project made a mistake somewhere.

However, if it is true, and neutrinos can travel faster than light, we’ll have to make some modifications to our understanding of relativity.

Note: Special thanks to David Aspnes, Distinguished University Professor of Physics at NC State, and Dan Stancil, Alcoa Distinguished Professor of Electrical and Computer Engineering at NC State, for taking the time to talk with me about this subject. Any mistakes found in this post are mine and mine alone.