“So, when will you be able to teleport me out to visit you?”
November 20, 2012 3 Comments
That’s a question my friends and family ask me (jokingly) on a regular basis, given that my graduate studies involve quantum mechanics and given that “quantum teleportation” is perhaps second in popularity only to “exoplanets” as a topic for popular science writing.
Seriously, there are so many articles about it. And that’s not a bad thing: I love anything that gives more exposure to the genuine awesomeness of quantum physics. Plus the field is undeniably hot right now: just as it seems like a new exoplanet is discovered every month, so to are different teams of physicists seemingly breaking teleportation records for size and distance.
But despite the current media exposure, the idea is, I think, still poorly understood. True, most folks are all well-informed enough by now to understand that when physicists say “quantum teleportation,” they don’t intend you to imagine them saying it in a Scottish accent, because just about every article on the subject dutifully clarifies that this ain’t quite the same thing as that shimmery trick Scotty does on “Star Trek.” But while everyone’s clear on what quantum teleportation is not, understanding just what it is can be a mite more difficult.
Part of the reason quantum teleportation is hard to understand is because it is the brilliant solution to a problem you would not have realized existed unless you already understood quantum mechanics. But if you were a quantum physicist before the invention of quantum teleportation in the 1990s, there’s a good chance you were extremely frustrated by the following facts: First, anything small enough to be governed by quantum mechanics (atoms or smaller, roughly speaking) is always in a very “fragile” state. Atoms, for example, come with a little built in arrow (for complicated reasons called a “spin”) which points in some direction, but will happily point in a different direction of you give it the slightest of bumps. So if you’re studying an atom and want to bring it to a colleague to consider, you can’t just pick it up with tiny tweezers, at least not if you were planning on studying anything that had to do with the state of it’s spin arrow.
One way around this problem would be to make a million copies of your atom, and then try carrying them one at a time over to your collaborator. Ah, but quantum mechanics has you there too: it turns out to be impossible to make an exact duplicate of a quantum object. The laws of physics will always conspire to cause some mistake to be introduced.
This is a frustrating situation. What good is it to try to study the state of a quantum system of it falls apart the minute you subject it to any intense scrutiny, and also won’t allow you to make backup copies? Well, physicists are clever people, so of course they invented many, many ways to get interesting information out of an atom without having to carry it around. Nevertheless, the fundamental inability to move these atoms reliably must have seemed like an unnecessary and frustrating complication.
Quantum teleportation is the solution to this problem. It is a procedure whereby you can take a quantum object like an atom, and move its properties onto a different atom that may be far far away. This is done by means of quantum entanglement, a topic elaborate enough that it will deserve its own (forthcoming) blog post. For the moment, suffice it to say that it is a kind of “kinship” which can be induced between particles such that what you do to one atom will influence the other, regardless of how far apart the two become.
Suppose you and your friend create a pair of entangled atoms, who share this special bond. Call them atoms A and B. Your friend takes atom A and goes to his lab, leaving you with the other half of the pair, which still feels some deep connection to the other. When you go to your own lab, with atom B still in your pocket, you find a third atom, “C“, which you think your friend would be interested in studying. By performing certain clever actions with both atom C (the one you which to share) and atom B (which is still entangled with A) you can use B as an intermediary to cause atom A to assume all the properties of your atom used to have– despite being all the way down the hall. I say “used to have” because of course, atom C is not allowed to keep all its features. Remember, you are not allowed to duplicate quantum systems. So the teleportation process inevitably changes atom C in order to give its properties to atom B. But it does leave anyone in possession of atom B with what is, for all intents and purposes, the exact same atom you were just looking at. In many ways, teleportation is like a long range, voluntary version of atomic identity theft.
So how is this helpful? Well, aside from sharing atoms that are found lying around (which physicists do not have to do as much as I may have just suggested) the system can also be used to share information. You could, for example, plan a code with a friend, where an atom whose spin points up stands for “0” and an atom whose spin points down stands for “1.” Thus, by preparing a bunch of your own atoms so that they encode some binary message, you could then send this message along by “teleporting” the states of these atoms onto some atoms that your friend keeps around for just such an emergency. For various reasons, communication this way could be done in principle in a much more efficient and secure manner than just about any other known way of sending signals. Hence, when physicists practice teleporting quantum states some 90 miles across the Canary Islands, they aren’t just doing it for the bragging rights. Some day, they hope to be able to use such techniques to communicate with satellite networks, and hence, to connect atoms around the entire globe.
And what of the prospects of beaming ones’s self to the Enterprise? The answer may surprise you, given the fact that the first words out of a physicists mouth when they mention quantum teleportation are always “It’s not like Star Trek!” In point of fact, nothing stops one from using this procedure on whole collections of atoms at once, and if you can transfer all the properties of every atom in an arrangement onto other atoms, you could indeed send whole objects from place to place. Quantum teleportation is not fundamentally forbidden from sending an apple across the room, so long as you have an identical pile of all the atoms in an apple waiting on the other side to receive their new apple-like identities.
But don’t sell your airline stock too soon: while not literally forbidden by physics, such a feat would require a level of engineering precision and computational power entirely unknown to mankind. There are billions of billions of atoms (actually many, many times more than that) in an apple, and each one would have to be individually tracked and manipulated if they were to be teleported. It’s not even clear that they could be teleported while still held together in an apple. They might have to be individually separated before they could be sent, and what’s the point of teleporting an atomized apple?
Nevertheless, scientists will keep pushing that boundary, as a group from China and Germany did this very week (original paper here). These fellows were able to take a cloud containing some ten million atoms and send them to a different cloud 150 meters away. The cloud set a record by being big enough, at 1mm in diameter, to be visible with the naked eye.
Unfortunately, ten million atoms may sound like a lot, but it is a long, long way from the billions of billions of atoms in the aforementioned apple. Calling this a step towards apple teleportation is a bit like calling it a step towards paying down the national debt when someone at the treasury department finds a penny on the ground. But ten million atoms can be used to store one really long binary message, and the size of the cloud helps make it less likely that the message will be lost to the intrinsic fragility of quantum states.
So to summarize, if you’ve just made friends with a physicist in hopes that he will be able to transport you to the coast for your summer vacation this year free of charge, it may be time to re-evaluate your relationship. On the other hand, if you decide you still want to stay friends with him, I have good news: a whole new wave of telecommunications capabilities may be coming your way courtesy of quantum teleportation in the not-too-distant future, so the two of you will have no trouble staying in touch!