Monday, February 2, 2015

Quantum teleportation

I recentlyish finished Scott Aaronson's Quantum Computing Since Democritus.  I enjoyed it quite a bit and it managed to explain some things very well.  One of those things was Quantum teleportation - something I've seen mentioned in the popular science press time and again but which always seems to be conveyed very badly.

Before I get to that explanation, though, I'm going to have to explain two things first: qubits and the no-cloning theorem.  Qubits or Quantum bits are the quantum version of classical bits.  Imagine that if a regular bit is an arrow that can point to the left or to the right to represent 1 or 0 a qubit is an arrow that can point in any direction.  That's totally misleading in all sorts of ways but it will work well enough for the rest of this post.  I'm here to explain quantum teleportation after all.

If you have a regular bit you can do a lot of things with it.  For instance you can copy it.  This very blog post, composed of classical bits, has been copied to your web browser dear reader and from there into your very head.  That's a very nice thing about classical bits but sadly you can't do that with quantum bits.  And qubits are fragile things when exposed to the world, prone to collapse or decoherence (depending on your religion).  You can spread one between multiple locations (via entanglement) but you've got to keep it isolated in each place or the whole thing is lost.

But what if I want to share my qubut with my friend across town?  However you try to send it you have to worry about stray atoms or photons ruining the process.  With classical bits you have sends zillions of electrons for one bit and try again if that doesn't work but the non-copyability of qubits prevents that here.  Unless you have a nice isolated fiber-optic cable or something you're out of luck.

That's where quantum teleportation comes in!  If I have a qubit that's in my house and I want to get it to my buddies house I need a couple of things.  I need to send my buddy a couple of bits of classical information but I can do that over the internet or something.  Also you need a second cubit which is already shared between the two places.  Why are you trying to send this first qubit if you're already sharing a second qubit?  Maybe the first one is really awesome and you need it over there now but the second qubit is just some Joe Schmoe qubit that you put in place a month ago.  But in any event you can spend that shared qubit and that nice easy classical information transfer to send your qubit over to your buddy.  So your qubit can teleport from your house to your buddies without ever having to brave the dangerous outside world.

No comments:

Post a Comment

Book review: The Righteous Mind

The Righteous Mind: Why Good People Are Divided by Politics and Religion  by Jonathan Haidt is a book about morality.  It's about the wa...