Mixed Signals
Any sufficiently advanced information is indistinguishable from noise
Friday 13 January 2006, 6:05 PM
Rupert Goodwins' Diary
Tuesday 10/1/2006
One of my favourite treats is getting to write about weird science. As I've yet to make a good commercial case for a Weird Science Web site — the market in advertising for lab coats and protein depolarizing death rays being somewhat soft at the moment — I have to get what I can, where I can. So thanks indeed to Toshiba Research and Cambridge University for coming up with a semiconductor gallium, indium and arsenic quantum dot entangled photon generator. It comes no weirder than this — and I mean that most sincerely.
Not only are quantum dots by far the coolest thing you can make with a handful of atoms, as they give you the chance to control electrons in ways nature never intended, but the resultant light is high on the list of the strangest things in the universe. Entangled photons are like Shakespearian lovers, of one soul but separated by cruel fate (in this case, Toshiba researchers). Hurt one, and the other weeps. Create a pair of them and send them to the opposite ends of the universe: when you specify the state of one, the other is instantaneously affected.
This might seem to break the ultimate law, that nothing can be transmitted faster than the speed of light. Physicists — especially the supremely patient Dr Andrew Shields, who for the second time found himself on the phone to a rather puzzled Rupert — explain patiently that the information about what's happened to the second photon doesn't make sense until you know by ordinary means what you did to the first, so you can't actually use this to annoy Einstein.
Nevertheless, there is something exceptionally strange going on. What mechanism can allow a photon to know what's happening to its soulmate, even if they're forbidden to communicate by the structure of the universe? There are some wild theories, none of which I understand. But this leads to the second joy of weird science — it leads you to some exceptional people. In this case, if you find your curiosity tickled by entanglement, you should seek out John Bell.
Bell was a mild-mannered, bearded Belfast man who did more than anyone else to penetrate into the very strangest parts of quantum physics — and he did so with humour and genius. For example, he wrote a paper concerning a colleague of his, "Bertlmann's Socks and the Nature of Reality". Bertlemann was in the habit of wearing different coloured socks, and Bell pointed out that equipped with a knowledge of Bertlemann's nature and seeing that the sock closest to you was pink, you would instantly know that the other sock was not pink. Even if Bertlemann wore his socks a light year apart, this knowledge would be transmitted instantaneously.
And so it may be with entangled photons. We'd have a better chance of knowing more today if Bell hadn't died of a stroke in 1990 at the criminally early age of 62. For all the physicists talk of locally-reversible time, the version we live in seems unfairly stuck in one direction.
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