Black holes as quantum-information mirrors

This is the tale of Alice, Bob and a black hole.

Alice and Bob are a couple with a big communication problem — they only talk using quantum information systems. Usually this involves sending encrypted messages via quantum dots or entangled photons, which are unreliable at the best of times.

But now Caltech’s John Preskill believes that they should try to exchange messages via a black hole. The idea is that Alice sends her message into the black hole where it gets mixed up in whatever goes on inside the event horizon. The event horizon marks the distance from the black hole from which nothing — not even light — can escape, so you would be forgiven for thinking that her message would be lost for ever.

Not so says Preskill — the information is slowly transmitted out of the black hole in the form of Hawking radiation. This is radiation that is thought to slowly leak out of a black hole, eventually causing the black hole to vanish.

All Bob has do is gather the Hawking radiation and use it to build a quantum state that is entangled with the state of the black hole inside the event horizon. Eventually, Alice’s message will leak out with the Hawking radiation, and the entanglement will allow Bob to read it –or something like that!

But, like all Alice and Bob’s other quantum conversations, there are problems — the process would take a very long time, and no-one has actually been able to see Hawking radiation from a black hole. Alice and Bob need to talk this over before they agree to Preskill’s scheme.