black hole
Representational imageistock

It may be possible to retrieve information from a black hole through quantum teleportation, physicists have said. In a paper appearing on arxiv.org, researchers say that in special circumstances, a qubit (a unit of quantum information) could be recovered after entering a black hole.

One of the major conundrums in theoretical physics is what happens to information that goes into a black hole. Known as the information paradox, or black hole information problem, the general theory of relativity says anything pulled into a black hole would be lost forever, but quantum mechanics says information cannot be completely lost.

In the paper, Aidan Chatwin-Davies, Adam Jermyn, and Sean Carroll of the California Institute of Technology in Pasadena, say it is possible to retrieve information using Hawking radiation and quantum teleportation. The information retrieved is quantum information, so can exist in all possible states.

The researchers use the analogy of Alice, who is sitting outside a black hole. Explaining what they found, Chatwin-Davies told IBTimes UK: "Alice first collects one particle of Hawking radiation and then tosses a single particle that she used to encode one qubit of information into the black hole.

"Crucially, the nature of Hawking radiation is such that Hawking particles emitted by the black hole have strong correlations with the inside of the black hole and its properties. Then, by carefully measuring properties of the black hole from the outside once the particle that she tossed in has fallen into the black hole, Alice can exploit these correlations to put the state of the Hawking particle that she holds into the state of the particle that she tossed in.

"The algorithm restores, or rather transfers, the state of the qubit that fell into the black hole into a system that exists outside the hole, namely, the Hawking particle that Alice collects. A good way to think of it is that any information-theoretic task that Alice could have performed with her original qubit she could perform with the Hawking particle at the end of the algorithm."

So the information that has been lost can be retrieved. However, authors note their algorithm only works for one qubit: "These days, the prevailing opinion is that in theory it should be possible to retrieve complete information about anything that falls into a black hole," Chatwin-Davies said.

"An inherent limitation of the protocol that we propose, however, is that it only works for one qubit at a time. Alice could not toss two or more qubits in at once and recover complete information about them using our method."

He said that while the study does not answer the black hole information problem, they showed that it is possible to retrieve a qubit. But more importantly, he added, it shows a better understanding of black holes is needed if the information paradox is to be answered.