Quantum eavesdropping is possible across a black hole鈥檚 event horizon, one of the most impermeable cosmic boundaries 鈥� at least in one direction.
at the University of Chicago, Illinois, wanted to know how the structure of space-time, the fabric of our reality, influences quantum objects. This led him and his colleagues to a thought experiment where two people, Alice and Bob, end up separated by one of space-time鈥檚 most extreme objects.
Alice has a quantum object and puts it into a 鈥�superposition鈥�, which combines two of its ordinary states in a way that can only exist in the quantum realm. Bob wants to covertly obtain information about Alice鈥檚 object. 鈥淗e doesn鈥檛 want to get caught, but he鈥檚 clever and he realises that he could hide inside of a black hole,鈥� says Danielson. Physicists agree that information can鈥檛 escape a black hole, but Danielson and his team wanted to know whether Bob could still somehow achieve his goal. They arrived at a mathematically rigorous 鈥測es鈥� by turning to quantum information theory.
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Quantum superpositions are fragile, so outside influences, such as small disturbances from the environment or measurements by nefarious agents like Bob, can make them change from the superposition to a single, un-mixed state. This process is called decoherence.
The researchers proved mathematically that the information that Bob can obtain from inside the black hole exactly matches the extent of decoherence that Alice would observe in her object on the outside. Danielson presented the work on 17 March at the in California.
Their calculations showed that when Bob disrupts the quantum state of Alice鈥檚 object, it would always look to Alice like he was extracting as much information from it as is allowed by physics. Danielson says this tells us two things about black holes. Strikingly, it implies that every black hole has to decohere superpositions in its vicinity. This is because if Alice could infer that the decoherence she sees is due to Bob鈥檚 eavesdropping, then she would be learning something definitive about the black hole鈥檚 interior, which must always stay hidden according to our best theories of gravity. It also tells us that black holes must be collapsing quantum states around them by extracting as much information possible.
at Vanderbilt University in Tennessee says researchers already knew that black holes are champions of extreme cosmic behaviours, but the new work points to 鈥測et another way in which they are the best at everything they do: black holes can destroy quantum superpositions with their strong gravity, and they do so in the fastest possible way鈥�.
The team also suggests this process requires that extremely low-energy particles exist everywhere in space around black holes. Lupsasca says studying these particles should be part of efforts to formulate a theory of quantum gravity 鈥� the elusive combination of quantum theory and Albert Einstein鈥檚 theory of general relativity into a theory of everything.
鈥淪ince black holes play such an important role in the major open questions about quantum gravity, it is important to have a clear understanding of how their quantum effects differ from those of ordinary bodies,鈥� says at the University of Arizona. He says candidate theories for quantum gravity could be tested and benchmarked against the researchers鈥� calculations.
For Danielson, the new work is a step towards connecting the structure of space-time to the flow of quantum information. For instance, if Bob hid inside a spherical shell made from ordinary matter instead of a black hole, the information flow between him and Alice would differ. This may mean that the structure of space-time isn鈥檛 fundamental, but a consequence of the laws that govern how quantum information can be exchanged. 鈥淭here鈥檚 a hope that this might give us some insight into the way in which space and time themselves could emerge from information theoretic principles,鈥� says Danielson.
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