Understanding the interactions between quantum physics and gravity within a black hole is one of the thorniest problems in physics, but quantum computers could soon offer an answer.
A quantum computer capable of simulating the exotic behaviour of a black hole and answering questions at the frontier of modern physics could be possible in the next decade, according to one of the world’s leading theoretical physicists.
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| Black holes hold many mysteries Vadim Sadovski/Shutterstock |
One long-standing goal for physicists like Juan Maldacena at the Institute for Advanced Study in Princeton, New Jersey, is to unite the theories of quantum mechanics and gravity. Maldacena and others think black holes might contain clues about a theory of quantum gravity, because they are both very small and incredibly dense, but they are impossible to study up close and hard to simulate in detail on existing computers due to their complexity.
Even simplified models, such as one developed in the 1990s that consists of a tiny universe containing a single black hole, have so far proved beyond our computational capabilities.
“These types of universes, because they are very small, the quantum gravity effects are relatively large,” says Maldacena. “So maybe some lessons that can be understood for quantum gravity in these types of model universes can be extracted and learned in order to apply it to our own universe.”
Quantum computers could offer an edge over ordinary ones for simulating this toy universe, by producing finely controlled interactions between their quantum bits, or qubits.
Maldacena has crunched the numbers on how powerful a quantum computer would need to be to achieve this, finding that it would require around 7000 qubits.
However, these qubits would need error correction, as they are susceptible to interference and noise, which can distort the results. Quantum error correction is still a burgeoning field, but a reasonable estimate is that each error-corrected qubit will require 1000 others. That brings Maldacena’s total closer to 1 million qubits.
Companies like IBM and Google are aiming to build devices this large in the next 10 years or so, but they still have some way to go. The current record holder is IBM’s Osprey computer, which has 433 non-error-corrected qubits.
Less powerful machines could also be useful, though. Maldacena says that this particular simulation might not need perfect error correction, as long as you don’t mind ignoring some of the features of a black hole, so the total number of qubits might be more reasonable. “The number of qubits is comparable to the numbers you would need for breaking RSA encryption, for example,” he says, referring to one of the major techniques for keeping data secure.
To verify that such a computer has simulated a truly quantum black hole, Maldacena says researchers should be able to look for patterns of energy in certain frequencies, similar to those seen in gravitational waves produced by black holes in our own universe, which are predicted by general relativity. Seeing these in the simulated black holes would be an indication we have created something that behaves like a black hole, he says.
Although some numerical calculations for this simple model can be run on classical computers, they are limited in the sorts of information they can produce and are frozen in points of time, says Masanori Hanada at the University of Surrey, UK. Implementing this model on a quantum computer would allow for the model to be run forwards in time, which could help us understand how quantum gravity works through time in our own universe, he says.
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