The “information paradox” of black holes has puzzled astronomers and physicists for years. Famed physicist Stephen Hawking devoted his last days to uncovering their secrets. But, researchers using a seven-qubit quantum computer have now started to unlock this mystery.
As matter falls into a black hole, all of the information about the particles — including their energy and momentum — gets scrambled together with all the other matter and energy within the black hole, seemingly lost forever. However, this is not supposed to happen — the laws of quantum mechanics state that information can never be lost, even when it enters the event horizon of a black hole, the boundary from which even light cannot escape.
In the mid-1970’s, Hawking realized that black holes can evaporate over time, releasing their energy into space, until they disappear. As particle/anti-particle pairs are generated near the event horizon of a black hole, it is possible for one member of the pair to fall into the black hole, while the other escapes to space. Over time, the body would lose mass. However, this is an extremely slow process — a black hole the size of our Sun would take 10⁶⁷ years (that’s a one with 67 zeros after it) to evaporate. This is trillions of times longer than the age of the Universe.
This process became known as Hawking radiation. This presented a problem — even if information were stored within black holes, how could it be saved following evaporation? Once a black hole losses nearly half its mass, it should be possible to measure quantum states within it, but even this represents an unreasonably long period of time.
Pairs of entangled quantum bits are so closely linked to each other that the state of one automatically determines the state of the other, regardless of how distant the two are from each other. Albert Einstein referred to this process as “spooky action at a distance.” Measurements of this process reveals the “teleportation” of quantum information from one particle to its quantum partner. Ironically, the more thoroughly information is scrambled within a black hole, the easier it should be to recover reliably through teleportation.
Quantum computers, a revolutionary form of computing which uses qubits of information (instead of bits). These qubits can exist not just as ones and zeros, but use the properties of a quantum system such as the spin of an electron or the polarization of a photon, to store and process information.
“Quantum theory, developed in the early 1900’s, revolutionized physics and chemistry by successfully explaining the weird behavior of tiny particles like atoms and electrons. In the late twentieth century it was discovered that it applied not just to these particles, but to information itself,” IBM explains in an introduction to quantum computing.
Researchers realized that by “dropping” a qubit — a quantum bit — of information into a virtual black hole, and measuring the Hawking radiation shown to be emitted from outside its boundary, it should be possible to measure the information contained within the black hole itself.
“One can recover the information dropped into the black hole by doing a massive quantum calculation on these outgoing Hawking photons. This is expected to be really, really hard, but if quantum mechanics is to be believed, it should, in principle, be possible. That’s exactly what we are doing here, but for a tiny three-qubit ‘black hole’ inside a seven-qubit quantum computer,” said Norman Yao, assistant professor of physics at UC Berkeley.
Out-of-time-ordered correlation functions (OTOC’s) are measurements of pairs of quantum states that differ in the timing of when perturbations, or kicks, are applied to their systems. In order to truly understand these, one must look both forward and backward in time, in order to measure the effect the second “kick” has on the first.
A scrambling quantum circuit was created on three qubits of the quantum computer, and the decay of OTOC’s were measured. Scrambling was shown to have occurred due to predicted effects, similar to processes predicted to happen within black holes. Environmental “noise” can also result in the decay of quantum states within particles, but this was accounted for in the study.
“One possible application for our protocol is related to the benchmarking of quantum computers, where one might be able to use this technique to diagnose more complicated forms of noise and decoherence in quantum processors,” Yao explained.
The results of this research might not only be applied to cosmology and astronomy, but also to the studies of high-energy systems, studies of condensed matter, and molecular and optical physics.