Source: Sci News

Scrambling is what happens when matter disappears inside a black hole.

The information attached to that matter — the identities of all its constituents, down to the energy and momentum of its most elementary particles — is chaotically mixed with all the other matter and information inside, seemingly making it impossible to retrieve.

This leads to a so-called ‘black hole information paradox,’ since quantum mechanics says that information is never lost, even when that information disappears inside a black hole.

So, while some theoretical physicists claim that information falling through the event horizon of a black hole is lost forever, others argue that this information can be reconstructed, but only after waiting an inordinate amount of time — until the black hole has shrunk to nearly half its original size.

Black holes shrink because they emit Hawking radiation, which is caused by quantum mechanical fluctuations at the very edge of the black hole and is named after the late physicist Stephen Hawking.

Unfortunately, a black hole the mass of our Sun would take about 10⁶⁷ years to evaporate — far, far longer than the age of the Universe.

However, it may be possible to retrieve this infalling information significantly faster by measuring subtle entanglements between the black hole and the Hawking radiation it emits.

Two bits of information — like the quantum bits, or qubits, in a quantum computer — are entangled when they are so closely linked that the quantum state of one automatically determines the state of the other, no matter how far apart they are.

Physicists sometimes refer to this as spooky action at a distance, and measurements of entangled qubits can lead to the teleportation of quantum information from one qubit to another.

“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 team member Dr. Norman Yao, a physicist at the University of California Berkeley.

“By dropping an entangled qubit into a black hole and querying the emerging Hawking radiation, you could theoretically determine the state of a qubit inside the black hole, providing a window into the abyss.”

In their experiments, the researchers effectively measured out-of-time-ordered correlation functions (OTOCs), which are created by comparing two quantum states that differ in the timing of when certain kicks or perturbations are applied. The key is being able to evolve a quantum state both forward and backward in time to understand the effect of that second kick on the first kick.

The team created a scrambling quantum circuit on three qubits within a seven-qubit trapped-ion quantum computer and characterized the resulting decay of the OTOC.

While the decay of the OTOC is typically taken as a strong indication that scrambling has occurred, to prove that they had to show that the OTOC didn’t simply decay because of decoherence — that is, that it wasn’t just poorly shielded from the noise of the outside world, which also causes quantum states to fall apart.

The scientists proved that the greater the accuracy with which they could retrieve the entangled or teleported information, the more stringently they could put a lower limit on the amount of scrambling that had occurred in the OTOC.

They measured a teleportation fidelity of about 80%, meaning that perhaps half of the quantum state was scrambled and the other half decayed by decoherence. Nevertheless, this was enough to demonstrate that genuine scrambling had indeed occurred in this three-qubit quantum circuit.

“With scrambling, one particle’s information gets blended or spread out into the entire system. It seems lost, but it’s actually still hidden in the correlations between the different particles,” said Kevin Landsman, a graduate student at the Joint Quantum Institute.

“Regardless of whether real black holes are very good scramblers, studying quantum scrambling in the lab could provide useful insights for the future development of quantum computing or quantum simulation,” said Dr. Chris Monroe, a physicist at the University of Maryland.

The research is published in the journal Nature.