Researchers Perform Experiments to Differentiate Between Quantum Scrambling and True Information Loss

Scientists at the Joint Quantum Institute have executed an experimental test for quantum scrambling, a disordered shuffling of information stored among a group of quantum particles.

Their experiments performed on a collection of seven atomic ions, published in the March 7^th, 2019 issue of Nature, show an innovative means to differentiate between scrambling—which retains the amount of information in a quantum system yet jumbles it up—and true information loss. Someday, the protocol could help assess the calculations of quantum computers, which tap the quantum physical rules to process information in innovative ways.

“In terms of the difficulty of quantum algorithms that have been run, we’re toward the top of that list,” stated Kevin Landsman, a graduate student at JQI and the lead author of the new paper. “This is a very complicated experiment to run, and it takes a very high level of control.”

The team of researchers, including JQI Fellow and UMD Distinguished University Professor Christopher Monroe and JQI Fellow Norbert Linke, carried out their scrambling tests by cautiously controlling the quantum behavior of seven charged atomic ions by the application of well-timed sequences of laser pulses. They discovered that it is possible to correctly diagnose whether information had been scrambled through the entire system of seven atoms with an accuracy of approximately 80%.

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.

Kevin Landsman, Graduate Student, Joint Quantum Institute.

Quantum scrambling is somewhat similar to shuffling a fresh deck of cards. Initially, the cards are ordered in a sequence, from ace to king, and the suits appear one after another. As soon as it is shuffled adequately, the deck seems mixed up; however, most importantly, there is a means to reverse that process. For painstakingly keeping a track of how the cards were exchanged for each shuffle, it would be simple (yet tedious) to “unshuffle” the deck by performing a repeat of all those swaps and exchanges in reverse.

Quantum scrambling is analogous to this since it involves mixing up the information stored into a set of atoms and can even be reversed, which is an important difference between scrambling and real, irreversible information loss. Landsman and his team harnessed this fact to their benefit in the new test by scrambling up a first set of atoms and executing a related scrambling operation on a second set. A mismatch between the two operations would signify that the process was not scrambling, leading to failure of the final step of the method.

That final step was dependent on quantum teleportation—a technique for transferring information between two quantum particles that are prospectively very far from each other. In the new experiment, the teleportation is over moderate distances—a mere 35 μm separates the first atom from the seventh—however, it is the signature through which the team identifies scrambling: a successful teleportation of information from one atom to another indicates that the state of the first atom is distributed across all of the atoms—something that occurs only if the information is scrambled. In the event of loss of the information, successful teleportation would not be feasible. Hence, in the case of an arbitrary process, the scrambling properties of which might be unknown, this technique could be used to test whether—or even to what extent—it scrambles.

According to the researchers, earlier tests for scrambling failed to capture the variation between hidden and lost information, mainly because individual atoms appear quite similar in both cases. The new protocol, first put forward by theorists Beni Yoshida of the Perimeter Institute in Canada and Norman Yao at the University of California, Berkeley, differentiates between the two cases by taking correlations between specific particles into account in the form of teleportation.

When our colleague Norm Yao told us about this teleportation litmus test for scrambling and how it needed at least seven qubits capable of running many quantum operations in a sequence, we knew that our quantum computer was uniquely-suited for the job.

Norbert Linke, Fellow, Joint Quantum Institute.

Originally, the experiment was based on the physics behind black holes. For a long time, researchers have made speculations of what happens when something falls into a black hole, specifically if that thing is a quantum particle. The basic rules of quantum physics propose that irrespective of what a black hole does to a quantum particle, it should be reversible—an estimation that appears to be inaccurate due to the tendency of a black hole to crush things into an infinitely small point and emitting out radiation. However, with the lack of a real black hole into which things can be thrown into, scientists have been stuck just predicting.

Quantum scrambling is one indication of how information can get into a black hole and come out as random-looking radiation. It could be that it is not random at all, and black holes are just exceptional scramblers. The study discusses this motivation and even an interpretation of the experiment that involves comparing quantum teleportation to information that goes through a wormhole.

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.

Christopher Monroe, Distinguished Professor, University of Maryland.