Paradoxical quantum phenomenon measured for the first time

Newswise —

Certain phenomena are connected, while others are not. Assume you choose a person at random from a group who is considerably above average height. It is probable that they will also exceed the average weight. Numerically, one variable carries some data regarding the other.

The realm of quantum mechanics permits even more robust correlations between various variables: distinct components or particles within a vast quantum framework can "share" a specific quantity of data. There exist intriguing theoretical forecasts concerning this phenomenon: remarkably, the degree of this "reciprocal information" relies not on the magnitude of the system but solely on its exterior. This startling outcome has been validated experimentally at TU Wien and disclosed in Nature Physics. The Max-Planck-Institut für Quantenoptik in Garching, FU Berlin, ETH Zürich, and New York University provided theoretical insights into the experiment and its interpretation.

Quantum information: More strongly connected than classical physics allows

"Let us envision a container of gas in which minute particles move and act in a classically conventional manner, akin to tiny spheres," explains Mohammadamin Tajik, the primary author of the present study and a member of the Vienna Center for Quantum Science and Technology (VCQ) - Atominstitut at TU Wien. "If the system is in balance, the particles situated in various portions of the container are oblivious of one another. They can be regarded as entirely distinct from each other. Thus, it is safe to conclude that the shared information between these two particles is non-existent, i.e., zero."

However, the scenario changes in the quantum domain: When particles behave in a quantum manner, it is possible that they can no longer be viewed as distinct from each other. They are mathematically linked, and describing one particle without mentioning the other loses its meaning.

Mohammadamin Tajik clarifies, "In such situations, there has been a theoretical projection regarding the shared mutual information among various subsystems of a many-body quantum system for quite some time. In a quantum gas of this sort, the shared mutual information surpasses zero and is independent of the subsystem's size but rather relies only on the outer boundary surface of the subsystem."

This theoretical prediction may seem counterintuitive since the classical world operates differently. For instance, the information contained within a book is influenced by its volume, not merely the surface area of the book cover. However, in the quantum realm, information is frequently connected to the surface area of the system, rather than its volume, leading to some perplexing outcomes.

Measurements with ultracold atoms

A team of international researchers, led by Prof. Jörg Schmiedmayer, has recently provided the first experimental evidence supporting the theoretical projection that mutual information in a many-body quantum system scales with surface area, rather than volume. To conduct this research, they analyzed a cluster of ultracold atoms that were cooled to a temperature just above absolute zero and immobilized on an atom chip. At these incredibly low temperatures, the particles' quantum properties become increasingly significant, and information spreads out more extensively in the system. As a result, the connection between the different components of the entire system becomes more relevant. In such a scenario, the quantum field theory can be used to explain the system's behavior.

Jörg Schmiedmayer explains, "Conducting this experiment was extremely challenging since we required comprehensive information about our quantum system, as precise as quantum physics permits. To accomplish this, we developed a specialized tomography technique. We obtained the necessary information by slightly altering the atoms and then observing the resulting dynamics. It's comparable to tossing a rock into a pond and then using the resulting waves to obtain information about the liquid and the pond's state."

Unless the temperature of the system reaches absolute zero, which is not feasible, the "shared information" has a limited range in the quantum world. This range is related to the "coherence length," which determines how far particles can quantumly behave similarly and hence have knowledge of each other. Mohammadamin Tajik explains, "This also explains why shared information is insignificant in a classical gas. In a classical many-body system, coherence disappears, and the particles no longer have any knowledge of their neighboring particles." The experiment also validated the influence of temperature and coherence length on mutual information.

The results of the experiment are highly relevant to several research areas, including solid-state physics and the quantum physics of gravity, as quantum information plays a crucial role in various technological applications of quantum physics.