Quantum tool opens the door to new phenomena

The temperature profiles obtained by the researchers show that particles that in
The temperature profiles obtained by the researchers show that particles that interact strongly with the environment are "hot" (red) and those that interact little are "cold" (blue). The entanglement is therefore particularly strong where the interaction between the particles is strong. © Helene Hainzer

Theoretical predictions of quantum field theory confirmed experimentally for the first time

Innsbruck scientists led by Peter Zoller have developed a new tool for determining entanglement in many-particle systems and demonstrated it in an experiment. The method enables investigations of previously inaccessible physical phenomena and can contribute to a better understanding of quantum materials. The work has been published in the journal Nature.

Entanglement is a quantum phenomenon in which the properties of two or more particles combine in such a way that it is no longer possible to assign a specific state to each individual particle. Such a system can only be understood if all particles that share a certain state are considered simultaneously. The entanglement of the particles ultimately determines the properties of a material.

"Entanglement of many particles is the property that makes the difference," emphasizes Christian Kokail, one of the first authors of the work now published in Nature. "But it is also very difficult to determine." The scientists led by Peter Zoller at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW) now provide an approach that can significantly improve the investigation and understanding of entanglement in quantum materials. In order to describe large quantum systems and extract information about the existing entanglement, an unimaginably large number of measurements would have to be carried out. "We have developed a more efficient description that allows us to extract entanglement information from the system using random samples," explains theoretical physicist Rick van Bijnen.

In an ion trap quantum simulator with 51 particles, the scientists have imitated a real material by recreating it particle by particle in the laboratory and studying it in a controlled laboratory environment. Only very few research groups worldwide have the necessary control over as many particles as the Innsbruck experimental physicists led by Christian Roos and Rainer Blatt. "In order to guarantee this control, we have highly automated the experiment," explains Manoj Joshi. "Automating the measurement sequences and analyzing them helps us to maintain control over the particles." In the experiment, the quantum simulator is used as a kind of co-processor for a classical computer, which outsources difficult computing tasks to the quantum computer. For the first time, the scientists were able to observe effects in the experiment that had previously only been described theoretically. "We have combined knowledge and methods here that we have laboriously developed together over the past few years. It is impressive to see that it is now possible to do these things with our resources," says a delighted Christian Kokail, who recently joined the Institute for Theoretical Atomic Molecular and Optical Physics at Harvard.

Shortcut via temperature profiles

In a quantum material, the particles can be more or less strongly entangled. Measurements on a strongly entangled particle only provide random results. If the results of the measurements fluctuate very strongly - i.e. if they are purely random - then scientists speak of a "hot" quantum object. If the probability of a certain result increases, it is a "cold" object. Only the measurement of all entangled objects reveals the actual state. In systems consisting of a large number of particles, the effort required for the measurement increases enormously. Quantum field theory has predicted that a temperature profile can be assigned to parts of a system consisting of many entangled particles. These profiles can be used to deduce the degree of entanglement of the particles.

In the Innsbruck quantum simulator, these temperature profiles are determined via a feedback loop between the computer and the quantum system, whereby the computer constantly generates new profiles and compares them with the actual measurements in the experiment. The temperature profiles obtained by the researchers show that particles that interact strongly with the environment are "hot" and those that interact little are "cold". "This corresponds exactly to the expectation that entanglement is particularly high where the interaction between the particles is strong," says Christian Kokail.

Opening the door to new areas of physics

"The methods we have developed provide a powerful tool for investigating large-scale entanglement in correlated quantum matter. This opens the door to researching a new group of physical phenomena with quantum simulators that already exist today," quantum mastermind Peter Zoller is convinced. "With classical computers, such simulations can no longer be calculated with reasonable effort." The methods can also be used to test new theories on such platforms.

The results were published in the scientific journal Nature. The research was financially supported by the Austrian Science Fund FWF, the Austrian Research Promotion Agency FFG, the European Union, the Federation of Austrian Industries Tyrol and others.

Publication: Exploring Large-Scale Entanglement in Quantum Simulation. Manoj K. Joshi*, Christian Kokail*, Rick van Bijnen*, Florian Kranzl, Torsten V. Zache, Rainer Blatt, Christian F. Roos, and Peter Zoller. Nature 2023 DOI: 10.1038/s41586’023 -06768-0 [arXiv: 2306.00057 ]
*these authors contributed equally