# Research Highlights

## Unlocking Quantum Secrets: Ultracold Atoms Pave the Way for Quantum Field Simulation

In the realm of quantum physics, where particles can exist in multiple states simultaneously and fields pervade spacetime, a groundbreaking revolution is underway. Advances in ultracold-atom experiments have unlocked a new avenue for simulating quantum fields, offering us unprecedented insights into the mysteries of the quantum world. Experiments, recently conducted at the Technical University of Vienna in collaboration with an international group of theoretical physicists including Spyros Sotiriadis, assistant professor at the University of Crete, have demonstrated the remarkable ability of ultracold atoms to mimic two physical phenomena predicted by quantum field theory and general relativity: a paradoxical property of quantum information known as the “area law” and the bending of light trajectories when travelling through curved space-time.

The revolution of ultracold atom experiments

At the heart of these cutting-edge experiments lies the remarkable control scientists have gained over ultracold atoms, a state of matter where atoms are cooled to temperatures just a fraction above absolute zero. This extreme chill slows down atoms, bringing them to a near-motionless state and enabling the observation of astonishing quantum effects. By harnessing the power of ultracold atoms, scientists have now transformed them into quantum simulators capable of emulating the behaviour of quantum fields.

Quantum field theory: the theoretical language of particle physics

Quantum field theory is a theoretical framework that combines quantum mechanics and special relativity to describe the behaviour of fundamental particles and their interactions. It provides a mathematical formalism for understanding the quantum nature of fields, which are pervasive in the universe and can be thought of as continuous quantities that exist throughout space and time. Quantum field theory has played a central role in theoretical physics, particle physics, and the development of our current understanding of the physical world.

The paradoxical “area law” of quantum information

In the context of quantum theory, the term "area law" refers to a specific scaling behaviour of the entanglement entropy or mutual information, which are measures of the information content of quantum systems. The area law states that in many physically relevant scenarios, the entanglement entropy or mutual information between two spatially separated regions of a quantum system tends to scale with the surface area of their boundary. This behaviour is in contrast to a "volume law" where the entanglement or mutual information scales with the volume of the regions. The area law implies that the entanglement or information content of a quantum system is predominantly influenced by particle or field interactions taking place near the interface of the regions, rather than their entire volume, and has profound implications for the study of quantum information, quantum field theory, and condensed matter physics. It underlies the efficiency of certain algorithms for the simulation of quantum systems in standard computers, and plays a crucial role in understanding the emergence of classical behaviour from quantum physics.

Light bending in curved spacetime

In the realm of general relativity, the fabric of spacetime is not static but dynamic, capable of bending and warping under the influence of massive objects. This curvature of spacetime leads to the phenomenon of light bending, where the path of light rays is altered as they traverse regions with strong gravitational fields. As light follows the curved contours of spacetime, it appears to be deflected from its straight trajectory, showcasing the intricate interplay between gravity and the propagation of light. This captivating phenomenon has not only provided evidence for Einstein's theory of general relativity but has also allowed astronomers to observe and study distant celestial objects, unveiling the secrets of the universe.

Ultracold atomic gases serving as quantum field simulators

Through ingenious experimental setups, scientists have now engineered ultracold-atom systems that exhibit striking parallels to these two phenomena encountered in quantum field theory. The collective behaviour of such systems, known as ‘quantum gases’, is effectively described by models of quantum field theory. Quantum field parameters like the mass of particles and speed of light are associated to emergent characteristics of the atomic gas like the spatial extent of atomic correlations and the speed of information propagation, respectively. Similarly, the externally controlled potential that confines the gas can be manipulated to mimic the effect of gravity on the geometry of spacetime as felt by photons in general relativity. By carefully monitoring these systems, researchers have successfully observed the analogues of the area law scaling of mutual information and bending of light trajectories in curved geometry. These fascinating phenomena were once confined to the realm of theoretical predictions or astronomical observations, but can now be witnessed and studied in the controlled environment of ultracold-atom experiments.

The implications of these achievements extend far beyond the laboratory. By leveraging ultracold-atom based quantum simulations, researchers can unlock profound insights into the behaviour of quantum fields and their influence on fundamental particles. This knowledge holds the potential to revolutionise fields such as materials science, quantum computing, and high-energy physics. Moreover, it enables us to explore and understand exotic physical phenomena, paving the way for breakthroughs in technology and our understanding of the intricate workings of the universe.

References:

[1] Experimental Observation of Curved Light-Cones in a Quantum Field Simulator,

M. Tajik, M. Gluza, N. Sebe, P. Schüttelkopf, F. Cataldini, J. Sabino, F. Møller, S.-C. Ji, S. Erne, G. Guarnieri, S. Sotiriadis, J. Eisert, J. Schmiedmayer, PNAS 120, e2301287120 (2023) https://doi.org/10.1073/pnas.2301287120

[2] Verification of the Area Law of Mutual Information in a Quantum Field Simulator,

M. Tajik, I. Kukuljan, S. Sotiriadis, B. Rauer, T. Schweigler, F. Cataldini, J. Sabino, F. Møller, P. Schüttelkopf, S.-C. Ji, D. Sels, E. Demler, J. Schmiedmayer, Nat. Phys. (2023). https://doi.org/10.1038/s41567-023-02027-1

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