The theory of relativity has proven effective in explaining phenomena on a cosmic scale, such as gravitational waves resulting from the collision of black holes. On the other hand, quantum theory has been successful in describing the behavior of particles on a microscopic level, like individual electrons within an atom. However, the challenge lies in combining these two theories seamlessly, which remains an outstanding task in scientific research.

The difficulty in achieving this synthesis is partly due to the intricate mathematics involved. Moreover, conducting suitable experiments presents a significant hurdle. To observe the simultaneous influence of both relativity and quantum effects, one would need to create scenarios where phenomena from both theories are evident. For instance, this could involve a spacetime that is curved by massive objects, while quantum properties like the dual nature of light are also observable.

In a noteworthy development at TU Wien in Vienna, Austria, researchers have made progress in addressing this challenge. They have devised a technique known as a “quantum simulator” to delve into these questions. Rather than directly investigating the actual system of interest, which involves quantum particles in curved spacetime, they construct a “model system.” By analyzing this analogous system, valuable insights about the original system can be obtained. The researchers have demonstrated the effectiveness of this quantum simulator, yielding promising results.

This significant achievement is the outcome of a collaborative effort among physicists from the University of Crete, Nanyang Technological University, and FU Berlin. Their findings have been published in the esteemed journal Proceedings of the National Academy of Sciences (PNAS).

## Learning from one system about another

The concept underlying the quantum simulator is straightforward: Many physical systems exhibit similarities. Despite being composed of distinct particles or operating on different scales, these systems may adhere to the same underlying laws and equations. Consequently, by studying one system, valuable insights about another system can be gained, even if they initially appear unrelated.

“We choose a quantum system that we can manipulate and control with great precision in experimental settings,” explains Professor Jörg Schmiedmayer from the Atomic Institute at TU Wien. “In our case, we work with ultracold atomic clouds that are held and manipulated using an atom chip employing electromagnetic fields.”

By appropriately configuring these atomic clouds, their characteristics can be mapped onto another quantum system. Consequently, by observing and measuring the atomic cloud model system, one can acquire knowledge about the target system—similar to how the behavior of a pendulum can be understood by analyzing the motion of a mass connected to a metal spring. Despite being distinct physical systems, they can be correlated and provide insights into each other.

## The gravitational lensing effect

“We have successfully demonstrated the ability to generate effects that mimic the curvature of spacetime using this method,” explains Mohammadamin Tajik, the first author of the research paper and affiliated with the Vienna Center for Quantum Science and Technology (VCQ) at TU Wien.

In the absence of any external influences, light propagates along a structure known as a “light cone.” As a fundamental constant, the speed of light remains consistent, meaning that it travels the same distance in each direction over equal time intervals. However, when subjected to the gravitational pull of massive objects like the sun, these light cones become distorted. The paths taken by light in curved spacetime deviate from perfectly straight lines, resulting in what is known as the “gravitational lens effect.”

Remarkably, a similar phenomenon has now been observed in atomic clouds. Instead of examining the speed of light, researchers focused on the speed of sound within these clouds. “We have created a system in which an effect akin to spacetime curvature or gravitational lensing manifests itself, while being a quantum system that can be described using quantum field theories,” Mohammadamin Tajik explains. “This breakthrough provides us with an entirely new tool for investigating the relationship between relativity and quantum theory.”

## A model system for quantum gravity

The conducted experiments have successfully demonstrated that the atomic clouds exhibit the expected characteristics observed in relativistic cosmic systems, including the shape of light cones, lensing effects, and reflections. This not only provides valuable data for fundamental theoretical research but also holds relevance for solid-state physics and the exploration of new materials, as they often involve similar underlying principles and can benefit from these experiments.

The researchers aim to enhance their control over the atomic clouds to obtain even more extensive data. By precisely manipulating the interactions between particles, they can recreate highly complex physical scenarios that are beyond the capabilities of even the most powerful supercomputers to calculate.

Consequently, the quantum simulator serves as an additional and novel source of information for quantum research, complementing theoretical calculations, computer simulations, and direct experiments. Through the study of these atomic clouds, the research team anticipates the discovery of previously unknown phenomena occurring on a cosmic and relativistic scale. Without investigating these minute particles, such phenomena may have remained concealed, underscoring the importance of exploring quantum systems to unveil new aspects of the universe.

Source: Vienna University of Technology