Bending space-time in the quantum simulator

Relativity works well when you want to explain phenomena at the scale of the universe: for example, gravitational waves generated when black holes collide; quantum theory works well in describing phenomena at the scale of particles (for example, the behavior of individual electrons in atoms). However, combining the two in a fully satisfactory way has not yet been achieved. The search for a "quantum theory of gravity" is considered to be one of the major unsolved tasks in science.

This is because the mathematics in this field is very complex and, at the same time, conducting proper experiments is difficult: we have to create situations in which relativistic phenomena play an important role (e.g., spacetime bent by heavy matter) and, at the same time, quantum effects become apparent (e.g., the dual particle and wave nature of light).

At TU Wien in Vienna, Austria, a new approach has now been developed for this purpose: a "quantum simulator" is used to probe the nature of these problems. Instead of directly studying the system of interest (i.e., quantum particles in curved spacetime), one creates a "model system" and then, by analogy, learns about some of the actual systems of interest from that system. Researchers have now shown that this quantum simulator works very well.

The results of this international collaboration, "Experimental observation of curved light-cones in a quantum field simulator," involving physicists from the University of Crete, Nanyang Technological University and the University of Berlin, are now published in the Proceedings of the National Academy of Sciences (PNAS).

The basic idea of the quantum simulator is simple: many physical systems are similar. Even if they are completely different kinds of particles or different scales of physical systems: at first glance, they have little to do with each other, but these systems may obey the same laws and equations at a much deeper level. This means that one can learn something by studying a particular system.

"We take a quantum system that we know can be well controlled, tuned in an experiment," says Prof. Jörg Schmiedmayer of the Institute for Atomic Research at the Technical University of Vienna: "In our case, these are ultracold atomic clouds, held by an atomic chip with electromagnetic fields to hold and manipulate."

"Suppose you properly tune these atomic clouds so that their properties can be translated into another quantum system. In this case, you can learn something about the other system from measurements of the atomic cloud model system: just like you can learn about the oscillations of a pendulum from the oscillations of a mass attached to a metal spring - they are two different physical systems, but one can be transformed into the other. "

"We have now been able to show that we can produce effects and use them in this way to model curvature similar to that of spacetime." said Mohammadamin Tajik of the Vienna Center for Quantum Science and Technology (VCQ), the University of Vienna, and first author of this paper.

The gravitational lensing effect is an example of an effect explained by the theory of relativity. With quantum particles, a similar effect can be studied.

In a vacuum, light travels along a so-called "light cone". The speed of light is constant; in equal time, light travels the same distance in each direction. However, if light is affected by heavy masses (such as the gravitational force of the sun), these light cones will bend. In curved spacetime, the path of light is no longer perfectly straight - this is called the "gravitational lensing effect".

The same thing can now be seen in the atomic cloud: instead of studying the speed of light, people are studying the speed of sound. "Now we have a system where there is an effect corresponding to the curvature of spacetime or gravitational lensing; but at the same time, it is a quantum system that you can describe in terms of quantum field theory." Mohammadamin Tajik said, "With this, we have a whole new tool to study the connection between relativity and quantum theory."

The experimental setup.

The experiments show that the shape of the light cone, lensing effects, reflections and other phenomena can be demonstrated in these atomic clouds, which is exactly what is expected in a relativistic cosmic system. This has important implications for generating new data for fundamental theoretical studies: solid-state physics and the search for new materials also encounter questions with similar structure and can therefore be answered by such experiments.

"We now want to better control these atomic clouds to determine more far-reaching data. For example, the interactions between particles can still be changed in a very targeted way." In this way, Jörg Schmiedmayer explains, the quantum simulator can reproduce physical situations that are so complex that they cannot be calculated even with supercomputers.

Thus, the quantum simulator will become a new and additional source of information for quantum research: in addition to theoretical calculations, computer simulations and direct experiments. In studying atomic clouds, the team hopes to discover new phenomena that may have been completely unknown up to now, and that also occur in the cosmic, relativistic context.

However, without the observation of tiny particles, they may never have been discovered.

Reference links:

[1] https://www.pnas.org/doi/10.1073/pnas.2301287120

[2]https://arxiv.org/pdf/2209.09132.pdf