Testing gravity with large quantum systems: what we know today
One of the greatest unsolved mysteries in physics is the unification of quantum mechanics and gravity. The core challenge arises from the fact that quantum systems, typically among the smallest physical systems, have extremely small masses. Measuring the gravitational field of such tiny masses is beyond the reach of current technology. As a result, even the seemingly simple question of how gravity behaves when a particle is in a quantum superposition remains unanswered experimentally. While numerous theoretical approaches have been proposed to unify quantum mechanics and gravity, the absence of experimental evidence makes it difficult to determine the correct path forward.
"My research areas include quantum information, quantum optics, linear and non-linear quantum dynamics, quantum behaviour of gravity and weak forces."
Sofia Qvarfort is an assistant professor at Fysikum, Stockholm University and Nordita through the Wallenberg Initiative for Networks and Quantum Information. She works on quantum physics for the development of quantum technologies such as quantum sensing for gravity and weak forces. She also works on developing analytical models for quantum systems with non-linear dynamics.
Exploring gravitational effects of quantum superpositions
Traditionally, the search for quantum gravity has focused on high-energy physics. The idea is that by probing nature at increasingly higher energies, quantum effects of gravity might eventually become accessible. However, current particle accelerators operate at energy scales around 104 GeV, while the Planck scale where quantum gravity effects are expected to become significant is at 1019 GeV. Bridging this gap remains an enormous technological challenge.
An alternative approach is to explore the gravitational effects of quantum superpositions in larger, more massive systems. If we could place a sufficiently massive quantum system in superposition and measure its gravitational field, what would we observe? Recent advancements in cooling techniques have made it possible to bring increasingly large quantum systems to their ground state, allowing quantum effects to become more pronounced. As the mass of such systems continues to increase, we may eventually reach a regime where their gravitational fields can be probed—or even where entanglement due to gravity can be tested.
In this review, we provide a snapshot of the current state of the field by gathering key ideas and proposals for such experiments. We also offer a pedagogical introduction to the fundamental challenges of unifying quantum mechanics and gravity, along with an overview of the theoretical tools required to model massive quantum systems. We hope this review will be valuable to anyone interested in the topic, particularly students looking to deepen their understanding of this fascinating frontier in fundamental physics.
More information
Sofia Qvarfort, Associate Professor and Postdoctoral Researcher at the Department of Physics and Nordita
Meet Sofia Qvarfort, an amazing physicist - video interview by Student Ambassador Furaha Bayibsa
Massive quantum systems as interfaces of quantum mechanics and gravity, research article in Physical Review Journals
Last updated: March 10, 2025
Source: Gunilla Häggström, Communications Officer, Fysikum