In a groundbreaking study recently published in Scientific Reports, researchers have demonstrated the power of digital quantum simulation to model particle creation in an expanding universe. This simulation utilized IBM’s quantum computers to explore quantum field theory in curved spacetime (QFTCS), marking a significant leap forward in the study of cosmology and quantum physics. The research paves the way for using quantum computing to investigate complex, cosmological events that were previously difficult to simulate.
While quantum gravity, a complete theory that would merge quantum mechanics and general relativity, remains elusive, QFTCS offers an alternative path to understanding the quantum nature of phenomena occurring in curved or dynamic spacetimes—like those encountered during cosmic inflation. This approach treats spacetime as a classical backdrop while treating matter and force fields quantum mechanically. By simulating quantum effects in curved spacetime without the need for a complete theory of quantum gravity, QFTCS allows physicists to model events like Hawking radiation and particle creation in expanding spacetimes. However, testing these predictions has proven challenging due to the experimental difficulties involved.
The Role of Quantum Simulations in Cosmology
In the past, researchers have used analog quantum simulations—such as Bose-Einstein condensates—to observe phenomena predicted by semi-classical theories, such as the production of particles in expanding spacetimes. However, the field of digital quantum simulation had not yet been fully explored. The new study breaks new ground by implementing a quantum digital simulation, which leverages quantum computers to simulate and predict particle creation in an expanding universe more accurately.
Marco Díaz Maceda, the lead author of the study and a graduate student at Universidad Autónoma de Madrid, emphasized the significance of the research, explaining, “I believe quantum computing has a promising future for advancing research in physics. I have always loved studying the universe and its phenomena, so I was naturally drawn to quantum fields in curved spacetime. This research represents a fascinating intersection of these two fields, making it a natural and inspiring choice for me.”
Overcoming the Challenges of Quantum Computing: Error Mitigation
The field of quantum computing is still in its nascent stages, and current quantum computers are far from perfect. The Noisy Intermediate-Scale Quantum (NISQ) era, where quantum devices contain tens or a few hundred qubits, poses significant challenges. One of the main obstacles is noise, meaning the qubits and quantum gates are highly sensitive to environmental disturbances, resulting in errors in quantum calculations. The limited scale of current quantum processors also makes the implementation of advanced quantum error correction codes (QECCs) impractical, as they require a significant number of physical qubits to represent a single logical qubit.
In light of these challenges, the researchers employed error mitigation techniques rather than the more resource-intensive error correction approaches. Error mitigation works by understanding how errors scale with noise and using that knowledge to estimate what the results would be without the noise. By applying techniques like zero-noise extrapolation (ZNE), the researchers were able to improve the reliability of their simulations.
Maceda further explained the importance of this approach: “We used only four qubits, one for each possible state of the field. However, since our circuit involved a large number of quantum gates, errors accumulated throughout the execution. To obtain reliable results, we applied error mitigation techniques, which helped improve the fidelity of our computations.”
Particle Creation in Curved Spacetime: The Theoretical Framework
In classical quantum field theory, a flat spacetime (known as Minkowski space) is typically assumed, and the physics remains relatively straightforward. However, in an expanding or curved spacetime, the dynamics of the system change dramatically. This was particularly relevant when modeling the early universe when cosmic inflation led to the creation of particles from the vacuum state.
As spacetime expands (as it did during the inflationary period of the early universe), the vacuum or zero-point energy state becomes excited. This phenomenon, referred to as particle creation, is a central feature of cosmological theories and is believed to have occurred during the universe’s early stages. The quantum field, which was initially in a vacuum state, undergoes fluctuations, leading to the creation of particles as spacetime stretches and evolves.
To simulate this particle creation, the researchers employed the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, a cosmological model that describes a homogeneous and isotropic expanding universe. This model is particularly useful for studying the early universe and cosmological events, as it captures the uniform expansion of spacetime. The quantum field they focused on was a massive scalar field governed by a modified Klein-Gordon equation that accounts for the curvature and expansion of spacetime.
The core mathematical tool used to describe particle creation is the Bogoliubov transformation. These transformations allow researchers to calculate how the number of particles in a given state evolves during the process of particle creation in a changing spacetime. This is crucial for simulating the initial and final states of particle creation, which the researchers replicated using the quantum computer.
Quantum Circuit Implementation: Bringing Theory to Life
The researchers designed a quantum circuit using IBM’s 127-qubit Eagle processor to simulate the particle creation process. The circuit was programmed to simulate the early universe starting from the vacuum state, where no particles exist, and then model the excitation of the quantum field as spacetime expands.
Maceda described the process of designing the quantum circuit, explaining, “The first step in designing the quantum circuit was to determine the time evolution operator of the system. This was achieved by relating the initial and final states through Bogoliubov transformations.” This allowed them to relate the quantum states at different times and, ultimately, to calculate how many particles would be created in this dynamic environment.
The next step involved assigning specific quantum states (representing the different excitation levels of the scalar field) to the quantum computer’s qubits. This encoding process mapped the field states to physical qubits, with each qubit representing a particular excitation level: the ground state, one excitation each in the positive and negative modes, and one in both modes.
“Finally, applying techniques developed by my mentor Dr. Sabín, we mapped the time evolution operator to unitary operations acting on these qubits, ensuring that their evolution accurately reflected the dynamics of the scalar field in an expanding universe,” said Maceda.
The complexity of the quantum simulation required the use of hundreds of quantum gates to accurately model the time evolution of the quantum field and the resulting particle creation.
Quantum Error Mitigation: Zero-Noise Extrapolation
Error mitigation played a key role in ensuring the success of the simulation. The team used zero-noise extrapolation (ZNE), a technique where controlled noise is deliberately added to the system. By measuring the system’s response to this noise, researchers can extrapolate backward to estimate the result in the absence of noise. This method proved essential in improving the fidelity of the quantum simulation, helping mitigate the high levels of noise typically present in quantum computers.
Outcomes and Future Prospects
The quantum simulation successfully demonstrated particle creation in an expanding universe, with results that closely aligned with theoretical predictions. While the simulations still exhibited higher levels of noise, the study’s success showed the potential of quantum computing to simulate cosmological phenomena. More importantly, the researchers’ application of error mitigation techniques showed that these simulations could become even more accurate in the future as quantum technology continues to advance.
Maceda discussed the broader implications of their work, stating, “Our work provides a new way to simulate particle creation in the early universe, offering deeper insights into fundamental processes that shape the cosmos.”
Looking ahead, the researchers are optimistic about the future role of digital quantum simulations in cosmology and physics. They noted that these simulations could become valuable tools for understanding complex systems and phenomena that are currently difficult or impossible to study with classical computers.
“This is a really exciting field, and digital quantum simulations are already proving to be viable for investigating cosmological phenomena,” Maceda said. “They are allowing us to simulate and study fundamental processes like particle creation, gravitational entanglement, and black hole evaporation, which were previously out of reach.”
As quantum computing technology continues to evolve, it is clear that quantum simulations will become an increasingly powerful tool for tackling the most complex questions in physics and cosmology.
Reference: Marco D. Maceda et al, Digital quantum simulation of cosmological particle creation with IBM quantum computers, Scientific Reports (2025). DOI: 10.1038/s41598-025-87015-6