The Pauli exclusion principle is a cornerstone of the Standard Model of particle physics and is essential for the structure and stability of matter. Now an international collaboration of physicists has carried out one of the most stringent experimental tests to date of this foundational rule of quantum physics and has found no evidence of its violation. Using the VIP-2 experiment, the team has set the strongest limits so far for possible violations involving electrons in atomic systems, significantly constraining a range of speculative theories beyond the Standard Model, including those that suggest electrons have internal structure, and so-called 'Quon models.' Their experiment, which was partially supported by the Foundational Questions Institute, FQxI , was reported in Scientific Reports in November 2025.
Austrian-Swiss physicist Wolfgang Pauli outlined the exclusion principle in 1925. It states that two identical 'fermions' (a class of particles that includes electrons) cannot occupy the same quantum state. It explains why electrons fill atomic shells, why solids have rigidity, and why dense objects such as white dwarf stars do not collapse under gravity.
"If the Pauli exclusion principle were violated, even at an extremely small level, the consequences would cascade from atomic physics all the way to astrophysics," says Catalina Curceanu.
However, since its inception, physicists have been searching for signs that the Pauli exclusion principle may be violated in extreme conditions. "If the Pauli exclusion principle were violated, even at an extremely small level, the consequences would cascade from atomic physics all the way to astrophysics," says FQxI member and physicist Catalina Curceanu of the Italian National Institute for Nuclear Physics (INFN), in Frascati, who is the spokesperson of the VIP-2 collaboration.
Forbidden Transitions
Curceanu and her colleagues are pushing Pauli's exclusion principle to the strongest limits. To test the principle, the international collaboration used an ultra-sensitive underground experiment at the Gran Sasso National Laboratory in Italy, called the VIP-2 experiment, which focuses on electrons inside copper atoms. A violation of the exclusion principle would allow electrons to undergo normally forbidden atomic transitions, producing X-rays at precisely defined energies not seen in standard atomic transitions.
To search for these signals, the VIP-2 team injected a large number of 'new' electrons into copper targets. "Introducing electrons that have not previously interacted with the atomic system, in a so called 'open-system configuration,' is crucial," explains lead author Alessio Porcelli of Jagiellonian University, in Kraków, Poland, INFN, and the University of Antofagasta, in Chile. "Quantum mechanics forbids violations from appearing in closed systems. By injecting new electrons, we avoid this limitation and carry out a clean and decisive test of the Pauli exclusion principle."
"This is the strongest experimental constraint ever achieved for electrons in open systems," says Alessio Porcelli.
The experiment then monitored the copper target for several years using low-noise X-ray detectors, installed deep underground to shield them from cosmic radiation. The researchers searched for the tell-tale X-ray lines that would indicate forbidden transitions.
No such signals were observed. This allows the team to set the most stringent upper limit so far on possible Pauli exclusion principle violations for electrons in atomic transitions, corresponding to a probability smaller than two parts in 1043. "This is the strongest experimental constraint ever achieved for electrons in open systems," says Porcelli.
Constraining Quons
Although the Standard Model of particle physics assumes that the Pauli exclusion principle is exact, many proposed extensions do not. By showing that no violation appears even at this extreme sensitivity, the VIP-2 result sharply restricts the landscape of viable new theories. In particular, the findings strongly constrain the so-called Quon model. In conventional quantum mechanics, particles are either fermions or bosons, with no intermediate behaviour. The Quon model relaxes this strict distinction, allowing particles to behave almost like fermions while occasionally violating the Pauli exclusion principle.
"Our result places very stringent constraints on possible deviations from standard fermionic behaviour for electrons, strongly restricting the alternative Quon models," says Kristian Piscicchia.
"Our result places very stringent constraints on possible deviations from standard fermionic behaviour for electrons, strongly restricting the alternative Quon models," says Kristian Piscicchia of the Enrico Fermi Research Center, in Rome, Italy and INFN, a leading member of the collaboration.
No Hidden Structure
The result also challenges ideas suggesting that electrons might have a hidden internal structure. Any such substructure would be expected to weaken the Pauli exclusion principle slightly––an effect that VIP-2 does not observe.
Even quantum gravity theories are affected. Some approaches that attempt to reconcile quantum mechanics with Einstein's general theory of relativity predict violations of the Pauli Exclusion Principle at levels now excluded by the experiment.
"Any viable extension of quantum theory must reproduce the Pauli exclusion principle with extraordinary precision," says Curceanu. "Our result significantly narrows the type of new physics that could appear, while also guiding the next generation of even more sensitive experiments."
If deeper laws underlie quantum mechanics, they must be compatible with the Pauli exclusion principle at an exceptionally high level of precision. The VIP-2 experiment represents an important step in systematically testing this foundational rule.
"This work was made possible by FQxI's commitment to exploring the deepest foundations of physics," adds Curceanu. "Their support allowed us to pursue a demanding experiment aimed squarely at testing the limits of quantum theory."
Building on these results, the next-generation VIP-3 experiment will push the sensitivity of such tests even further, continuing the search for possible deviations and sharpening our understanding of the quantum world.
This work was partially supported through FQxI's Consciousness in the Physical World program
