Quantum computers are considered one of the most important technologies of the twenty-first century, which is why Germany’s federal government is investing billions of euros in their development. The enormous potential of these computers derives from their ability to solve problems that even the most powerful conventional computers cannot solve. For example, they can factorize incredibly large numbers. However, while these computers have a great deal of potential, the technology upon which they are based is very fragile. A team of international researchers, including Freie Universität quantum physics professor Jens Eisert, recently published a study in Nature Physics exploring the precise limitations of fault-tolerant quantum computing. Their results could significantly inform strategic political and industrial decisions in the future.

Unlike conventional computers, quantum computers are not subject to the traditional, classical laws of physics. They are based on the principles of quantum mechanics and make use of quantum bits or “qubits.” While classical bits are limited to either a zero or one state, a qubit can be in a zero state, a one state, or in a superposition of both. By using qubits in superposition, scientists can manipulate many basis states of classical zeros and ones at once – and therein lies the potential computing power of these systems. Quantum computing opens up a multitude of possibilities in areas such as materials research, chemistry, optimization, or machine learning. The field of quantum computing is moving fast – the first systems featuring over 1,000 qubits are already a reality.

“However, quantum computers have one considerable weakness,” says Eisert, quantum physicist at Freie Universität and leader of the research group behind the study. “They are very sensitive to even the smallest disruption in their environment. Even the tiniest external disruption can result in a loss of quantum information (‘decoherence’), largely nullifying the system’s computing advantage. To a certain extent, quantum computers are the Goldilocks of computers – they need everything to be just right.”

To date, research in quantum computing has followed one of two strategies for dealing with this extreme sensitivity. One approach is quantum error correction, whereby logical quantum information is stored in many qubits distributed across additional supporting qubits in order to compensate for potential errors. This strategy can be used to develop fully fault tolerant architectures. However, it is technically demanding and requires exceptionally large systems. The other approach is called the near-term regime; here, quantum physicists accept that errors will occur and develop systems that can operate as reliably as possible despite noise, decoherence, and other errors.

The research team investigated this second approach for dealing with sensitivity and came to the conclusion that such near-term quantum computing can only be used to carry out complex calculations to a limited extent. The decisive factor was the accuracy and reliability of the individual operations, i.e., the gate fidelity. Gate fidelity measures how accurately a quantum gate performs its intended operation compared to an ideal, noise-free version of that gate. Regardless of these limitations, if the gate fidelity is high enough, a quantum computer can still be used to perform large, practically relevant calculations.

“This in turn gives rise to an exciting regime ready to be further explored,” says Eisert. “Our study provides not just a theoretical limit for near-term quantum computing, but also a specific direction for how we can develop quantum computers in the future.”

The study included researchers from Sorbonne University, the University of Chicago, the Fraunhofer Heinrich Hertz Institute, the University of Lyon, the Helmholtz-Zentrum Berlin, and the Massachusetts Institute of Technology.

Their work combines theoretical physics with applied mathematics. Eisert also emphasizes how the study reinforces the importance of Berlin in the field of quantum computing. Initiatives such as Berlin Quantum and the Berlin University Alliance are underscoring Berlin’s role as a key player in quantum technologies.

Mele, A.A., Angrisani, A., Ghosh, S. et al. “Noise-Induced Shallow Circuits and the Absence of Barren Plateaus.” Nature Physics. (2026). https://doi.org/10.1038/s41567-026-03245-z