Quantum Computing: A Fast-Paced Field

Quantum computers operate using so-called qubits, which can exist in a superposition of multiple states. This allows scientists to work with superpositions of computational basis states and opens up entirely new computational possibilities – for example in materials research, optimization problems, or machine learning.

The problem: Extreme Sensitivity

“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 address this issue, quantum computing research follows two main strategies: one relies on complex quantum error correction requiring large-scale systems, while the other – the near-term approach – accepts noise and aims to compute as reliably as possible despite errors.

Limits of Quantum Computers

Eisert and colleagues investigated this second approach as part of an interdisciplinary collaboration in theoretical physics and applied mathematics, together with researchers from Sorbonne Université in Paris, the University of Chicago, the Fraunhofer Heinrich Hertz Institute, the University of Lyon, the Helmholtz-Zentrum Berlin, and the Massachusetts Institute of Technology.

The team found 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, known as 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.”

Publication in Nature Physics: Noise-induced shallow circuits and the absence of barren plateaus  

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