The promise of quantum computing is to solve complex problems faster and more energy-efficiently than today's supercomputers – from optimizing logistics processes to simulating molecules. This goal is coming within reach as the number of qubits – the computational units of quantum computing – increases. But in addition to technological challenges of scaling, there is another, less-considered issue: In the New Journal of Physics (DOI: 10.1088/1367-2630/ae6e68), researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) demonstrate that in extreme cases, the so-called quantum Zeno effect can nearly halt computational processes as the number of qubits increases – a dreaded phenomenon comparable to a traditional computer "freezing".
"The quantum Zeno effect is a previously overlooked obstacle to a certain class of quantum computers," says Dr. Gernot Schaller, head of Quantum Technologies at HZDR's Institute of Theoretical Physics. These so-called adiabatic quantum computers operate according to a special principle: Their qubits are always in their ground state, the lowest energy state. To solve a computational problem, the qubits' energy landscape is gradually altered – slowly enough for them to adapt continuously and follow the changing ground state. Once the transformation is complete, the ground state immediately encodes the solution to the problem.
"Adiabatic algorithms are considered robust and can be executed by quantum computers largely independently of the hardware that is used," explains Institute Director Prof. Ralf Schützhold. It therefore does not matter whether the qubits are generated using solid-state superconductors or individual ions confined in electromagnetic traps. Both hardware variants are already being used to test adiabatic algorithms that can be programmed elegantly and with relative ease.
When disturbances become an issue
"However, a quantum computer can only function properly if its qubits are not disturbed too much," Schützhold emphasizes. Shielding against electromagnetic radiation and cooling to temperatures near absolute zero – minus 273.15 degrees Celsius – helps protect the qubits from such disturbances. Only then can they assume all possible states between zero and one – a property known as superposition. Their quantum-mechanical link, called entanglement, is also highly sensitive to external disturbances. Only the interplay of superposition and entanglement makes it possible to solve complex problems extremely fast.
"But despite all these measures, environmental impacts on the qubits can never be fully eliminated," says Schützhold. According to the theoretical model developed by Schützhold's team, adiabatic quantum computers become increasingly sensitive to disturbances as they scale up and the number of qubits increases. This is because the more qubits are linked, the smaller the changes in the energy landscape they must follow. "This is where the quantum Zeno effect kicks in," says Schaller. Even tiny environmental influences can then affect the quantum states of the qubits. "Each disturbance acts like an unwanted measurement, slowing down the system's evolution," says Schaller. "In the worst case, a calculation could even freeze completely."
To better understand the principle, let's compare it with baking: A cake will only turn out right if it is allowed to rise in the oven undisturbed. If you keep opening the oven door to check if it's done, you disrupt the baking process – the cake stays flat or even sinks. The quantum Zeno effect works in a similar way: Every disturbance disrupts the natural evolution of the quantum state. If this happens too often, the system will no longer be able to reach the desired final state. In the most extreme case, the computing process will nearly grind to a halt.
However, quantum computer developers can take action to mitigate the quantum Zeno effect, for instance by shielding against electromagnetic radiation and heat. In our cake analogy, this would be akin to putting a padlock on the oven door. Schützhold also proposes active measures to protect the process: "Using the spin-echo method, we can apply coherent pulses to reduce the coupling of qubits to their environment." Back in the kitchen, the oven would rapidly heat up for brief periods to compensate for every time the door is opened. "Our study shows that we can only develop powerful quantum computers when we factor in environmental impacts from the very beginning," Schützhold summarizes.
Publication:
N. Ahmadiniaz, D. Kraft, G. Schaller, R. Schützhold: Quantum Zeno effect versus adiabatic quantum computing and quantum annealing, in New Journal of Physics (2026) (DOI: 10.1088/1367-2630/ae6e68)
Further information:
Dr. Gernot Schaller | Head of Quantum Technologies
Institute of Theoretical Physics at HZDR
