Quantum Zeno effect could slow scaled-up qubit computers
HZDR researchers say environmental disturbances may cause some adiabatic quantum computers to stall as qubit counts grow.
By Priya Raghavan · Science Reporter
3 min read
Researchers at the Helmholtz-Zentrum Dresden-Rossendorf say the quantum Zeno effect could become a serious obstacle for some quantum computers as their qubit counts rise. In a study published in the New Journal of Physics, the team reports that frequent small disturbances may slow, or in extreme cases nearly stop, certain calculations.
The work focuses on adiabatic quantum computers, a class of machines designed to solve problems by keeping qubits in their lowest-energy condition while gradually changing the system. According to HZDR, the final low-energy state then represents the answer to the computational problem.
Quantum computing is being pursued for tasks such as logistics optimization and molecular simulation, where researchers expect it could outperform current supercomputers. HZDR said progress depends in part on increasing the number of qubits, the basic units used in quantum computation.
What is the quantum Zeno effect?
The quantum Zeno effect is a quantum-physics phenomenon in which repeated interactions that resemble measurements can hinder a system’s normal evolution. In the HZDR model, environmental disturbances can play that role for qubits, repeatedly interfering with their changing quantum state.
Gernot Schaller, head of Quantum Technologies at HZDR’s Institute of Theoretical Physics, described the effect as an underexamined barrier for adiabatic quantum computing. The issue, according to HZDR, becomes more pronounced as more qubits are connected and the energy changes they must follow become smaller.
Ralf Schützhold, director of the institute, said adiabatic algorithms are viewed as robust and can run on different hardware platforms. HZDR noted that such algorithms can be tested on systems including superconducting solid-state devices and individual ions held in electromagnetic traps.
Why do disturbances matter for qubits?
Qubits require tight protection from their surroundings because the quantum features used for computation are fragile. HZDR said shielding from electromagnetic radiation and cooling close to absolute zero help preserve superposition, in which qubits can occupy states between zero and one, and entanglement, the quantum link between them.
Those protections cannot remove every environmental influence, according to Schützhold. The HZDR team’s theoretical model suggests that as adiabatic systems scale up, even small outside effects can increasingly interfere with the computation.
Schaller said each disturbance can act like an unintended measurement that slows the system’s progress. In the most severe case described by the researchers, the calculation may come close to freezing before the system reaches the desired final state.
HZDR compared the problem to repeatedly opening an oven while a cake is baking: each check disrupts the process, and too many interruptions can prevent the intended result. For a quantum computer, the comparable interruption is an environmental interaction that disturbs the qubits’ evolution.
How could developers reduce the risk?
The researchers said stronger protection against heat and electromagnetic radiation could help reduce the effect. Schützhold also pointed to active methods, including spin echo, in which coherent pulses are used to lessen the coupling between qubits and their environment.
The study, by Naser Ahmadiniaz and colleagues, is titled “Quantum Zeno effect versus adiabatic quantum computing and quantum annealing.” HZDR said the findings show that environmental effects need to be considered from the start when designing more powerful quantum computers.
This story draws on original reporting from Phys.org.