By Holt Hackney
Quantum computers remain constrained by their sensitivity to errors and environmental disturbances. The longer a quantum operation takes, the greater the opportunity for errors to disrupt a computation.
Researchers at Chalmers University of Technology in Sweden have developed a method that they say can perform a range of advanced quantum operations more than 1,000 times faster than previous approaches. The work addresses a significant challenge in developing fault-tolerant quantum computers.
Quantum computing could eventually have applications in areas including drug discovery, energy systems, cryptography, artificial intelligence and logistics. But realizing those applications at scale will require quantum computers to become substantially more reliable.
One of the central challenges is managing computational errors. Even relatively small environmental disturbances, including electrical noise, cosmic radiation and temperature changes, can affect quantum systems.
Conventional computers also encounter errors, but established error-correction methods can detect and correct them. Quantum systems present a more difficult problem because their fundamental building blocks, or qubits, are particularly sensitive to disturbances.
“The fundamental building blocks of quantum computers, known as qubits, are so sensitive that even the smallest disturbance can cause the quantum state to deviate from the target, resulting in the loss of information. If too many errors accumulate before they can be corrected, the computation can fail,” said Lei Du, a researcher in applied quantum physics at Chalmers and lead author of the theoretical study published in Physical Review Letters.
An Alternative Approach to Protecting Quantum Information
Researchers seeking more resilient quantum systems are exploring different ways of protecting quantum information. One approach involves bosonic quantum codes.
“Rather than storing quantum information in individual qubits, bosonic codes encode information in the microwave fields found within superconducting circuits. This approach has been shown to provide stronger protection against certain types of errors,” said Tangyou Huang, a researcher in quantum technology at Chalmers and co-author of the study.
The difficulty is that quantum operations using bosonic codes can be complicated and time-consuming to create and control.
Previous approaches have constructed quantum states incrementally, requiring systems to pass through thousands of repeated driving cycles. The longer this process takes, the more opportunity there is for environmental disturbances to introduce errors.
Du and Huang developed an approach designed to perform these operations much faster.
“Our method shows that a diverse range of quantum operations on bosonic states can be completed within a single driving cycle, rather than the several thousand cycles that have been required previously,” Du said. “This makes the operations both faster and more efficient, while reducing the risk that disturbances will corrupt the information before the process is finished.”
Addressing a Quantum Computing Bottleneck
The approach relies on so-called quantum lattice gates, a recently proposed universal quantum gate set developed by the same research team.
The gates effectively provide building blocks that allow certain operations to be performed within a single driving cycle instead of being assembled through thousands of individual cycles.
“You can think of it like building a large Lego castle,” Huang said. “Instead of assembling it brick by brick and risking mistakes along the way, quantum lattice gates act like pre-built Lego modules that can be connected quickly and efficiently.”
The method is particularly relevant to superconducting quantum computers, one of several architectures being developed for large-scale quantum computing. Chalmers is using superconducting technology in its development of a 100-qubit quantum computer.
Another potential advantage is compatibility with existing superconducting quantum circuit platforms.
“We are already discussing possible experimental realisations with colleagues at Chalmers, and we hope to see a demonstration of the method in the near future,” Huang said.
That distinction is important: the current work is theoretical, and experimental validation remains a next step.
“Our results address one of the major bottlenecks in the field: how to quickly and reliably create and control the error-correcting quantum states that could play an important role in future quantum computers,” Du said.
What the Technology Does
Bosonic quantum codes store quantum information in states such as microwave or optical resonators rather than individual qubits. Researchers are studying them as tools for quantum error correction because they can provide protection against certain types of errors.
Quantum lattice gates provide elementary building blocks for controlling these bosonic quantum states. More complex quantum operations can then be constructed from the gates.
The Chalmers method uses a technique known as Floquet control, in which a quantum system is driven by periodic control signals. Previous Floquet-based approaches have often required many driving cycles. The researchers’ method is designed to implement quantum lattice gates within a single driving cycle, allowing some operations to be performed more than 1,000 times faster.