Quantum computers could eventually transform areas such as drug discovery, energy technology, cryptography, artificial intelligence, and logistics. Before that can happen, however, these machines need to become much more dependable.

Why Quantum Computers Are So Error-Prone

A major challenge is that quantum computations can be disrupted by extremely small environmental effects. Electrical noise, cosmic radiation, and overheating can all introduce errors while information is being processed.

Traditional computers can experience errors too, but decades of development have produced reliable error correction methods that can quickly detect and repair them. Quantum systems are much harder to protect because the information they use is extraordinarily delicate.

"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," says Lei Du, researcher in Applied Quantum Physics at Chalmers University of Technology in Sweden, and lead author of the theoretical study published in the journal Physical Review Letters.

A Different Way to Protect Quantum Information

To make quantum computing more resilient and eventually fault-tolerant, researchers are investigating new ways to shield quantum information from errors. One promising strategy uses so-called bosonic quantum codes*.

Instead of assigning quantum information to individual qubits, this approach stores it in microwave fields inside superconducting circuits.

"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," explains Tangyou Huang, researcher in Quantum Technology at Chalmers and co-author of the study.

Quantum Operations More Than 1,000 Times Faster

Working with bosonic quantum codes is not simple. Creating and controlling the required quantum states has traditionally involved guiding a quantum system through thousands of repeated driving cycles.

That process can take considerable time, and every additional cycle creates another opportunity for outside disturbances to interfere with the calculation. In quantum computing, speed is therefore closely tied to reliability.

Chalmers researchers Lei Du and Tangyou Huang have now proposed a different strategy. Rather than constructing the desired quantum states one small piece at a time, their method can perform a wide variety of 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. This makes the operations both faster and more efficient, while reducing the risk that disturbances will corrupt the information before the process is finished. It represents an important step towards fault-tolerant quantum computers," says Lei Du.

Quantum Lattice Gates Provide a Shortcut

The new approach is built around Quantum lattice gates, a recently proposed universal set of quantum gates developed by the same research team.

These gates act somewhat like shortcuts. Instead of requiring a long sequence of repeated control steps, they can allow the intended quantum operation to be completed in just one driving cycle. That could make the process faster, simpler, and less vulnerable to errors.

"You can think of it like building a large Lego castle. 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," says Tangyou Huang.

Designed for Superconducting Quantum Computers

The technique is especially well suited to superconducting quantum computers, which are among the most prominent technologies being developed in the international push toward large-scale quantum computing.

Chalmers University of Technology is also using superconducting technology as it develops a 100-qubit quantum computer.

"A key advantage of our approach is that it can be implemented using existing superconducting quantum circuit platforms. We are already discussing possible experimental realizations with colleagues at Chalmers, and we hope to see a demonstration of the method in the near future," says Tangyou Huang.

The researchers say the work addresses a central problem facing the field: efficiently producing and controlling quantum states that are capable of helping correct errors.

"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," says Lei Du.

More About Bosonic Codes, Quantum Lattice Gates and Floquet Control

*Bosonic quantum codes store quantum information in the states of, for example, microwave or optical resonators rather than in individual qubits. They are considered a promising tool for quantum error correction because they can provide built-in protection against certain types of errors.

Quantum operations are needed to process that stored information. Quantum lattice gates are a recently proposed collection of basic building blocks that can be used to control bosonic quantum states. By combining and designing these gates, researchers can carry out a wide range of more complicated quantum operations.

One method for implementing these operations is known as Floquet control, which uses periodic control signals to drive a quantum system. Earlier Floquet-based techniques have generally depended on slower processes involving many repeated driving cycles. The approach developed at Chalmers can instead perform quantum lattice gates directly in a single driving cycle, making some operations more than a thousand times faster.

More About the Research

The scientific paper "Single-Period Floquet Control of Bosonic Codes with Quantum Lattice Gates" has been published in Physical Review Letters. The authors are Tangyou Huang, Lei Du, and Lingzhen Guo. The researchers are affiliated with Chalmers University of Technology in Sweden and Tianjin University in China.

The research was funded by the National Natural Science Foundation of China (NSFC), the Wallenberg Centre for Quantum Technology (WACQT), and the Knut and Alice Wallenberg Foundation.