Breakthrough may clear major hurdle for quantum computers

A research team from Chalmers University of Technology has now developed a unique system that addresses this dilemma, paving the way for longer computation times and more robust quantum computers.

For quantum computers to make a real impact in society, researchers must first overcome major challenges. Errors and noise from sources such as electromagnetic interference or magnetic fluctuations cause sensitive qubits to lose their quantum states, limiting their ability to continue calculations. Consequently, the duration for which a quantum computer can work on a problem is currently constrained. Moreover, for a quantum computer to tackle complex problems, researchers must find a way to control quantum states. Like a car without a steering wheel, quantum states are somewhat useless without an efficient control system.

The research field faces a trade-off: systems that allow for efficient error correction and longer computation times are less capable of controlling quantum states, and vice versa. However, the Chalmers University of Technology research team has managed to find a way to combat this dilemma, marking a significant advancement in the quest for practical and effective quantum computing.

“We have created a system that enables extremely complex operations on a multi-state quantum system, at an unprecedented speed,” says Simone Gasparinetti, leader of the 202Q-lab at Chalmers University of Technology and senior author of the study.

While the building blocks of classical computers, bits, have either the value 1 or 0, the fundamental units of quantum computers, qubits, can exist in a superposition, holding both values simultaneously in any combination. This phenomenon, called superposition, allows quantum computers to perform simultaneous calculations, resulting in immense computing potential.

However, qubits encoded in physical systems are extremely sensitive to errors, prompting researchers to find methods to detect and correct these errors. The system developed by the Chalmers researchers is based on continuous-variable quantum computing and uses harmonic oscillators to encode information linearly. These oscillators consist of thin strips of superconducting material patterned on an insulating substrate to form microwave resonators, a technology compatible with the most advanced superconducting quantum computers. This method, known in the field, departs from the two-quantum state principle by offering a larger number of physical quantum states, thereby enhancing the resilience of quantum computers against errors and noise.

“Think of a qubit as a blue lamp that, quantum mechanically, can be both switched on and off simultaneously. In contrast, a continuous variable quantum system is like an infinite rainbow, offering a seamless gradient of colours. This illustrates its ability to access a vast number of states, providing far richer possibilities than the qubit’s two states," says Axel Eriksson, researcher in quantum technology at Chalmers University of Technology and lead author of the study.

Although continuous-variable quantum computing using harmonic oscillators enhances error correction, its linear nature does not support complex operations. Previous attempts to integrate harmonic oscillators with control systems like superconducting quantum systems have been thwarted by the Kerr effect, which scrambles the many quantum states of the oscillator, negating the intended benefits.

The Chalmers researchers overcame this by placing a control system device inside the oscillator, effectively circumventing the Kerr effect and addressing the trade-off problem. This innovative system preserves the advantages of harmonic oscillators, such as a resource-efficient path towards fault tolerance, while allowing precise control of quantum states at high speed. Detailed in an article published in Nature Communications, this breakthrough may pave the way for more robust quantum computers.

“Our community has often tried to keep superconducting elements away from quantum oscillators, not to scramble the fragile quantum states. In this work, we have challenged this paradigm. By embedding a controlling device at the heart of the oscillator we were able to avoid scrambling the many quantum states while at the same time being able to control and manipulate them. As a result, we demonstrated a novel set of gate operations performed at very high speed,” says Simone Gasparinetti. 

Image caption: The circuit diagram to the left illustrates how the Chalmers researcher team was able to turn on and off different operations by sending microwave pulses (wiggly arrow) to the control system embedded in the oscillator. The researchers used the system to generate a so-called cubic phase state which is a quantum resource for quantum error correction. The blue areas to the right are so called Wigner negative regions – a clear signature of the quantum properties of the state. Credit: Timo Hillman