The trade-off problem of quantum computing will be solved by a new system

The potential of quantum computers is currently thwarted by a trade-off problem. Quantum systems that can perform complex operations are less tolerant of errors and noise, while systems that are more secure against noise are harder and slower to calculate with.

Now a research team at Chalmers University of Technology in Sweden has created a novel system that overcomes this dilemma, paving the way for longer computation times and more robust quantum computers.

For the impact of quantum computers to be felt in society, quantum researchers must first overcome some major obstacles. So far, errors and noise generated by electromagnetic interference or magnetic fluctuations, for example, cause sensitive qubits to lose their quantum states – and subsequently their ability to continue computation. The amount of time a quantum computer can work on a problem has thus far been limited.

Additionally, for quantum computers to be able to tackle complex problems, quantum researchers need to find a way to control quantum states. Like a car without a steering wheel, quantum states can be considered somewhat useless if there is no efficient control system to govern them.

However, the research field is facing a trade-off problem. Quantum systems that allow efficient error correction and long computation times, on the one hand, lack in their ability to control quantum states – and vice versa. But now, a research team from Chalmers University of Technology has found a way to fight this dilemma.

“We have created a system that allows extremely complex operations on multi-state quantum systems to be performed at unprecedented speeds,” says Simone Gasparinetti, head of the 202Q-Lab at Chalmers University of Technology and senior author of the study.

deviates from the two-quantum-state theory

While the building blocks of a classical computer, bits, have a value of either 1 or 0, the most common building blocks of a quantum computer, qubits, can have the value 1 and 0 at the same time – in any combination. This phenomenon is called superposition and is one of the key elements that enables quantum computers to perform simultaneous calculations, resulting in much higher computing power.

However, qubits encoded in physical systems are extremely sensitive to errors, which has led researchers in this field to search for ways to detect and correct these errors. The system created by the Chalmers researchers is based on so-called continuous-variable quantum computing and uses harmonic oscillators, a type of microscopic component, to encode information linearly.

The oscillators used in the study consist of thin strips of superconducting material mounted on an insulating substrate to form microwave resonators. This technology is fully compatible with the most advanced superconducting quantum computers.

This method is already known in the field and differs from the two-quantum state theory because it provides a much larger number of physical quantum states, thus making quantum computers much better equipped against errors and noise.

“Think of a qubit as a blue lamp that can be switched on and off simultaneously via quantum mechanics. In contrast, a continuously changing quantum system is like an infinite rainbow, providing a seamless gradient of colours. This reflects its ability to reach a vast number of states, providing much richer possibilities than the two states of a qubit,” says Axel Eriksson, a quantum technology researcher at Chalmers University of Technology and lead author of the study.

The method tackles the trade-off problem between operational complexity and fault tolerance

Although continuous-variable quantum computing based on harmonic oscillators enables better error correction, its linear nature does not allow complex operations to be performed.

Attempts have been made to combine harmonic oscillators with control systems such as superconducting quantum systems, but this has been hampered by the so-called Kerr-effect. The Kerr-effect in turn disorganizes many of the quantum states introduced by the oscillator, cancelling out the desired effect.

By placing a control system device inside the oscillator, the Chalmers researchers were able to circumvent the Kerr-effect and tackle the trade-off problem. This system offers a solution that preserves the benefits of the harmonic oscillator, such as a resource-efficient route toward fault tolerance, while enabling precise control of quantum states at high speeds.

This system is described in an article published in. Nature Communications This could pave the way for even stronger quantum computers.

“Our community has often tried to keep superconducting elements away from quantum oscillators, so as not to disturb the delicate quantum states. In this work, we have challenged this paradigm. By embedding a control device in the center of the oscillator we were able to keep many quantum states from becoming chaotic, while also being able to control and manipulate them.

“As a result, we demonstrated a new set of gate operations that can be performed at very high speeds,” says Gasparinetti.