Science & Technology

A major step towards stability in quantum computing

Artistic impression of a superconducting resonator coupled with its quantum-mechanical environment (Heikka Valja.)
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Quantum computing has long faced a major hindrance — the ability to preserve information requires controlling for energy losses and shifts. Researchers have found a different approach which may provide a solution to this problem. 

Quantum computers require the ability to store information for a long period in order to solve problems faster than a regular computer. The difficulty this poses is that energy loses can change the state of qubits — the quantum computing equivalent to bits in conventional computing — resulting in the destruction of this information.

Artistic impression of a superconducting resonator coupled it its quantum-mechanical environment. Figure (Heikka Valja)

Therefore, one of the major problems that scientists have attempted to crack in quantum computing has been the prevention of said energy losses. Dr Mikko Mottonen and his team have taken a different approach to the problem. What if, instead of preventing energy losses, they work around them?

As Mottonen explains: “Years ago we realized that quantum computers actually need dissipation to operate efficiently. The trick is to have it only when you need it.”

Photo of a centimetre-sized silicon chip, which has two parallel superconducting resonators and quantum-circuit refrigerators connected to them (Kuan Yen Tan)

In a paper to be published on 11 March 2019 in Nature Physics, the scientists — hailing from Aalto University and the University of Oulu —  demonstrate that they can increase the dissipation rate, on demand, by a factor of thousand in a high-quality superconducting resonator — just like the ones used in prototype quantum computers.

Dr Matti Silveri, the paper’s primary author, reveals that the results of most scientific significance were unexpected: “To our great surprise, we observed a shift in the resonator frequency when we turned on the dissipation. 

“This discovery took us on a journey to 70 years in the past when Nobelist Willis Lamb made his first observations of small energy shifts in hydrogen atoms. We see the same physics, but for the first time in engineered quantum systems.”

Lamb’s observations were revolutionary at that time. They showed that modelling the hydrogen atom alone was not enough. Electromagnetic fields must be accounted for, even though their energy is zero. This phenomenon is now confirmed also in quantum circuits.

Helium boiling at QDC Labs (Mikko Raskinen/Aalto University)

The key to the new observation was that dissipation — hence the energy shift — can be turned on and off. Control of such energy shifts is critical for the implementation of quantum logic and quantum computers.

Mottonen highlights the significance of this discovery: “Building a large-scale quantum computer is one of the greatest challenges of our society.

“The quantum-circuit refrigerator that we recently invented was the key to achieve this tunability of dissipation. Future quantum computers need a similar feature to be able to control energy loss on demand.”

Main image: Artistic impression of a superconducting resonator coupled with its quantum-mechanical environment (Heikka Valja.)

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