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Individual defects in superconducting quantum circuits imaged for the first time

Individual defects in superconducting quantum circuits have been imaged for the first time, thanks to research by scientists at the National Â鶹ÒùÔºical Laboratory (NPL) in collaboration with Chalmers University of Technology and Royal Holloway University of London.

In a paper in Science Advances, scientists at NPL achieved a pivotal step in understanding tiny material defects known as two-level system (TLS) defects in superconducting quantum circuits for the first time.

The significant advancement makes it possible to locate, image and eventually mitigate these defects and can lead to stable and reliable quantum computers capable of revolutionizing the different fields of cybersecurity, optimization, drug discovery, and clean energy.

A major challenge for quantum computers is decoherence of the fragile quantum states. Decoherence happens when quantum information stored in the computer leaks out and is irretrievably lost. For quantum processors built using superconducting circuits—one of the most promising platforms used by several industry leaders—the primary source of decoherence is these TLS defects residing in the circuits.

While researchers have known about these defects for over five decades, it has never been possible to locate individual defects and study how each of them contributes to decoherence in a live quantum circuit.

Now, scientists at NPL have built a new instrument capable of locating and studying individual TLS defects for the first time. The instrument combines advanced microscopy techniques with live quantum circuits and operates in a light-tight, dark chamber cooled to just above absolute zero to minimize thermal fluctuations.

The scientists believe that this new tool will help them study these defects to first chemically identify and eventually get rid of them, paving the path for building reliable and stable quantum computers ready to outperform classical computers.

Dr. Riju Banerjee, a senior scientist at NPL and one of the lead authors of the paper said, "For years people have believed that TLS defects perturb quantum circuits. It is remarkable to finally be able to visualize the fluctuations and decoherence each TLS defect causes as it interacts with the circuit.

"The first data that we have taken using our new instrument looks as if there's an actual noisy liquid sloshing around the circuit, and individual defects appear as rings, just like ripples from raindrops on a pond."

"We now have a new tool with which we can learn so much more about these nasty defects that plague quantum circuits. It can now help us to find ways to get rid of these defects in the future," said Dr. Sebastian de Graaf, principal scientist at NPL.