Zhang Zhihao
physicists to humanity’s treasure trove of knowledge.”
According to Xue, understanding the elusive quantum anomalous Hall effect is like unpacking a Russian nesting doll; one must slowly take apart each layer of concepts before tackling the core idea and appreciating its true significance.
A good starting point is the Hall effect, discovered in 1879 by Edwin Hall, a physicist from the United States. It states that electrons moving in a conductor can also move transversely when a magnetic field is applied perpendicular to the direction of the electric current.
This phenomenon is now widely used in a range of gadgets and everyday devices, from auto ignitions and speedometers to computer keyboards and factory robots.
More than a century after Hall’s discovery, scientists found a new version of the effect that can be applied at the quantum level, hence the name quantum Hall effect.
Apart from helping its discoverer, Klaus von Klitzing, to win the Nobel Prize in 1985, the effect also led to the 2006 discovery of topological insulators — strange materials that can conduct electricity on their external surfaces, but not those inside.
Scientists hope these exotic materials may eventually lead to faster, more efficient computer chips, or even more-stable and powerful quantum computers. The materials are already being used as virtual laboratories to test predictions about undiscovered states of matter and the laws of physics.
However, they often require a number of extreme lab conditions to function, such as a very strong magnetic field and temperatures close to absolute zero, or -273 C, and the harsh conditions severely limit their practicality.
“The most beautiful part of the quantum anomalous Hall effect is that it can produce the same features as the quantum Hall effect without the need for a strong external magnetic field,” said Lyu Li, Xue’s colleague and a researcher at the Institute of Physics at the CAS.
As a result, the quantum anomalous Hall effect eliminates one of the biggest obstacles preventing the new materials from revolutionizing electronic engineering, such as creating the next generation of energy-efficient transistors for electronics, Lyu said.
Anyone who owns a mobile phone or laptop may find their machine giving off heat after prolonged use. This can lead to a range of issues, from sweaty fingerprints to a catastrophic system meltdown.
Overheating is the single largest obstacle to the development of computers, because circuits are becoming smaller and more densely packed. That means engineers have to use various methods, such as internal fans or pumping cooling water, to keep the circuits from overheating.
“Fundamentally speaking, our computers overheat and slow down because the electrons in their circuits are moving without specific paths and are constantly bumping into obstacles, wasting energy and giving off heat in the process,” Xue said.
“It’s like driving a car through a chaotic and crowded market. However, in the quantum anomalous Hall state, electrons move like cars on a highway — they can travel smoothly without much resistance and are mutually undisturbed.”
If the quantum anomalous Hall effect is applied to mobile phones or computer circuits, “it significantly reduces heat dissipation and makes the machine safer, faster and more compact”, he added.
Today’s supercomputers consume huge amounts of electricity because of the necessary cooling systems. “Imagine cutting out most of the cooling equipment, while at the same time being able to pack more circuits onto the chips without fearing that the heat will melt them — the boost in computing power would be enormous,” Xue said.
The world has already witnessed this phenomenon. After all, modern mobile phones have more computing power than the IBM Deep Blue supercomputer that beat Garry Kasparov in a historic chess match in 1997.
Xue hopes his work will lead to more game-changing materials and inventions for the computing and new energy industries, as well as helping China to gain an edge in the next wave of the information technology revolution.
However, the quantum anomalous Hall effect still requires temperatures close to absolute zero. “Our next goal is to raise the temperature at which the effect can take place. If it can take place at room temperature, then it will have wide practical uses,” Xue said.
In a video lecture, Steven Girvin, a physics professor at Yale University, said that while scientists are trying out these new phenomena in condensed matter physics, the experiments remain “very, very challenging”.
“We are still at a very early stage (of the technology),” he said. “Then again, it is too early to say it is not going to work.”
When Xue and his team announced the discovery of the quantum anomalous Hall effect, some physicists — who were also racing to uncover new phenomena — doubted the findings because the effect requires an extremely challenging material.
“We needed something that is inherently magnetic, does not conduct electricity on the inside but can somehow conduct electricity on its surface,” Xue said.
“It is like finding a super athlete who combines the speed of a sprinter, the strength of a weight lifter and the agility of a figure skater.”
Finding the right material that embodies those traits is hard enough on paper, producing it is a worldclass challenge because the specimen is extremely sensitive to impurities and defects, and must be perfectly flat right down to the atomic level.
“It is like creating a sheet of perfectly flat paper the size of a track field,” Xue said.
Wang Yayu, chair of Tsinghua’s physics department, noted that Xue organized an “all-star team of scientists” to achieve the desired level of precision. The experts came from several related fields and institutes, and even built their own equipment for the lab experiments.
After testing more than 1,000 samples in four years, the team finally confirmed the discovery of the effect in December 2012.
“The result was so perfect that some of our foreign peers could not believe it, but when I showed them our methods and raw data, they were convinced,” Xue said.
In 2014, a physics lab at the University of Tokyo replicated Xue’s experiments and confirmed his findings. Labs from other top universities, from the Massachusetts Institute of Technology to Stanford and Princeton, also validated Xue’s discovery in the years that followed.
“The key to scientific success is to focus on a big problem, and then push ourselves rigorously to the limit in pursuit of absolute perfection,” Xue said. “This is also one of the best ways to foster competitive, worldclass young talent.”