Finding causes and effects

Oct 14, 2019

Giulio Chiribella_1

A pioneering researcher in quantum information science, computer science Professor Giulio Chiribella at the University of Hong Kong opened the door to an exciting new development in the realm of artificial intelligence.


Giulio Chiribella and David Ebler
Professor Chiribella (left) and his student Daniel Ebler published their latest research in prestigious journal Nature Communications.

Latest research by him and his student Daniel Ebler, published in Nature Communications, shows a powerful advantage of quantum algorithms over all classical algorithms in discovering cause-effect relations, a task that has countless applications in science and technology. Determining if a treatment causes recovery from an illness, identifying which gene is responsible for a certain hereditary condition, or establishing the causes of global warming are just a few examples of applications where cause-effect relations play centre stage. 

“Classical algorithms for testing cause-effect relations are widely used in statistics and artificial intelligence. The mathematical framework underpinning such algorithms is relatively recent, and dates back to the work of Judea Pearl and collaborators in the 1980-1990s. The importance of this work was recognised in 2011, when Pearl received the Turing Award, the highest distinction in computer science,” said Professor Chiribella. “Recently, researchers around the world started thinking how to extend Pearl’s framework to new scenarios governed by the laws of quantum mechanics.”

Quantum mechanics is one of the most fundamental theories of physics. It describes nature at the smallest scales of atoms and subatomic particles. Professor Chiribella’s team addressed the challenging question of how to discover cause-effect relations in the world of quantum particles. 

“How one quantum variable affects another quantum variable, and how to establish their cause-effect relation are exciting questions and they were completely unexplored until just a few years ago,” added Professor Chiribella. 


The variables and cause-effect relations between them

This graph shows the variables and cause-effect relations between them.


Quantum revolution

For centuries, physics has developed along the conceptual framework laid down by giants like Galileo Galilei and Isaac Newton. This framework, classical physics, describes accurately most of the phenomena we see in our everyday life. But if we zoom in to the level of atoms and particles, a new range of phenomena starts to emerge. “Everything we see around us is made of quantum particles, and their behaviour is governed by laws that are radically different from the classical laws posited by Galilei and Newton. If we want to understand the universe we live in, quantum mechanics is indispensable.”  The rise of quantum physics, started a century ago, has not only been a revolution in science, but also a revolution in engineering. “Quantum mechanics is at the basis of some of today’s most used technologies, such as transistors and lasers, and a new generation of quantum technologies is currently under way. In the past 40 years, our technology has developed to the point that we can manipulate information at the level of individual quantum particles,” Professor Chiribella explained.  “We can now write a message in the state of a single photon, a particle of light. Continuing in this direction, in the future we will be able to communicate and process information using quantum particles. This will make our communication infrastructure more secure, and our computers faster.”


Bringing causality to the quantum world 

Causality has been a very important concept since the beginning of modern science and engineering, but it was only when Pearl and coworkers came up with a mathematical theory in the 1980-1990s that it became a scientific discipline of its own.

Pearl’s work was inspirational to Professor Chiribella, who was doing research on how the fundamental laws of quantum mechanics can be turned into advantages for future information technologies. “When I found out about Pearl’s ideas on causality, I immediately wondered whether quantum mechanics could give us new, faster ways to discover cause-effect relations.”

His team discovered that a puzzling feature of quantum mechanics, known as quantum superposition, can indeed speed-up the discovery of cause-effect relations. Roughly, the idea is to perform many tests at the same time, making them interfere with each other. Exploiting the interference of multiple tests, is possible to increase the probability of correctly guessing whether a variable causes another variable. A quantum algorithm based on this principle can identify cause-effect relations much faster than every classical algorithm.

 “Our result is only scratching the surface of a vast range of applications of quantum algorithms to the new science of causality,” says Professor Chiribella. Like Pearl, he believes that the ability to exploit the causal relationships between different variables is the future of A.I. “The next level of research in A.I is to enable machines to reason in terms of causes and effects, so that they can come up with the right sequence of actions to obtain a desired outcome. This complex reasoning will be based on the ability to identify and exploit causal relations among many variables,” Professor Chiribella said.


From art to science

Professor Chiribella was awarded Croucher Senior Research Fellowship

Professor Chiribella was awarded Croucher Senior Research Fellowship to further his research on quantum causality in 2018.


Professor Chiribella is a highly successful academic. In 2011, he was awarded the prestigious Hermann Weyl Prize for pioneering research in quantum information. While at HKU, he was awarded the title of CIFAR-Azrieli Global Scholar 2016, becoming first scientist from an Asian institution to receive this distinction. In 2018, he was awarded a Croucher Senior Research Fellowship to further his research on quantum causality. 

Still, he revealed to us that science has not always been his main priority. While still a university student, Professor Chiribella wanted to become a music composer, who could capture the complexity of our world through music. He changed his mind after taking courses in quantum physics at the University of Pavia in Italy. “I became completely fascinated by quantum mechanics,” he recalled. “It is magical because it’s very different from everything we are used to in everyday life, and its ramifications are extremely profound. The rules of quantum mechanics are simple, but their consequences are sophisticate and often unexpected. It is a bit like the game of chess: it takes only a few minutes to learn the rules, but it takes a lifetime to explore their consequences. This is also the same situation as in music, where a few notes can be used to build very complex pieces. Perhaps becoming a quantum scientist has been just a new way to realise my old dream.”

Since early 2000s he has devoted himself full-time to research and teaching in the intriguing aspects of quantum physics. His academic career has taken him to different parts of the world.  He has been first postdoctoral fellow the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and then Associate Professor at Tsinghua University, Beijing, and Professor at Oxford University. 

“I feel blessed to have been able to experience so many different research environments and to connect with some of the world’s finest minds” he said. “In each place I learned a different approach to research, and all these experiences have immensely enriched my professional and personal life.”


Cross-boundary research  

Professor Chiribella and his research group

Professor Chiribella hopes to build a research group with young researchers from all over the world.

Coming from Europe, China was very much a place of novelty to him. “My move to Tsinghua University in 2012 has been the start of an amazing, life-changing experience,” he told us. What made his time there even more memorable was “the extraordinary students and generous government programs supporting basic research”. He joined HKU in 2015, some of his students at Tsinghua followed him to Hong Kong, wishing to continue doing ground-breaking research on the foundations of quantum mechanics. Three of them are now postdoctoral fellows overseas, but still they remain part of the extended research family that the Italian professor has formed over the years.

Looking back, he is pleased to see the network of talented, driven researchers coming from the fields of computer science and physics. Hong Kong is also to him the perfect place to keep up cherished ties with colleagues, former students, and fellow researchers from all around the world.

“Hong Kong is the perfect base to connect with my esteemed colleagues from China and overseas. It is easy for everyone to come and live in this convenient city, be it for an academic job or even just short research visit,” said Professor Chiribella, who is also supervising doctoral students at Oxford University. His cosmopolitan approach testifies to the global nature of research today. Undoubtedly he is open to input from and interactions with talented scientists from all over the world. “I like to have a research group where young researchers from any country can feel at home and give their very best.”