# Prof Giulio Chiribella: exploring the fundamentals of quantum mechanics

Professor Giulio Chiribella (Croucher Senior Research Fellowship 2018) is a leading figure in quantum information science and the fundamentals of quantum mechanics. His interests extend beyond the theoretical foundations of quantum mechanics to real world issues of communications and computation.

Giulio Chiribella is Professor at the University of Hong Kong (HKU) and CIFAR-Azrieli Global Scholar in Quantum Information Science. He is interested in how the fundamental laws of quantum mechanics can be advantageous to future information technologies, and conversely, in what the power of quantum technologies reveals about fundamental physics.

“Our area of investigation delves into the very heart of quantum physics: the core ideas and its core structures. Quantum theory challenges the traditional notion of cause and effect, enabling new exotic scenarios that were unimaginable in classical physics. That is what we are looking at now,” Chiribella explains.

### Switches and Gates

Chiribella’s team is working on a “quantum switch" which combines quantum gates in many different orders at the same time. In 2009, the team devised a way to realise this situation using a single qubit as a switch that controls the causal order of events experienced by a particle that acts as a second qubit. The technique they use involves creating a special type of quantum superposition: the superposition of the order of events in time.

Superpositions of quantum states are well known: a spin, for example, can be placed in a superposition of up and down states. When the control-switch qubit is in state 0, the particle goes through gate A first, and then through gate B. When the control qubit is in state 1, the order is inverted, with B first and then A. But if that control qubit is in a superposition of 0 and 1, the particle experiences a superposition of both sequences AB and BA, so there is no defined order to the particle’s traversal of the gates.

The simple analogy is a train running on tracks governed by a two-way switch, which means it passes through station A or B first but with the switch in superposition it is impossible to say whether the train passes through station A or B first, and appears to pass through both simultaneously.

Chiribella expands, “this means that we are not forced to use, say, gate A first and gate B later, or vice-versa, but we can create a situation in which the two orders AB and BA coexist. In layman’s terms, A happens before B and, at the same time, B happens before A. There is no way to distinguish between or predict the outcome.”

Chiribella’s team is demonstrating how this new way of combining gates has advantages for quantum computation and quantum communication. In a recent breakthrough in November 2017, the seemingly impossible was achieved: “using the superposition of different orders, a way was found to combine two communication lines, that have zero capacity to transmit data, forming a new communication line that can transmit information.” This achievement has led to the collaboration of Chiribella’s team with an experimental group at the University of Vienna.

The team are looking forward to developing a new paradigm of quantum communication with superposition of orders. Quantum communication is now a significant area of interest around the world with groups competing to develop quantum cryptography. In June 2017, a Chinese satellite transmitted entangled protons between ground stations separated by 1,200km, clearing a major hurdle on the way to ultrasecure communications.

### Quantum communications

Besides exotic superpositions in time, the study of causal relations in quantum mechanics may have broad applications in the emerging area of quantum communication networks. “In a complex communication network, it is important to figure out who is communicating with whom,” Chiribella says. This directly relates to the fundamentals of detecting causal relationships. The problem is to understand how the input in one node of the network affects the output at another node.

Even in standard situations where the causal relations can be defined, quantum mechanics has surprises in store. In the classical world, one can discover how variable X affects variable Y by changing the value of X and observing the response of Y. But in quantum mechanics, the variables are replaced by quantum systems, which exhibit notoriously puzzling features like quantum superposition and entanglement .

“Taking advantage of these features,” explains Chiribella, “we can sometimes identify causal relationships more quickly. In some situations, we can figure out the correct causal relationship with a few experiments, and with a probability of error that is exponentially smaller than the probability of error we would have in the classical world.”

“The big revolutions come from innovation at the fundamental level,” he says listing the computer as an example, which came about from research on the foundations of mathematics, lasers from the study of the fundamental interactions between matter and light, and semi-conductors from the investigation of the quantum properties of solids.

Chiribella reflects that we must keep an eye on practical applications of this technology, but it is also important to dive deep into the fundamentals and allow room for creative thinking. He is a keen believer that progress in our society is a result of our innate curiosity.

*Professor Giulio Chiribella obtained his PhD in 2007 from the University of Pavia, Italy. He was an associate professor at the University of Hong Kong and at Tsinghua University, Beijing, where he held a fellowship of the Young 1000 Talents Program of China. Earlier on, he was a postdoctoral fellow at the Perimeter Institute for Theoretical Physics. In 2010, he was awarded the Hermann Weyl Prize for applications of group theory in quantum estimation. In 2018, he was awarded a Croucher Senior Research Fellowship to further his work in the field of quantum science.*

To view Prof Chiribella’s Croucher profile, please click here.