A quantum is the smallest unit of an object. In a world of quantum scales, such as clusters of atoms or molecules, the properties of quantum objects are very different from those of macroscopic objects in our everyday lives. Scientists are using these specificities to develop faster devices for storing, calculating and exchanging information. At the Institute of Applied Physics and Materials Engineering of the University of Macau, Chair Professor Tang Zikang, Associate Professor Yin Hao, Assistant Professor Lai Tun and Assistant Professor Sun Pengzhan formed the Quantum Information Research Group to develop new information processing methods based on the principles of quantum mechanics, hoping to make breakthroughs in the fields of computing and communication technology.
Explore the world at the quantum scale
The national 14th Five-Year Plan released in 2021 pointed out that China will implement a number of forward-looking and strategic major national science and technology projects in frontier fields such as quantum information. In the United States, funding for quantum information research has increased significantly since the National Quantum Initiative Act came into effect in 2018. The United Kingdom, Japan, Germany and other countries have also formulated quantum technology policies.
With the support of these cutting-edge technologies, experts around the world are developing quantum computing, a key application in quantum information research. A normal computer can only perform one calculation at a time. In contrast, quantum computers can perform multiple calculations at the same time, storing information in qubits, each of which can not only represent 0 or 1, but also represent "superposition" states of "0 and 1" in different proportions.
Professor Yin Hao's research involves quantum computing using classical coherent beams
In front of a whiteboard full of mathematical icons in his office, Professor Tang Zikang, a physicist who developed the world's thinnest single-walled carbon nanotube, explained the reason why his team studied classical coherent beams for quantum information processing: "Quantum computing has unlimited potential, and it can solve calculations that ordinary classical computers take decades to complete, or even cannot solve, and have become a research topic that countries are eager to explore. Our team also wants to make breakthroughs in cutting-edge research areas that the world is interested in."
Therefore, the Institute has formed a quantum information research group, specializing in quantum information and computing, nonlinear quantum transmission, and new material systems based on topology in solid-state to develop innovative results.
Classical coherent beams are used for quantum computing
Prof Yin has been researching quantum information processing and quantum computing at UM for a long time. He said: "To achieve quantum computing with a degree of perfection and practical use, many difficult problems need to be overcome, such as the qubit state is very fragile and sensitive, and a slight change in temperature can change its state. Therefore, it is often expensive to process and study qubits at close to absolute low temperatures."
Professor Yin also said that in the world of the quantum scale, a particle can be connected with another particle in a strange way, no matter how far apart, as long as one particle is affected, the state of the other particle will automatically change, this phenomenon is called quantum entanglement. "Quantum algorithms usually require two particles to be stably 'entangled', which involves complex and difficult processes."
Quantum Data Research Group, Institute of Applied Physics and Materials Engineering, from left: Professor Yin Hao, Professor Tang Zikang, Professor Sun Pengzhan and Professor Lai Fen
In response to this problem, Prof. Yin and Prof. Tang explored the use of classical coherent beam technology to replace qubits with macroscopic light states (i.e., observable and measurable properties of light at the macroscopic level), and successfully applied them to an efficient licensed quantum algorithm to decompose a large integer into two prime numbers (i.e. "integer factorization"), which was quite exciting. "Many scientists think that this algorithm can only be used through quantum entanglement, but we have shown that this is not necessarily the case."
Professors Tang and Yin use two properties of macroscopic light, such as the "polarization" and "amplitude" of the beam, to realize the function of qubits. Next, they "entangled" the states of these two properties, so that the two showed a mapping relationship, as if the two particles were in quantum entanglement.
Using Shor's algorithm, a quantum algorithm, the team is trying to achieve integer factorization using coherent photons without using a cryogenic environment. Professor Yin said, "Using quantum algorithms in this way is much less costly and complex than quantum entanglement for quantum mechanics norms. Since many cryptography technologies involve large integer factorization, if quantum algorithms can be widely used to decouple large integers, it will make information transfer activities such as banking transactions or private communications safer and more convenient."
New materials for sensors for communication devices
Quantum information research also involves materials that have the potential to be used in a new generation of sensors. At present, communication equipment such as mobile phones, mobile communication base stations and satellites have many sensors. Professor Lai, who has studied the Hall effect for many years, said: "For some materials, when an alternating current electric field is applied in a specific crystal direction, these materials will form a special 'nonlinear Hall effect'. By measuring this voltage, communication equipment can detect the surrounding electromagnetic field. For example, the sensor of a mobile phone can recognize the surrounding wireless electromagnetic wave signal by measuring the Hall voltage."
Nonlinear Hall-effect materials based on Berry-linked polarization tensors can efficiently detect electromagnetic waves in the terahertz band
In recent years, the importance of terahertz communication in 6G mobile communication network technology has become increasingly prominent. As the core device of the terahertz communication system, the performance of the terahertz detector determines the application value of the terahertz system. Current terahertz detectors have some common and difficult problems. Therefore, "nonlinear Hall-effect" materials that are not affected by material scattering and other external conditions have attracted a lot of attention in recent years, and are expected to improve the performance and reliability of future communication equipment.
A research team that Professor Lai has worked on has found that one of the nonlinear Hall effects stems from a material property called the Berry-linked polarization tensor. "Since joining UM, I have continued to study this nonlinear Hall effect, which can be applied to sensors in quantum communication devices to accurately detect and absorb nearby electromagnetic signals," he said.
Tiny channels convey complex information
General electronic products use the flow of electrons to exchange information. Professor Sun uses the flow of molecular ions to achieve the same goal. "I used two-dimensional crystals such as graphene to construct a very tiny channel that selectively allows single molecular ions to pass through to transmit information."
Many scholars believe that most of the classical physical principles cannot be applied to objects at the quantum scale. However, the overseas research team that Professor Sun had participated in found that in the sub-nanometer channel they built, the "Kelvin equation" that describes the accumulation of liquid in tiny pipes (the "capillary condensation phenomenon") is still valid. "I continue to understand the physical properties of these tiny channels at UM and help develop techniques to precisely control the flow of molecular ions," said Prof Sun.
Two-dimensional crystals such as graphene selectively pass atoms such as hydrogen atoms and subatomic particles such as protons through their crystal lattice
In addition, Professor Sun has recently begun to construct two-dimensional "heterogeneous" channels at the nanoscale and at the finer sub-nanoscale. "We can assemble channels with different physical properties from various two-dimensional crystals like playing with Lego bricks, so as to further develop new conceptual communication measures to exchange very complex information with molecular ions."
Quantum research helps build a smart society
Quantum information research is expected to revolutionize key technologies in areas such as communications and computing, and will play a revolutionary role in building a smarter society. Looking ahead, Prof Tong said, "Our Institute is actively stepping up cooperation with other universities and institutions to promote quantum information research, including exploring new physical phenomena and developing new technologies to broaden and enhance the applicability of quantum information research results."