Creating a New Quantum Computer Platform by Controlling Spin of Single Atoms

In the invisible world of quantum mechanics, different laws of physics operate compared to the macroscopic world that we live in. Quantum superposition and entanglement are some of the representative phenomena that occur in the smallest scales of the universe. When commercialized, quantum computers utilizing these phenomena are expected to rapidly perform vast amounts of calculations that current computers cannot handle. While the basic unit of computation in conventional computers, the bit, can only represent states of 0 or 1, the basic unit of quantum computers, the qubit, can have superpositions of both 0 and 1.

In May, the IBS Center for Quantum Nanoscience successfully developed a new qubit platform using the electron spin of single atoms. Korean researchers were the first in the world to achieve this new representation of qubits, opening up new possibilities for quantum computers. We spoke with PHARK Soo Hyon, a research fellow at the IBS Center for Quantum Nanoscience to learn more about their research.

Q: Please introduce yourself.

A: After completing my doctoral studies in Physics at Seoul National University in 2006, I embarked on a research career at the Department of Oxide Electronics Engineering Research at Seoul National University, focusing on the surface and initial growth of oxides. From 2010 to 2015, I conducted research on magnetic nanostructures at the Max Planck Institute in Germany. The research topics from that period continue to influence my work to this day.

Upon returning to Korea, I had various roles in the early establishment of the IBS Center for Quantum Nanoscience in 2017. Before the center officially commenced research activities, I conducted research at the IBM Research Laboratory in San Jose, USA from 2019 to 2020, focusing on the magnetic properties of atomic chain structures and quantum state control of single-atom spins using high-frequency signals. Building on the ideas gained during that time, the Center for Quantum Nanoscience now utilizes scanning tunneling microscopy (STM) and electron spin resonance (ESR) to investigate the quantum mechanical properties of solid surfaces and nanostructures at atomic resolution.

Q: Please introduce the IBS Center for Quantum Nanoscience, where you work.

A: The IBS Center for Quantum Nanoscience was established in January 2017 and currently consists of approximately 35 research personnel and over 10 administrative and technical support staff members who collaborate closely with each other.

The center focuses on investigating the quantum mechanical properties of individual atoms and molecules placed on solid surfaces. These physical and chemical properties are applied to nanostructures composed of atoms and molecules, and their potential applications in fields such as quantum information, computing, and sensing are explored. The Center’s primary research involves utilizing scanning tunneling microscopy (STM) to leverage quantum mechanical phenomena. Using STM, the electronic states of individual atoms, molecules, or artificially created structures can be controlled and measured. Currently, I lead a research team at the Center for Quantum Nanoscience, overseeing the 'Atomic-Scale Electron Spin Qubit' project.

Q: After living as a researcher abroad, did you have a specific reason for deciding to return to Korea and join the IBS Center for Quantum Nanoscience?

A: Simply put, since I completed my graduate studies in Korea, I always had a desire to return to my homeland if the opportunity arose. For approximately 5 years at the Max Planck Institute, I conducted research on atomic resolution imaging of nanomagnets, a field that was scarcely explored in Korea at that time. Thus, I felt that with my expertise, I could contribute to pioneering new areas of research upon returning to Korea.

Q: Please introduce some key research achievements of the Center for Quantum Nanoscience.

A: In 2018, we successfully measured the nuclear spin state of a single atom on a solid surface by combining STM and ESR techniques. In 2019, we achieved the control and measurement of the electron spin state of a single atom (titanium) on the surface by applying microwave pulses.

Building upon these successes, in May 2023, we also succeeded in controlling the qubit of a single atom spin on the surface. Furthermore, we implemented a multi-qubit system capable of controlling multiple qubits simultaneously. All three of these research results have been published in Science.

Q: Please introduce the research on electron spin qubits published in Science this year.

A: In this study, we created a structure with multiple titanium atoms placed on the surface of a thin insulator made of magnesium oxide, which was then used to form a multi-qubit platform. This novel approach to creating qubits opens up endless possibilities for future development of quantum computers.

Q: How was the research conducted?

A: The research team first succeeded in manipulating the precise positions of each atom using the probe of the STM. Through this, we managed to create a structure where multiple titanium atoms can interact with each other's spins. Subsequently, we placed the probe on one titanium atom to serve as a sensor, and then we remotely controlled the other titanium atoms to simultaneously control and measure them Each titanium atom serves as an individual qubit and interacts with each other according to the laws of quantum mechanics. Leveraging this, we implemented the 'CNOT' and 'Toffoli' gates, which are the fundamental operations in quantum information processing.

Q: What was the reason for developing a completely new atomic-scale spin qubit, which is different from other forms of qubits?

A: It's true that currently, superconducting junction qubits and ion trap qubits are leading the way from a commercialization perspective. However, in academia, the development of quantum computers is still at a stage where it's unclear who will emerge as the winner when it comes to practical applications. As it's still in the laboratory research stage, research groups worldwide are developing and advancing qubit platforms based on their respective expertise.

Our research team has world-class expertise in the manipulation and measurement of individual atoms and molecules using STM. Therefore, it was natural for us to develop a qubit platform utilizing the spins of individual atoms and molecules.

Q: What are the advantages of atomic-scale spin qubits compared to other types of qubits?

A: Since atomic-scale spin qubits utilize the spin of individual atoms, it is possible to achieve precise manipulation of individual atom’s spins by controlling their positions, as well as arranging multiple atomic spins in desired configurations. This ability to precisely control information exchange between qubits at the atomic level is a significant advantage.

Furthermore, the space occupied by each qubit is only about 1 square nanometer, making it significantly smaller compared to other types of qubits. As technology advances, this compactness presents a unique advantage in increasing qubit density.

Q: Were there any difficulties during the research process?

A: The main challenge arose from the structural limitations of the experimental method using STM (Scanning Tunneling Microscopy). STM utilizes a probe with a tip consisting of a single atom to manipulate the positions of surface atoms and measure signals with atomic resolution. So far, direct control and measurement were only possible for atomic spins very close to the probe. However, to create qubits, it is necessary to control and measure multiple atoms individually. The most significant technological advancement in this research was devising a method to control and measure qubits at a distance from the probe and applying it to various qubit structures. This resulted in the successful development of "remote qubits."

Q: What was the response from the academic community after the publication of the paper?

A: This research was of interest to researchers in both the field of quantum computing and those studying atoms and molecules using STM. Those conducting research using STM seemed to highly appreciate the creation of a new quantum information platform at the atomic scale. It's seen as an advanced application of STM technology and an expansion of its scope.

For those who are already researching in the field of quantum computing, it seems to be perceived as a "new attempt" for now. Since other qubit platforms have already made significant progress, there may be various opinions regarding the potential development of atomic-scale spin qubits.

Q: How is the possibility of commercialization for atomic-scale spin qubits?

A: It's cautious to predict the future of our quantum platform, as the field is just at the beginning stage. Commercialization would require more research on the reliability and degree of integration. To make a computer, you naturally need a huge number of qubits, but currently, our research group has only succeeded in implementing three qubits. Moreover, one of the most critical factors for qubits is reliability in information processing and storage. For example, let’s consider storing information and later verifying it. You would need confidence that the information remains the same as when initially stored and is still trustworthy. Subsequent research is also needed on how accurately information can be processed and stored for an extended period.

However, we can shape the research direction of our platform by considering the established integration density and reliability in existing quantum platforms, which are mainly determined by their physical and chemical properties.

Q: What are the goals of subsequent research?

A: As mentioned earlier, the goals include increasing the number of qubits and ensuring a sufficiently long operation time to perform multiple quantum operations continuously. With the multiple qubit structure and control method introduced in this study, we anticipate being able to connect and operate up to 5-6 qubits. While this may seem significantly less compared to the 100+ qubits integrated into leading superconducting or ion trap qubits, it's not a simple comparison due to differences in research timelines.

Quantum platform development is taking place worldwide in various material systems. Excluding superconducting materials and ion traps, the number of qubits that can be controlled and measured simultaneously does not exceed a few in most cases. Considering this, our research is not lagging behind that much compared to the overall quantum platform research.

Q: What are the plans for future research?

A: In the future, we plan to create a 5-6 qubit system and apply various fundamental quantum algorithms that have been theoretically proposed. Through this, we aim to assess the practicality and potential of atomic-scale spin qubits and explore directions for advancement.

Furthermore, future research will focus on improving and refining the methods of connection and measurement between qubits, with the aim of developing a platform capable of controlling more than 10 qubits simultaneously. Research to enhance reliability will also continue.

Q: What support is needed for future research?

A: Roughly speaking, from the inception of a new concept or idea to its commercialization, research goes through three main stages: 1) Laboratory research, 2) Practicality research through collaboration with experts in basic science and technology application fields, and 3) Commercialization research conducted by companies.

The laboratory research phase, which institutions like ours are responsible for, involves understanding the properties of unknown subjects and establishing novel methods. Naturally, numerous attempts may fail during this process. However, the knowledge gained accumulates, leading to better ideas and eventually to the discovery of highly useful subjects and methods. Therefore, sustained long-term interest and warm support are essential. Just as leading countries in quantum information science have provided steady support for superconducting and ion trap-based quantum platforms over the past 30 years, we hope for continuous interest and support not only when there are achievements but also during periods of trial and error.