Identifying Quantum Scars for Stable Control of Quantum Chaos

Quantum mechanics is the study of phenomena in a tiny microcosm, such as atoms and electrons. Although it may seem complex because of dealing with the invisible world, quantum technology utilizes the principles of quantum mechanics that promise to resolve uncertainties and limitations in various industries, including sensors, semiconductors, and computers. It is expected to enable the development of faster and more secure information technology. For example, quantum sensors, which measure physical quantities based on the principles of quantum mechanics, can measure subtle signals that classical sensors cannot measure thanks to their high sensitivity and resolution. They can be utilized in various fields, such as basic science, information and communication technology (ICT), urban engineering, health care, and defense. As a result, leading companies worldwide and advanced countries, including the United States, Germany, and China, are investing significantly in research to commercialize quantum technology to secure national security and gain a competitive edge in the industry.

Amidst these developments, a research team led by Professor Park Moon-jip of the Department of Physics at Hanyang University has recently gained attention in the academic community by theoretically proving the existence of a stable “quantum scar state” in the photonic crystal quantum chaos system. The research findings by Professor Park’s team have provided a general methodology for effectively controlling quantum chaos, which is expected to be widely used in various quantum technologies, including quantum sensing.

Stable “Quantum Scars” Found Amid Unstable Quantum Chaos

The performances of tightrope walkers make their audiences dizzy. This is because it is common knowledge in physics that objects with high potential energy will fall to a stable position. However, in quantum mechanics, this common sense can be violated. The behavior of quantum mechanics in a small microscopic world exhibits a level of complexity that is often difficult to comprehend within the framework of classical physics.

In physics, a fixed point is a state in which an object is in equilibrium with force; as such, there is no movement. It is divided into two types: a stable fixed point and an unstable fixed point. A stable fixed point means a ground state with minimized potential energy, while an unstable fixed point means a top state where the balance of force is disrupted by a small deviation.

On the other hand, in quantum mechanics, it is possible for classically stable orbits to become unstable and vice versa. A chaotic system is said to be a system in which this instability is maximized. Particularly, quantum mechanics has proposed that particles can be stably located even at an unstable fixed point because of interference from particle-wave duality. This interesting quantum state is called the “quantum scar state.”

In a complex quantum chaos system, if a stable quantum scar state is implemented despite external interference, it can be widely used in quantum technologies, such as quantum sensing, and quantum computing, in the future. To realize technology based on quantum physics, it is crucial to stably store and control quantum mechanical information. For this reason, the physics community has been attempting to implement a quantum scar state by locking photons at unstable vertices inside microresonators.

Expanding the Existing Problem of Quantum Chaos to Many-Body Systems

The research team succeeded in theoretically identifying the quantum scar state inside the photonic crystal by using photonic crystals made up of numerous resonators arranged in a lattice structure to effectively control the movement of light.

The existing quantum scar state has only been proposed in a single resonator; this is the first time it has been proposed in a photonic crystal structure with multiple resonators. The team said, “When considering the quantum chaos problem in a lattice, the fact that it is possible to control the quantum scar state in a photonic crystal system has been introduced to the academic society.”

Research on Entangled Quantum Matters Will Be Conducted for Quantum Computing Application

The stability in quantum systems has been a significant obstacle to commercializing quantum computing, quantum sensing, and other applications utilizing quantum mechanics. The research of Professor Park’s team is significant in that it suggests the possibility of achieving stable control in general systems.

Professor Park Moon-jip said, “It is meaningful in that it not only presents a general methodology that effectively controls quantum chaos and can be widely used in quantum technology in the future, but also a new direction for mesoscopic system research that connects the boundaries between quantum mechanics and classical mechanics.”

Professor Park cited the “free research environment” as a driving force behind the research achievements. He recalled, “It all started from casual conversations during coffee breaks, where researchers from different disciplines in the laboratory would gather after lunch and freely exchange ideas. That chance encounter sparked the beginning of our research.” Professor Park emphasized the importance of a free and open environment for fostering good research.

Professor Park aims to expand the current study to include “entangled quantum matter.” He said, “To achieve the implementation of quantum scar states in quantum computing technologies, including quantum sensing, it is important to understand quantum scar states in entangled quantum matter.” He added, “Even though we may not know what results we will obtain, I believe that continuously exploring the unknown territory is a quality of a good scientist.”

The research, a collaboration between Professor Park Moon-jip’s research team, Pukyong National University, and the Institute for Basic Science, was published on May 4 in “Light: Science & Applications” (IF: 20.26, top 1.9% in the Journal Citation Reports [JCR] field), an authoritative journal in the field of optics.