Manipulation and Control of Quantum Many-Body Systems
Exactly 100 years ago, Heisenberg and others established "quantum mechanics"—a fundamental theory clarifying the physical laws of the microscopic world of molecules and atoms. The new concepts appearing in quantum mechanics seem to conflict sharply with the common sense of our everyday macroscopic world, and accepting its way of thinking is not easy. However, over the past 100 years, the correctness of quantum mechanics has been confirmed by numerous experiments, dramatically advancing our understanding of the natural world.
Entering this century, we have reached a stage where we no longer stop at passively "describing" or "understanding" the microscopic world, but rather seek to actively "manipulate and control" it. In fact, advances in experimental technology have made precise measurement and manipulation at the single-molecule and single-atom level possible. Furthermore, controlling "quantum many-body systems"—which are formed by many microscopic components following quantum mechanics—without losing their quantum nature is a major challenge toward the realization of quantum computers. Normally, if a quantum many-body system is left alone, it relaxes into a state of thermal equilibrium (this fact itself is a completely non-trivial fundamental problem in physics, and active research is still ongoing today!). Since quantum properties are largely lost in a state of thermal equilibrium, we must apply operations to the target quantum system and continue to drive it into a non-equilibrium state.
Theoretically studying challenging practical problems like quantum computers is extremely significant for the discipline of physics itself. For example, for the future development of quantum science and technology, it will be extremely useful to know what is possible and what is impossible in principle when a certain operation is applied to a quantum system. In the macroscopic world, thermodynamic theory predicts that the efficiency of a heat engine cannot exceed a certain universal upper limit (Carnot efficiency). Thermodynamics is precisely a theoretical framework that tells us "what can and cannot be done." How such a thermodynamic framework can be extended to the quantum world is one of the most important fundamental problems in theoretical physics. Additionally, a theoretical framework that comprehensively describes the "non-equilibrium state," which is essential for controlling quantum many-body systems, is currently unknown. Even from the standpoint of fundamental problems in physics, many fascinating questions remain regarding the manipulation and control of quantum many-body systems.
By theoretically studying cutting-edge technical problems, I strongly hope that the universal laws of non-equilibrium quantum many-body systems will be elucidated, even if only slightly, and that our understanding of physics will deepen further.