Thermodynamics that are widely applied to heat engines and chemical reactions and dominate our daily lives.The second law of thermodynamics, which is especially important, is the law of increasing entropy. For example, if hot coffee is left in the air at room temperature, it will cool down, but on the contrary, the cold coffee cannot heat up by itself. , Represents an irreversible change.

　On the other hand, the basic laws of micro components such as atoms and molecules that make up such macro phenomena (such as the Schrodinger equation of quantum mechanics) have the property of being symmetrical with respect to time inversion.That is, based on quantum mechanics, if it is possible to change the time when coffee cools, it is also possible to change the time when the cold coffee heats up by itself, which causes a contradiction.Therefore, how to understand the consistency between the macro-irreversible world and the micro-reversible law has been a major challenge in physics since the 19th century.

　In recent years, it has become possible to derive the theory of the second law of thermomechanics using quantum mechanics and statistical mechanics for this old difficult problem, but in this research, quantum mechanics of multiple systems without using the concept of statistical mechanics. The second law is derived based on the above, and it is a big feature that it directly connects two widely separated systems, quantum mechanics that governs the microscopic world and thermodynamics that governs our daily life.Furthermore, we succeeded in proving the generalization of the second law of thermodynamics called Fluctuation theorem with the same setting.

　This result is expected to be a major step toward understanding the origin of irreversibleness, also known as the "arrow of time," based solely on quantum mechanics.

Paper information:[PHYSICAL REVIEW LETTERS] Fluctuation Theorem for Many-Body Pure Quantum States