Our Research

Cosmology

   It is believed that a period of rapid accelerated expansion, called inflation, existed at the beginning of the universe, and that the quantum fluctuations that occurred during the inflationary period were the origin of the current large-scale structure of the universe, and this scenario is the standard model. Furthermore, observations of Type Ia supernovae and large-scale structures have revealed that our universe is still in the second period of accelerated expansion. Figure 1 below schematically illustrates the evolution of such a universe. By conducting theoretical research based on such a scenario, we are approaching the fundamental questions of what the origin of the accelerated expansion of the universe is, what the nature of dark matter is, and whether the structure of the universe was really created by the quantum fluctuations of space-time during inflation.

  In particular, one prediction that can be derived from the above Standard Model is primordial gravitational waves. Primordial gravitational waves have not been detected to date, but if detected, they could provide evidence for inflation and the quantum nature of gravity. Our laboratory has proposed a new theory for the detection of primordial gravitational waves based on the application of quantum decoherence (loss of quantum interference effects) induced by primordial gravitational waves. Recently, we have also been discussing a new detection theory of gravitational waves based on quantum sensing. Thus, we aim to provide a new window for gravitational wave detection using quantum information theory and quantum technology, which have been rapidly developed in recent years, to search for new physics and elucidate the early universe.

The Verification of the Quantum Nature of Gravity

  General relativity successfully describes gravity and accurately explains a wide range of observations, including gravitational lensing, black holes, and gravitational waves. However, because the effects of gravity are extremely weak, its quantum aspects have never been tested at the microscopic scales where quantum mechanical effects become significant. If gravity possesses quantum properties, then superpositions of spacetime itself should be possible. Verifying such a feature would be key to understanding the origin of spacetime and the universe, but no such experimental demonstration has been realized to date.

  In 2017, Bose et al., and independently Marletto & Vedral, proposed experimental schemes focusing on quantum entanglement induced by gravity, drawing significant attention. They claimed that if gravity obeys quantum mechanics, then it should generate quantum entanglement between two massive objects. Since entanglement can only be generated through quantum interactions, the observation of gravity-induced entanglement would serve as direct evidence that gravitational interactions obey the laws of quantum mechanics. Since then, many discussions have taken place regarding the feasibility of such experiments under realistic conditions. Based on this proposal, our group has quantitatively clarified the relationship between gravity-induced entanglement and quantum field theory of gravity. This work provides a quantitative resolution to paradoxes in quantum gravity [1].

  To prepare a massive object, sensitive enough for gravitational detection, in a quantum state, precise techniques such as quantum control and quantum measurement are essential. Our research focuses on optomechanical systems, which use laser light to precisely control macroscopic mirrors. By developing theoretical models for these systems, we are exploring practical approaches to detect gravity-induced quantum entanglement [2]. 

[1]  Sugiyama, Matsumura, Yamamoto PRD 106 125002 (2022)
[2]  Miki, Matsumura, Yamamoto PRD 110(2) 024057 (2024) 

Open Quantum Systems

   In quantum mechanics taught at the undergraduate level, quantum systems such as harmonic oscillators and spin systems are usually treated as “isolated systems. However, in the real world, no quantum system is completely isolated, and all quantum systems always interact with their surrounding environment. Quantum systems that take such interaction with the environment into account are called “open quantum systems,” and the theory that describes their behavior is the “theory of open quantum systems.

   The theory of open quantum systems contributes to our understanding of various phenomena such as the operating principle of lasers, spin relaxation, and analysis of decoherence, and is one of the most important research approaches in our laboratory. Open quantum systems are characterized by their quantum nature, which is easily broken by interaction with the environment. However, this property can be reversed and applied as a quantum sensor that responds with high sensitivity to environmental changes. For example, detection methods utilizing the high sensitivity of open quantum systems are attracting attention in the search for gravitational waves and dark matter.

 Moreover, the Gorini–Kossakowski–Sudarshan–Lindblad (GKSL) equation, one of the fundamental equations describing the time evolution of open quantum systems, has recently found applications in quantum-gravity research. By studying the theoretical framework of relativistic open quantum systems, we aim to advance our understanding of quantum gravity.