The nano-carbon materials (carbon nanotube, graphene, and so on) are intriguing stage to develop the material science and energy application based on nano-science and –technology. The optically generated bound electron-hole pair (exciton) plays dominant roles in the optical properties of carbon nanotubes. We demonstrate the experimental evidence of three-particle bound charged states of charged exciton (positive trion) in the hole-doped carbon nanotubes at room temperature, which is the first experimental evidence of high temperature stable charged exciton (trion) among various semiconducting materials and also enable us to control of quantum states with spin degree of freedom in carbon materials.

 It is extremely imperative to enhance the light emission from carbon nanotubes for optical applications in next-generation quantum information processing and low-energy consumption of quantum light sources. However, the luminescence quantum yields are fairly low at about 1%. To overcome this, we studied the luminescence properties of oxygen-doped carbon nanotubes. The large enhancement in the quantum yield of about 20% and very efficient energy up-converted luminescence are realized in oxygen-doped carbon nanotubes.

 These achievements provide us with important examples to demonstrate new optical functionalities beyond the intrinsic properties of nanomaterials by structural modification and act as the first step toward low energy–consumption quantum devices in quantum information processing.

 Since the discovery of graphene, emerging atomically thin materials have caused a paradigm shift in material and optical science. The extreme quantum confinement and locking of the valley and spin degrees of freedom (valley–spin) are realized in atomically thin semiconducting materials. New science and technological applications with different aspects of conventional electronics and photonics will be developed based on these emerging materials.

 We are studying the optical physics of atomically thin materials to realize novel photonics using the valley–spin degrees of freedom. The strong light–matter interaction observed even in very thin layers with only three atomic layer thicknesses can be utilized for these purposes. Moreover, the modulation of carrier doping and photoluminescence enhancement were realized using chemical carrier doping approaches in atomically thin materials. We also revealed the physical mechanism of the valley–spin polarization and obtained useful references for valley–spin control.

 These findings provide us with the basis for the generation, observation, and control of valley–spin degrees of freedom toward the proposed valley–spin photonics.

 The new route for controlling valley–spin degrees of freedom as quantum states has been discussed in previous studies. We have initiated new research on “valley–spin quantum optics” in the interdisciplinary fields of optical physics and materials science. Moreover, we are trying to make the science of the quantum optics apply to the valley–spin quantum photonics.

 A longer coherence time and variable controllability of valley–spin states would be useful for achieving a novel system of the valley–spin quantum optics, which can provide new routes for the application of quantum devices such as multiple-quantum bits and single-photon emitters.

 The study of high-performance and novel functional photovoltaic devices using emerging low-dimensional materials (nanocarbon, perovskite, and transition metal dichalcogenide) has attracted considerable attention toward a new stage of energy science. Therefore, the realization of high-performance solar-cell devices based on the quantum photoelectric conversion process using these nanomaterials is expected.

 We have investigated various types of solar cells, such as those made using perovskite and carbon nanotube/Si heterojunction, as model photovoltaic devices. The new strategy to improve the photovoltaic conversion efficiency in these devices is achieved by gaining a thorough understanding of the photoelectric conversion process. Therefore, high-performance photovoltaic solar cells have been realized in solar cells using novel nanostructures.

 Moreover, we are embarking on developing novel solar-cell devices using a quantum photovoltaic process such as shift-current phenomena.