The discovery of Na3Bi as a three-dimensional topological Dirac semimetal (TDS) represents a significant advancement in quantum materials research. This material exhibits 3D Dirac fermions with linear dispersion along all momentum directions, a unique electronic structure that makes it the 3D counterpart of graphene. Unlike 2D Dirac fermions in graphene or surface states of topological insulators, Na3Bi's Dirac fermions exist in the bulk and are protected by bulk crystal symmetry. This discovery establishes Na3Bi as the first known model system for 3D TDSs, offering a platform for studying quantum phase transitions between various topological states. Na3Bi's structure consists of stacked triple-layer groups with a specific rotational arrangement, leading to a unique 3D Brillouin zone. The material's electronic structure was determined using angle-resolved photoemission spectroscopy (ARPES), revealing a pair of 3D Dirac fermions near the Γ point with distinct anisotropy. These Dirac fermions are robust against surface modifications, indicating their protection by bulk symmetry. The material's electronic properties include giant diamagnetism, quantum magnetoresistance, and unique Landau level structures, making it a promising candidate for spintronics and quantum computing applications. The study also demonstrates that Na3Bi's Dirac fermions can be manipulated by surface doping, with the Fermi level tunable through potassium doping. This control over the electronic structure highlights the material's potential for applications in quantum devices. The discovery of Na3Bi as a TDS opens new avenues for exploring topological quantum states in three dimensions and could lead to novel quantum technologies. The research underscores the importance of understanding and harnessing the unique properties of topological materials for future technological advancements.The discovery of Na3Bi as a three-dimensional topological Dirac semimetal (TDS) represents a significant advancement in quantum materials research. This material exhibits 3D Dirac fermions with linear dispersion along all momentum directions, a unique electronic structure that makes it the 3D counterpart of graphene. Unlike 2D Dirac fermions in graphene or surface states of topological insulators, Na3Bi's Dirac fermions exist in the bulk and are protected by bulk crystal symmetry. This discovery establishes Na3Bi as the first known model system for 3D TDSs, offering a platform for studying quantum phase transitions between various topological states. Na3Bi's structure consists of stacked triple-layer groups with a specific rotational arrangement, leading to a unique 3D Brillouin zone. The material's electronic structure was determined using angle-resolved photoemission spectroscopy (ARPES), revealing a pair of 3D Dirac fermions near the Γ point with distinct anisotropy. These Dirac fermions are robust against surface modifications, indicating their protection by bulk symmetry. The material's electronic properties include giant diamagnetism, quantum magnetoresistance, and unique Landau level structures, making it a promising candidate for spintronics and quantum computing applications. The study also demonstrates that Na3Bi's Dirac fermions can be manipulated by surface doping, with the Fermi level tunable through potassium doping. This control over the electronic structure highlights the material's potential for applications in quantum devices. The discovery of Na3Bi as a TDS opens new avenues for exploring topological quantum states in three dimensions and could lead to novel quantum technologies. The research underscores the importance of understanding and harnessing the unique properties of topological materials for future technological advancements.