Researchers at the Wigner Research Centre for Physics predicted that single carbon atom acts as quantum bit in atomically-thin tungsten disulphide, reported in their recent Nature Communication paper. The results pave the way for realization of multi quantum bit logical operations and quantum information processes by defect spins in two-dimensional materials.
Carbon atom embedded into two-dimensional tungsten disulphide as quantum bits manipulated by laser excitation
Quantum technology encompasses methods and tools that harness the fundamental rules of quantum mechanics to realize computing, communication, sensing techniques and applications more accurately or efficiently than classical technologies. The basic unit or building block of these technologies are two-level quantum systems called quantum bits or qubits. Several types of quantum architectures have been proposed and designed as qubits, and among them, semiconductor-based solid-state point defects are promising for room temperature operation.
Point defects are very common atomic-like structures in crystals. They could be prepared during crystal growth or post-processing like ion beam irradiation. However, not all defects or host materials could serve as qubit, and ideal candidates should fulfil strict requirements. To achieve optimal performance, it is of vital importance to discover suitable qubit systems that can be efficiently initialized, manipulated and read out. To this end, not only experimentally atomically-precision characterization and control over defects are required but also comprehensive study of the electronic and magneto-optical properties of defect qubits. Recent advances in atomistic simulations of defects in solids make possible for an accurate description of their magneto-optical properties and even predict new types of defect qubits for a given application.
Based on a recent experimental achievement in deterministic activation of carbon substitutional defects in single-layer atomically-thin tungsten disulphide , Adam Gali group at Wigner Research Centre for Physics investigated the neutral charge state of the carbon defect as a qubit in this two-dimensional material. Using density-functionals theory, it was found that the inherited giant spin-orbit coupling mixes the excited states and results in phosphorescence at the telecom band wavelength. The work published in Nature Communications  establishes the quantum protocol of the scalable qubit with spin-photon interface at telecom wavelength region. Compared to the traditional three-dimensional materials, two-dimensional materials provide deterministic manipulation and control of defects and could also be integrated with other architectures. The result provides the prerequisite to realize multiqubit logical operations and quantum information processes based on defects in a two-dimensional material.
 Spin-dependent vibronic response of a carbon radical ion in two-dimensional WS2, Katherine A. Cochrane, Jun-Ho Lee, Christoph Kastl, Jonah B. Haber, Tianyi Zhang, Azimkhan Kozhakhmetov, Joshua A. Robinson, Mauricio Terrones, Jascha Repp, Jeffrey B. Neaton, Alexander Weber-Bargioni, and Bruno Schuler, Nature Communications 12, 7287 (2021).
 Carbon defect qubit in two-dimensional WS2 Song Li, Gergő Thiering, Péter Udvarhelyi, Viktor Ivády and Adam Gali, Nature Communications 13, 1210 (2022).