To make a solid-state qubit, a defect must be inserted into the solid material to control the spin states of the electrons. These quantum defects have the potential to enable integration of quantum information systems with conventional electronic devices. However, positioning the defects is challenging, especially when synthetic diamond is used as the solid material for the qubit. Instead of using synthetic diamond, researchers at the University of Texas at Dallas (UTD) explored the use of sheets of transition metal dichalcogenides (TMDs), a family of 2D materials, to make solid-state qubits. The researchers searched computationally through defects for a spin defect qubit in 2D monolayer TMDs, and performed a comprehensive characterization of fundamental qubit properties, including electronic, magnetic, vibrational, and optical properties, as well as thermodynamic stability. Among multiple advantages that the TMDs provided as a solid platform for hosting quantum defects, the researchers determined that defects consisting of transition metal atoms substituted at chalcogen sites with desirable spin-triplet ground state, zero-field splitting in the tens of gigahertz, and strong zero-phonon coupling to optical transitions in the highly desirable telecom band. The nitrogen-vacancy (NV) center (NCVC−1 defect complex in diamond) does not operate at telecom wavelengths for low-loss fiber transmission. “We need to move away from using diamond to using a system that is 2D, like paper, to control defect positions,” said professor Kyeongjae Cho, who led the research. The ability to control a qubit’s properties could lead to better ways to maintain the qubit conditions needed for quantum entanglement and long-distance communication over optical fiber networks — two essential processes for fast, long-distance quantum communications, he said. Quantum entanglement — the process that enables quantum computers to process huge amounts of information simultaneously — can only be maintained under precisely controlled conditions. Optimizing qubit properties using TMDs should help ensure the reliability of quantum information processing and communication, Cho said. In addition, the researchers found that TMDs offered tunable properties and the possibility of position control. The placement of defects in a 2D layer of material, versus defects buried in a 3D material, could be precisely controlled using a scanning tunneling microscope or focused electron beam lithography. From left: Researcher Xiuyao Lang, professor Kyeongjae Cho, and researcher Yaoqiao Hu. The researchers found that quantum computing qubits made from transition metal dichalcogenides, a family of 2D materials, can surmount the challenges of using synthetic diamonds for qubits. The researchers are holding atomic structure models of solids, with the white sticks representing the bonding between atoms, and the blue, red, and gray balls representing different atoms in solid structures. Courtesy of the University of Texas at Dallas. TMD qubits also support wavelengths in the infrared range — a requirement for communication over optical fiber networks. “While diamond qubits must first be converted to the correct infrared wavelength, TMD-based qubits do not need to be converted,” Cho said. 2D materials provide additional advantages, compared to 3D materials, including heterostructure engineering, reduced sensitivity to the nuclear spin environment, and ease of integration with photonic platforms. The team also demonstrated strain effects on radiative recombination lifetime and defect energy levels. The technique could be used to further engineer qubit properties and applications for sensitive quantum strain sensors. The UTD researchers collaborated with researchers from MIT and other institutions to develop a method for fabricating small, 2D TMD, single-crystal materials using geometric confinement. The confined-growth technique provides a path to obtaining wafer-scale, single-domain, 2D monolayer arrays and their heterostructures on arbitrary substrates. The results of this component of the research were published in the journal Nature (www.doi.org/10.1038/s41586-022-05524-0). With the discovery and characterization of promising spin-defect qubits in monolayer TMDs, the team introduces a path to achieving spin-defect qubits with 2D materials. The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-35048-0).