01 Introduction
Diamond is a crystal composed of carbon elements, possessing excellent mechanical hardness, high thermal conductivity, an extremely low thermal expansion coefficient, an extremely wide spectral transmission range, and excellent chemical stability. It is now widely used in cutting-edge technical fields such as mechanical processing, thermal management, optoelectronics, and aerospace. Quantum diamond refers to diamond containing high-quality nitrogen-vacancy (NV) centers, used in fields such as quantum computing, quantum communication, and quantum precision sensing. Its application principle is based on the excellent optical properties and paramagnetism of NV centers . With in-depth research on the quantum properties of diamond and advances in engineering technology, the application of diamond in the quantum field is increasingly approaching practical use. Diamond color centers not only serve as qubits in quantum computing, with extremely high stability and long coherence times, but also demonstrate superior performance in highly sensitive magnetic field, electric field, and temperature sensing. Currently, numerous academic teams worldwide are studying the application of its quantum properties, and an increasing number of companies are investing in the development of diamond quantum technology.
02 NV Center Structure and Principles
The most common form of diamond is the face-centered cubic structure, in which each carbon atom forms covalent bonds with four neighboring carbon atoms through sp3 hybrid orbitals, constructing a tetrahedral structure. This highly symmetrical strong covalent bond network gives diamond extremely high hardness, low compressibility, and chemical stability. When a carbon atom in the diamond lattice is replaced by a nitrogen atom, with a carbon vacancy present at an adjacent site, the nitrogen atom and the adjacent carbon vacancy form a system, namely the nitrogen-vacancy color center, abbreviated as NV center . In naturally formed diamond, NV center point defects are widely present, and NV centers can also be artificially prepared through various methods.
NV center structure
The NV center has C3v symmetry along the "nitrogen atom-vacancy" connection direction (the NV center axis). Typically, NV centers exist in two forms: NV- (negatively charged) and NV0 (electrically neutral), among which the electron spin state of NV- is easy to polarize and manipulate [4].
The diamond NV center can be regarded as an electron spin with spin S=1, whose ground state is a spin triplet. The initialization and readout of the NV center spin quantum state are both achieved through spin-dependent optical transition processes. Under laser irradiation, the ms=0 spin state is excited to an excited state and returns to the ground state by emitting fluorescent photons, so ms=0 is regarded as the "bright state"; while the ms=±1 spin states, after excitation, have a greater probability of returning to the ms=0 state through the spin singlet pathway, with correspondingly less radiation emitted, so the ms=±1 states are regarded as the "dark state." Therefore, the NV center spin state can be determined by measuring the fluorescence intensity. Based on the same process, a few microseconds of continuous laser polarization can pump the NV spin to the ms=0 state, achieving high-fidelity spin state initialization .
03 NV Center Preparation
To meet the precise requirements for NV centers in practical applications, researchers are committed to exploring various color center preparation methods. These methods include electron irradiation, chemical vapor deposition, ion implantation, and laser direct writing, each with unique advantages and applicable fields, providing diversified solutions for NV center preparation.
3.1 Electron Irradiation Method
The electron irradiation method uses a focused electron beam to irradiate nitrogen-containing diamond, and the electrons strike the lattice structure to form vacancies. During high-temperature annealing, the vacancies diffuse and combine with nitrogen atoms to generate NV centers. By controlling the number, kinetic energy, and irradiation time of electrons, the content and depth of NV centers can be adjusted. This technology can effectively prepare NV centers, and the product fluorescence brightness is relatively high, making it widely used in fields such as magnetic field detection and biomarkers .
NV center preparation by electron irradiation has the advantages of simple operation, low cost, and strong controllability. This method can control the concentration and distribution of vacancies over a large range. However, in the electron irradiation process, the electron energy and dose need to be precisely controlled to avoid severe damage to the diamond lattice caused by excessively high irradiation doses. The temperature and time control during annealing also need to be very precise to ensure efficient formation and uniform distribution of NV centers .
3.2 Chemical Vapor Deposition (CVD) Method
The CVD method is a technology that decomposes carbon-containing gases to crystallize and grow diamond on a substrate. The mixed gas decomposes under vacuum and high temperature into a plasma ball containing carbon-containing groups, which then deposits and crystallizes on the substrate to form diamond. During the growth process, corresponding color centers can be fabricated by doping impurities into the substrate or the gas source. Doping in the substrate facilitates the fabrication of shallow color centers, while doping in the gas source facilitates control of the color center preparation process. The CVD method can adjust the size of diamond crystals and the content of color centers. Single NV centers prepared by the CVD method not only have high purity, but their coherence time τ can also reach the millisecond scale .
3.3 Ion Implantation Method
NV center preparation by ion implantation uses precise ion beam technology to directly implant selected impurity ions, such as nitrogen or other types of ions, into the diamond crystal . The core idea of the ion implantation process is "first dope nitrogen, then create vacancies, then anneal." First, a mask is used to create the desired pattern on the diamond surface; then high-energy nitrogen ions (keV-MeV) bombard according to a set dose, implanting nitrogen atoms into the lattice; next, other impurity ions or high-energy protons bombard the lattice to knock carbon atoms out and form vacancies; finally, annealing is performed in an inert atmosphere, where vacancies migrate and are captured by nitrogen atoms to form NV center arrays, while annealing repairs the lattice damage caused during ion implantation .
3.4 Laser Direct Writing
The laser direct writing method essentially also uses a high-energy electron beam to form vacancies, but the high-energy electron beam is formed by the filamentation effect of femtosecond laser in air. In the filamentation region of the femtosecond laser, gas molecules such as oxygen and nitrogen in the air are ionized by the strong electromagnetic field and high-energy electron beam to form high-density plasma. The high-density plasma bombards the diamond lattice to form vacancies, and after annealing, the vacancies migrate to form NV centers .
The main advantage of laser direct writing in preparing NV centers in diamond is that it provides very high spatial resolution and localized control, allowing precise positioning and fabrication of NV centers at the micrometer or even nanometer scale without the need for complex masks. However, its preparation efficiency is relatively low, and thermal damage is more difficult to repair than the lattice damage caused by electron irradiation and ion implantation .
04 Quantum Diamond Applications
4.1 Quantum Sensing
Diamond NV centers, due to their excellent physical properties, exhibit extraordinary robustness in complex environments and have become revolutionary quantum sensors. Their core advantage lies in the ability to efficiently convert microscopic physical signals such as magnetism, electricity, heat, and strain into stable, optically readable fluorescence and spin state signals.
Magnetic sensors: Magnetic field sensing technology is widely used in many fields such as magnetic anomaly submarine detection, geomagnetic navigation, biomedicine, and geological exploration. Magnetic field sensing technology based on diamond NV centers can theoretically achieve magnetic measurement at the attotesla level, becoming one of the main directions for the future development of miniaturized solid-state atomic magnetic sensing technology.
Temperature sensors: Temperature reflects the magnitude of internal energy and heat in an object and is an important parameter describing the thermodynamic state of an object. Diamond NV center temperature measurement is mainly based on three principles, namely measuring the zero-field splitting value, measuring the zero-phonon line, and indirectly measuring temperature through magnetic field measurement.
Force sensors: Force sensors based on NV centers have high sensitivity, high resolution, and high compressive resistance at the micrometer or even nanometer scale. They can be used for non-destructive mechanical property research and to monitor microscopic mechanical processes.
Electric field sensors: Diamond NV center electric field measurement utilizes its spin properties. By precisely adjusting the magnetic field, the electric induction conversion is decoupled from magnetic fluctuations, providing a method for the sensor to switch from "magnetic" mode to "electric" mode. By exploring the interaction among Zeeman shift, local strain effects, and the Stark shift of the ground-state spin manifold, and using a better understanding of this interaction to sense electric fields.
4.2 Quantum Computing
The realization of quantum computing requires completing the controllable operation and readout, storage, and logic operations of qubits. NV centers can construct fully functional qubits at room temperature, and experiments have already realized the functions of all components required to create a quantum computer, such as quantum error correction, quantum coupling, and quantum repeaters. As qubits, NV centers can operate at room temperature and pressure, and the electron spin T1 can reach the ms scale. Through polarization and fluorescence readout, they have unique advantages .
In July 2026, the German quantum computing company SAXONQ announced the official launch of its commercial diamond NV center quantum computing systems: the 128-qubit SXQ128 and the 512-qubit SXQ512. These systems do not require cryogenic cooling or a vacuum environment and can operate normally at room temperature. SXQ128 and SXQ512 have 128 and 512 qubits, respectively, based on diamond NV center materials and multi-core architecture, and can be used for various quantum computing tasks, including chemical simulation, optimization, and quantum machine learning.
4.3 Quantum Communication
Quantum communication technology can be divided into quantum key distribution and quantum secure communication. The sender and receiver use single photons as carriers, with one part used as signals for transmission and the other part used as quantum keys to detect signal states, ensuring the security of transmitted information. The coherence time of NV center nuclear spins can reach the second scale, enabling long-term storage and efficient processing of quantum information, on which basis it is expected to realize quantum networks.