The scarcity of high-quality millimeter-wave coherent light sources constitutes a critical bottleneck for the development of 6G communications, deep-space exploration, and related fields. Conventional approaches, such as low-frequency synthesis and high-frequency beating, suffer from low output power and poor coherence, while cryogenic microwave coherent sources cannot operate at room temperature. Hence, there is an urgent demand for efficient, stable, millimeter-wave stimulated-emission coherent sources that function under ambient conditions. Leveraging the unique advantages of the nitrogen-vacancy (NV) center system—including long spin coherence times at room temperature, optical initializability, and precise tunability of Zeeman levels—combined with a high-quality-factor millimeter-wave resonator, we can construct a coupled system of light and artificial atoms operable at room temperature. This paper proposes a scheme for room-temperature millimeter-wave stimulated emission based on tuning the ground-state zero-field splitting of diamond NV centers via a strong axial magnetic field, aiming to achieve a high-quality coherent millimeter-wave radiation source.
Through Zeeman-level engineering of NV centers under tesla-level magnetic fields, we propose that by applying a strong axial magnetic field B_\textNV > 1.18\ \textT along the 111 crystallographic direction of the NV^- center, the radiative transition frequency of the NV-center artificial atom can be shifted into the millimeter-wave band. Subsequently, a millimeter-wave stimulated-emission system is constructed, comprising a 2 T superconducting magnet, an ensemble of NV centers in diamond, a high-quality-factor cylindrical resonant cavity, and a 532 nm pump laser. A diamond single crystal containing \sim 10^13 NV centers is placed at the center of the cavity and resonantly coupled to the cavity’s TE_01\delta mode. Optical pumping with 532-nm laser light enables population inversion of the NV centers. The stimulated-emission millimeter-wave photons can establish coherent oscillation inside the cavity, thereby yielding a coherent millimeter-wave output. Based on numerical simulations with typical parameters, the threshold conditions for achieving millimeter-wave stimulated emission are derived, and the linewidth of the obtained coherent radiation is evaluated. The results indicate that, using a cavity with a quality factor of Q = 5 \times 10^4 and a diamond sample with N = 4 \times 10^13 NV centers, the optimal pumping rate can be attained with a remarkably low laser-pump threshold of w_\textth \approx 270\ \texts^-1. Under these conditions, the collective coherence of the atomic ensemble is maximized, enabling the generation of coherent millimeter-wave radiation with a power exceeding 10^-6\ \textW (microwatt level) and a linewidth as narrow as 10^-4\ \textHz. Higher output power can be achieved by increasing the number of NV centers, improving the cavity quality factor, or raising the pump rate.
Specifically, this work demonstrates that, by applying a 2 T axial magnetic field and employing a high-quality resonator with Q = 5 \times 10^4, stimulated emission of millimeter waves at a frequency of 53.13\ \textGHz can be realized at room temperature. Moreover, by adjusting the strength of the applied magnetic field, it is theoretically feasible to achieve a fully tunable coherent millimeter-wave source covering the entire 30\ \text–\ 300\ \textGHz band. This provides a viable pathway toward a high-performance room-temperature millimeter-wave light source. The methodology adopted here—combining quantum Langevin equations with the mean-field approximation to describe stimulated-emission amplification—can be extended to other microwave gain-media systems for realizing various coherent radiation sources based on stimulated emission. In particular, the fact that the coherence properties of NV centers are only weakly affected by strong magnetic fields makes the proposed magnetic-field-tuning scheme for millimeter-wave stimulated emission from diamond color centers experimentally verifiable, paving the way for the generation of highly coherent wave sources across the entire microwave frequency range.