The quest for time-frequency precision has evolved significantly since 3500 BC, transitioning from rudimentary tools like sundials and hourglasses to today's optical atomic clocks, which now boast the highest precision among the seven fundamental physical quantities. These clocks play indispensable roles in generating standard time, advancing fundamental scientific research, and enabling space-based experiments. Despite achieving remarkable milestones—such as an error margin of one second over tens of billions of years—high-precision optical atomic clocks remain constrained by bulkiness, complexity, high costs, and portability challenges, limiting their practical deployment in non-laboratory environments.

To address these limitations, the research team led by Prof. Jingbiao Chen and Assistant Researcher Tiantian Shi from Peking University, in collaboration with Prof. Anhong Dang, has pioneered the development of a portable, turnkey Voigt optical atomic clock. Through theoretical and experimental investigations of the transmission spectrum of the Voigt anomalous dispersive optical filter (VADOF) under intense light and magnetic fields, a VADOF with optimized parameters was designed. This enabled the realization of a Voigt laser capable of autonomous alignment to atomic transition lines. Under these conditions, the Voigt optical clock—based on the Voigt laser—demonstrates the ability to instantaneously align with atomic transition lines and lock its frequency upon startup, even after prolonged storage or exposure to severe environmental shocks. This breakthrough validates the feasibility of deploying optical clocks in industrial and outdoor settings, while unlocking new possibilities for compact, autonomous atomic devices in both civilian and military applications. The relevant research results are published in Photonics Research, Volume 13, Issue 4, 2025. [Zijie Liu, Zhiyang Wang, Xiaomin Qin, Xiaolei Guan, Hangbo Shi, Shiying Cao, Suyang Wei, Jia Zhang, Zheng Xiao, Tiantian Shi, Anhong Dang, and Jingbiao Chen, "Turn-key Voigt optical frequency standard," Photon. Res. 13, 1083-1093 (2025)]

Fig. 1 (a) Experimental setup diagram of the Voigt optical frequency standard. (b) Frequency instability of the Voigt optical frequency standard.

Figure 1(a) illustrates the experimental configuration of the Voigt optical frequency standard. The system employs MTS to lock the frequency of the Voigt laser to the ⁸⁵Rb D2 transition 5²𝑆_1/2, 𝐹 = 3 → 5²𝑃_3/2, 𝐹 = 4. The Voigt optical frequency standard was subsequently compared with an optical frequency comb to evaluate its frequency stability through down-conversion from the optical to microwave domain. The results are presented in Figure 1(b). Frequency stability testing was divided into short-term and long-term evaluations. For short-term stability, a beat frequency measurement between two Voigt optical frequency standards achieved a stability of 8.50 × 10⁻¹⁴/𝜏^1/2.

Long-term stability was assessed by comparing the Voigt standard with the optical frequency comb. Within the first 100 seconds, the stability was limited by noise from the frequency comb and did not reflect the intrinsic performance of the Voigt standard. Beyond 100 seconds, the stability became dominated by the Voigt standard itself, revealing a long-term stability of 5.8 × 10⁻¹⁴ over 10,000 seconds. This result represents the best long-term stability achieved in the 780 nm band using a thermal-atom-based modulation transfer spectroscopy (MTS) locking scheme.

The key to the turnkey operation of the compact Voigt optical clock lies in the Voigt laser, which automatically aligns its frequency to atomic transition lines. As illustrated in Figure 2 (lower panel), the experimental setup of the Voigt laser is presented. To achieve precise alignment of the Voigt laser with atomic spectral lines, an in-depth investigation into the transmission characteristics of the Voigt atomic dispersive optical filter (VADOF) under intense light and magnetic fields was conducted. This ensured that the transmission spectrum peak of the optimized VADOF (Figure 2, upper panel) perfectly matched the target atomic transition line in the modulation transfer spectroscopy (MTS) locking scheme. Under these conditions, the output frequency of the Voigt laser remains confined within the Doppler-broadened linewidth of the rubidium atomic transition 5²𝑆_1/2, 𝐹 = 3 → 5²𝑃_3/2, 𝐹 = 4, unaffected by external parameters. This enables automatic frequency locking within 60 seconds upon startup, significantly simplifying operational procedures.

Figure. 2 Transmission spectrum characteristics of the Voigt atomic dispersive optical filter (VADOF), and the experimental configuration of Voigt laser.

The Voigt laser demonstrates inherent immunity to fluctuations in laser diode current and temperature. As shown in Figures 3(a)–(c), despite variations in diode current and temperature, the output wavelength of the Voigt laser fluctuates only within the range of 780.243–780.2445 nm, maintaining precise alignment with the ⁸⁵Rb D2 transition. To further validate the turnkey functionality, temperature stress tests were performed by incrementally raising the diode temperature by 2.5°C every 120 s. During the first 60 s of each cycle, the diode temperature was ramped, followed by 60 s of automatic relocking and stable operation (Figure 3d). Notably, the Voigt optical frequency standard temporarily lost lock during temperature shocks but promptly reacquired lock once the temperature stabilized, without requiring manual intervention. In contrast, conventional optical frequency standards based on external-cavity semiconductor lasers suffer from significant frequency drift under similar conditions, necessitating high-precision wavelength meters and manual recalibration. These results underscore the superior robustness of the Voigt optical clock, making it highly suitable for field applications.

Fig. 3 (a) Saturation absorption spectrum of ⁸⁵Rb atoms. (b) Variation in the output wavelength of the Voigt laser under changes in laser diode current. (c) Variation in the output wavelength of the Voigt laser under changes in laser diode temperature. (d) Temperature stress test: The laser diode temperature is increased by 2.5°C every 120 s, with the first 60 s allocated for temperature ramping and the subsequent 60 s for automatic relocking and stable operation of the Voigt optical frequency standard.

Following the validation of turnkey functionality, the frequency stability was optimized by analyzing dominant factors affecting short- and long-term performance. Short-term stability is primarily limited by residual amplitude noise at off-resonance error signal regions, attributable to etalon effects. This was mitigated through the adoption of wedge-shaped optical components. Additionally, enhancing the signal-to-noise ratio (SNR) by increasing the atomic density (via elevated reference cell temperatures) and optimizing pump-probe power ratios improved error signal amplitudes. As depicted in Figure 4, these measures achieved a record short-term frequency stability of 8.50 × 10⁻¹⁴𝜏^-1/2, reaching internationally leading levels.

Fig. 4 (a) Short-term frequency stability variations corresponding to changes in reference cell temperature. (b) Impact of pump-probe power ratios on short-term frequency stability. (c) Optimal beat-note frequency stability and linear fitting results.

For long-term stability, the impacts of temperature fluctuations in the electro-optic modulator (EOM) and reference cell, long-term power drift of the Voigt laser, and residual post-locking noise were investigated (Figure 5). The long-term stability within 1–100 s was dominated by noise transferred from the optical frequency comb, masking the intrinsic performance of the Voigt standard. Beyond 100 s, long-term temperature drift in the reference cell and power drift in the Voigt laser emerged as critical limiting factors. Future efforts will focus on optimizing laser power stability or implementing active power stabilization schemes to further enhance long-term frequency stability.

Fig. 5 Long-term frequency stability of the Voigt optical frequency standard compared with an optical frequency comb, along with contributions from various technical noise sources.

Future optimizations can be categorized into addressing short-term and long-term frequency instabilities. For short-term instability, enhancing the signal-to-noise ratio of the error signal is paramount. This can be achieved by implementing wedge-shaped EOMs and reference cells to reduce residual amplitude noise in the optical path. Additionally, expanding the beam diameters of the pump and probe lights can increase the number of interacting atoms and mitigate shot noise, thereby amplifying signal intensity. For long-term instability, improving the long-term power stability of the Voigt laser and enhancing the overall temperature stability of the optical clock system are critical steps. These advancements are expected to push the long-term frequency instability of the 780 nm thermal-atom-based optical frequency standard to the 10⁻¹⁴ level. Concurrently, further miniaturization of the optical frequency standard is essential to pave the way for its deployment in vehicular, maritime, and civilian applications.