Researchers have recently developed a "full differential spectroscopy" (FDS) method that could help make high-performance optical frequency standards smaller and more practical for portable applications. The method substantially suppresses background and noise while enhancing the clock transition signal, offering a new approach to developing miniaturized optical frequency standards.

The method was proposed and demonstrated by researchers at the National Time Service Center (NTSC) of the Chinese Academy of Sciences (CAS), in collaboration with the Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) of CAS.

The study was published in Communications Physics on Sept 23.

Chip-scale atomic clocks (CSACs) based on coherent population trapping (CPT) represent a milestone in precision timekeeping for applications with stringent requirements for size, weight, power and cost (SWAP+C), such as deep-space exploration, micro- and nano-satellite formation networking, mobile very-long-baseline interferometry (VLBI), and underwater resource exploration.

In recent years, optical frequency standards based on optical transitions (≥100 THz), rather than microwave transitions (~10 GHz), have attracted worldwide attention for their potential to achieve better frequency stability and accuracy while maintaining portability.

The FDS method uses counter-propagating pump and probe beams that interact with an atomic ensemble. The pump beam partially prepares the atomic ensemble in a quantum coherent dark state, while the two orthogonal polarization components of the counter-propagating probe beam undergo constructive or destructive interference with the dark state, simultaneously generating electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA). The two signals share the same Doppler-broadened background but have opposite phases.

With differential detection, a full differential spectrum is obtained, in which the Doppler-broadened background and common-mode noise are substantially suppressed, while the Doppler-free resonant signal is preserved and its amplitude enhanced. This yields a clock transition signal with a high signal-to-noise ratio, a key requirement for high-performance optical frequency standards.

Schematic of the experimental setup for full differential spectroscopy. (Image by YANG Tenghui)

"This method avoids conventional complex setups and arrangements such as additional reference optical beam, a bichromatic laser, an external magnetic field, or fine polarization control. It also operates near room temperature and consumes laser power as low as 100 μW level, which makes it an ideal way to implement more compact or even chip-scale optical frequency standards," said YANG Tenghui from NTSC, first author of the study.

In the experiment, the researchers observed mirror-symmetric EIT/EIA signals, while the measured differential (DIF) signal showed increased amplitude and a suppressed background, in good agreement with theoretical predictions. The results also showed that the noise of the differential signal was reduced by an order of magnitude compared with that of the EIT or EIA signal.

Using the FDS signal with its enhanced signal-to-noise ratio, the team then conducted a laser-locking experiment. A distributed Bragg reflector (DBR) laser was first injection-locked to a miniature Fabry–Pérot cavity for pre-stabilization and then locked to a rubidium vapor cell using the FDS method to realize a miniaturized optical frequency standard.

The resulting frequency stability reached 1.2 × 10^-13 at one second and remained below 10^-12 for up to 1,000 s, demonstrating the potential of the method for developing higher-performance optical frequency standards.

"Besides the miniaturized and high-performance optical frequency standard demonstrated here, the FDS method may also find application in precision spectroscopy, quantum sensors, all-optical switch and coherent manipulation of quantum states, just name a few," said YUN Peter from NTSC, co-corresponding author of the study.

LIANG Wei from SINANO is another co-corresponding author of the study. This work also received support from Professor Boudot Rodolphe of FEMTO-ST, France, and Dr. HAO Qiang of Nanyang Technological University, Singapore.