Chip-Scale Terahertz Carbonyl Sulfide (OCS) Clock: An Overview and Recent Studies on Long-Term Frequency Stability of OCS Transitions

This paper reviews the recent innovations of a miniature time-keeping device with high affordability: chip-scale molecular clock. It is based on the recent research progress of high-precision terahertz (THz) rotational spectrometers, especially those using CMOS integrated circuit technologies. The clock probes the rotational modes of carbonyl sulfide ( $^{16}$ O $^{12}$ C $^{32}$ S) molecule gas and then calibrates its MHz output according to the measured terahertz transition frequency of OCS. In contrast to cesium/rubidium atomic clocks, the THz OCS clock has fully electronic operations and, hence, a significantly simplified implementation. In particular, it is realizable on a waveguide-attached CMOS chip, which minimizes the form factor and cost. Based on a lab scale prototype with an Allan deviation in the 10 $^{-11}$ range, this paper for the first time studies two critical metrics related to the long-term stability of THz OCS clocks. These metrics are the clock sensitivities to temperature change of the OCS gas and external magnetic field. The measured average temperature coefficient of the clock, without ovenized temperature stabilization and temperature compensation, is 9.5 × 10 $^{-11}$ / $^\circ$ C in the range of 28 $\ ^\circ$ C–70 $\ ^\circ$ C. Next, the measured clock shift in response to a 75 G external magnetic field is $ 4 × 10 $^{-11}$ , with a theoretical value near 10 $^{-13}$ . Lastly, this paper also reviews the first reported CMOS prototype of the clock, which consumes only 66 mW dc power and achieves an Allan deviation of 3.8 × 10 $^{-10}$ ( $\tau$ = 1000 s). Approaches for performance improvements and potential monolithic chip implementation are discussed. These studies present the feasibility of a CMOS-based magnetic-shield/heater-free clock with high energy efficiency and sub- ppb stability over a wide range of operation condition.