The continued remarkable advances in optical clocks and oscillators has led to a parallel strong development of optical clock networks [1]. Such networks have the potential to support a wide range of applications from basic time/frequency dissemination, to clock-based geodesy, to tests of general relativity [1]. To support optical clocks/oscillators at their highest accuracy and precision, these networks must rely on optical signals between the nodes to carry the timing signals. Networks based on the reciprocal transmission across fiber optics have allowed frequency comparisons between remote clocks across Europe and Japan [2, 3]. However, there are many potential applications, both terrestrial and space-based, for which the clocks will not be connected via an optical fiber and the signals must be sent across free space optical links. To this end, we have been developing techniques for the free-space transfer of time and frequency [4-6]. Moreover, rather than pursuing only frequency comparison, we have developed an approach that allows time comparison and even real-time synchronization between nodes. Such a capability can support the wide variety of proposed applications of mobile optical clock networks.The continued remarkable advances in optical clocks and oscillators has led to a parallel strong development of optical clock networks [1]. Such networks have the potential to support a wide range of applications from basic time/frequency dissemination, to clock-based geodesy, to tests of general relativity [1]. To support optical clocks/oscillators at their highest accuracy and precision, these networks must rely on optical signals between the nodes to carry the timing signals. Networks based on the reciprocal transmission across fiber optics have allowed frequency comparisons between remote clocks across Europe and Japan [2, 3]. However, there are many potential applications, both terrestrial and space-based, for which the clocks will not be connected via an optical fiber and the signals must be sent across free space optical links. To this end, we have been developing techniques for the free-space transfer of time and frequency [4-6]. Moreover, rather than pursuing only frequency comparison, we have developed an approach that allows time comparison and even real-time synchronization between nodes. Such a capability can support the wide variety of proposed applications of mobile optical clock networks.

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)Jun 1, 20170 citations

Demonstration of chronometric leveling using transportable optical clocks beyond laser coherence limit

Yi Yuan, Kaifeng Cui, Daoxin Liu +6 more

An optical clock network requires the establishment of an optical frequency transmission link between multiple optical clocks, utilizing narrow-linewidth lasers. Despite achieving link noise levels of ${10}^{\ensuremath{-}20}$, the final accuracy is limited by the phase noise of the clock laser. Correlation spectroscopy has been developed to transmit frequency information between two optical clocks directly, enabling optical clock comparison beyond the phase noise limit of clock lasers, and significantly enhancing the measurement accuracy or shortening the measurement time. In this article, two compact transportable ${}^{40}\mathrm{Ca}{}^{+}$ clocks are employed to accomplish a correlation spectroscopy comparison, demonstrating a 10-cm level measurement accuracy of chronometric leveling using a mediocre clock laser with linewidth of 200 Hz. The relative frequency instability reaches $6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}/\sqrt{\ensuremath{\tau}/s}$, which is about 20 times better than the result with Rabi spectroscopy using the same clock laser. This method circumvents the harsh requirements on the performance of the clock laser, so that an ordinary stable laser can also be employed in the construction of an optical clock network, which is essential for field applications, especially for chronometric leveling.An optical clock network requires the establishment of an optical frequency transmission link between multiple optical clocks, utilizing narrow-linewidth lasers. Despite achieving link noise levels of ${10}^{\ensuremath{-}20}$, the final accuracy is limited by the phase noise of the clock laser. Correlation spectroscopy has been developed to transmit frequency information between two optical clocks directly, enabling optical clock comparison beyond the phase noise limit of clock lasers, and significantly enhancing the measurement accuracy or shortening the measurement time. In this article, two compact transportable ${}^{40}\mathrm{Ca}{}^{+}$ clocks are employed to accomplish a correlation spectroscopy comparison, demonstrating a 10-cm level measurement accuracy of chronometric leveling using a mediocre clock laser with linewidth of 200 Hz. The relative frequency instability reaches $6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}/\sqrt{\ensuremath{\tau}/s}$, which is about 20 times better than the result with Rabi spectroscopy using the same clock laser. This method circumvents the harsh requirements on the performance of the clock laser, so that an ordinary stable laser can also be employed in the construction of an optical clock network, which is essential for field applications, especially for chronometric leveling.

arXiv:2310.08835Physical Review AppliedApr 29, 202412 citations

Optical lattice clock

Masao Takamoto, Hidetoshi Katori

Conference on Lasers and Electro-OpticsJan 1, 20055 citations

40Ca+ optical clocks in China

Hua Guan, Baolin Zhang, Huaqing Zhang +4 more

In this review, the improvements made in the past two decades for 40Ca+ optical clocks in China are presented. Uncertainty of below 1.3×10−17 was achieved by applying the “magic” RF trapping frequency and introducing a generalized Ramsey excitation scheme. The stability of 40Ca+ optical clocks has been improved to 2.5×10−15/τ with an uptime rate of 93.8% by implementing two ameliorated lock algorithms in the Ramsey excitation scheme. A long-term clock frequency comparison shows a clock stability of 6.3 × 10−18 in an averaging time of 524 000 s. A robust and transportable clock installed in an air-conditioned car trailer can achieve almost the same performance of laboratory clocks. A height difference between transportable and laboratory clocks was measured with an uncertainty of 0.33 m and the absolute frequency of 40Ca+ optical clock transitions was remeasured as 411 042 129 776 400.41(23) Hz, with a fractional uncertainty of 5.6×10−16 based on a Cs fountain clock in the National Institute of Metrology after the transportable clock was transported from Wuhan to Beijing. The author predicts that transportable single-ion optical clocks especially based on the 40Ca+ will make a significant contribution to the construction of a world-wide optical clock network and the redefinition of the unit of time in the future.In this review, the improvements made in the past two decades for 40Ca+ optical clocks in China are presented. Uncertainty of below 1.3×10−17 was achieved by applying the “magic” RF trapping frequency and introducing a generalized Ramsey excitation scheme. The stability of 40Ca+ optical clocks has been improved to 2.5×10−15/τ with an uptime rate of 93.8% by implementing two ameliorated lock algorithms in the Ramsey excitation scheme. A long-term clock frequency comparison shows a clock stability of 6.3 × 10−18 in an averaging time of 524 000 s. A robust and transportable clock installed in an air-conditioned car trailer can achieve almost the same performance of laboratory clocks. A height difference between transportable and laboratory clocks was measured with an uncertainty of 0.33 m and the absolute frequency of 40Ca+ optical clock transitions was remeasured as 411 042 129 776 400.41(23) Hz, with a fractional uncertainty of 5.6×10−16 based on a Cs fountain clock in the National Institute of Metrology after the transportable clock was transported from Wuhan to Beijing. The author predicts that transportable single-ion optical clocks especially based on the 40Ca+ will make a significant contribution to the construction of a world-wide optical clock network and the redefinition of the unit of time in the future.

AVS Quantum ScienceOct 11, 20214 citations

Optical lattice clocks

S. Bize

Oct 10, 20230 citations

{}^{40}\mathrm{Ca}{}^{+}

clocks are employed to accomplish a correlation spectroscopy comparison, demonstrating a 10-cm level measurement accuracy of chronometric leveling using a mediocre clock laser with linewidth of 200 Hz. The relative frequency instability reaches $6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}/\sqrt{\ensuremath{\tau}/s}$, which is about 20 times better than the result with Rabi spectroscopy using the same clock laser. This method circumvents the harsh requirements on the performance of the clock laser, so that an ordinary stable laser can also be employed in the construction of an optical clock network, which is essential for field applications, especially for chronometric leveling.