Abstract Optical clocks are important for precise measurements in the field of physics. As reported, both the instability and uncertainty of optical lattice clocks are more than two orders of magnitude smaller than those of the best microwave clocks. Therefore, in the near future, optical clocks could be used to redefine the second. Nevertheless, an optical clock with reliability comparable to microwave clocks has not been achieved thus far. In this paper, we compared the frequencies of two Ca^+ optical clocks that were nearly continuously operated for 31 days. Through the comparison experiment, the frequency stability of a single clocks was found to be 6.3×10^-18 at an averaging time of 520 000 s and 7.9×10^-18 at averaging time of 262000 s, while the operating uptime rate reached more than 90% in the period of around 5 days. Thus, our experiment demonstrated that it is possible to increase the stability of single-ion optical clocks to the 10^-18 level, while still maintaining quasi-continuous operation with a high operating rate. This result further confirms that optical clocks can potentially be used to redefine the second.
Abstract Optical clocks are important for precise measurements in the field of physics. As reported, both the instability and uncertainty of optical lattice clocks are more than two orders of magnitude smaller than those of the best microwave clocks. Therefore, in the near future, optical clocks could be used to redefine the second. Nevertheless, an optical clock with reliability comparable to microwave clocks has not been achieved thus far. In this paper, we compared the frequencies of two Ca^+ optical clocks that were nearly continuously operated for 31 days. Through the comparison experiment, the frequency stability of a single clocks was found to be 6.3×10^-18 at an averaging time of 520 000 s and 7.9×10^-18 at averaging time of 262000 s, while the operating uptime rate reached more than 90% in the period of around 5 days. Thus, our experiment demonstrated that it is possible to increase the stability of single-ion optical clocks to the 10^-18 level, while still maintaining quasi-continuous operation with a high operating rate. This result further confirms that optical clocks can potentially be used to redefine the second.
Towards a transportable Ca+ optical clock with a systematic uncertainty of 4.8×10−18
We present a compact, long-term nearly continuous operation of a room-temperature Ca+ optical clock setup towards a transportable clock, achieving an overall systematic uncertainty of 4.8×10−18 and an uptime rate of 97.8% over an 8-day period. The active liquid-cooling scheme is adopted, combined with the precise temperature measurement with 13 temperature sensors both inside and outside the vacuum chamber to ensure the accurate evaluation of the thermal environment for the optical clock. The environmental temperature uncertainty is evaluated as 293.31(0.4) K, corresponding to a blackbody radiation (BBR) frequency shift uncertainty of 4.6×10−18, which is reduced more than two times compared to our previous work. Through the frequency comparison between the room temperature Ca+ optical clock and a cryogenic Ca+ optical clock, the overall uncertainty of the clock comparison is 7.5×10−18, including a statistic uncertainty of 4.9×10−18 and a systematic uncertainty of 5.7×10−18. This work provides a set of feasible implementations for high-precision transportable ion optical clocks.
Optical clocks are being developed worldwide and are at the forefront of frequency metrology. Today they outperform microwave clocks instability and inaccuracy by two orders of magnitude, raising discussion about the redefinition of the SI second. Their outstanding performances are paving the way to applications in different fields apart from metrology. Optical clocks can be used for geodetic measurements, such as chronometric levelling, and fundamental physics experiments. These applications require transportable, and even space-born optical clocks that are in development. Space optical clocks will serve as a high accurate references for intercontinental clock comparisons, and will also provide a unique test bench for fundamental physics theories, such as Einstein’s prediction of time dilation and equivalence principle.
This work presents the development and characterization of a transportable strontium optical lattice clock, realised in the framework of the ESA I-SOC (Space Optical Clock on the ISS) mission. The aim of the project is to investigate new technologies and techniques, in order to develop a robust apparatus as demonstrator for space applications. Special effort has been made to reduce the size and weight of the clock, so as to meet the stringent requirements of a space mission. The system reported here is one of the most compact optical clock to date, with a total volume of only 1 m3, excluding electronics.
The clock is operated with the strontium bosonic isotope 88Sr: this is preferable to the fermionic counterpart 87Sr, commonly used in optical lattice clocks, if simplicity and reliability are required. However, in terms of accuracy, bosonic clocks present some disadvantages compared with the fermionic ones, which have significantly slowed down their development for metrological applications.
We characterised the relevant operating parameters and their effect on the clock accuracy and instability. Exploiting a clock laser offering an outstanding stability, we were able to demonstrate a clock fractional instability of 3×10-18 , and an inaccuracy of 2×10-17: both values represent a factor of 30 improvement compared with previous works on bosonic optical clocks. These results suggest that, in the near future, bosonic clocks may become competitive to their fermionic counterparts. This is of interest for applications such as time scale realization, transportable clocks, or for test of fundamental physics. As an example, we measured the 88Sr-87Sr isotope shift, with an uncertainty of 12 mHz.
Microwave atomic clocks have been the de facto standards for precision time and frequency metrology over the past 50 years, finding widespread use in basic scientific studies, communications, and navigation. However, with its higher operating frequency, an atomic clock based on an optical transition can be much more stable. We demonstrate an all-optical atomic clock referenced to the 1.064-petahertz transition of a single trapped 199Hg+ ion. A clockwork based on a mode-locked femtosecond laser provides output pulses at a 1-gigahertz rate that are phase-coherently locked to the optical frequency. By comparison to a laser-cooled calcium optical standard, an upper limit for the fractional frequency instability of 7 x 10(-15) is measured in 1 second of averaging-a value substantially better than that of the world's best microwave atomic clocks.
Stability is one of most important performances of an atomic clock. Here we describe our recent work on improving the stability of our 40 Ca + optical clock. State preparation is adopted to transfer the ion to the ground-state magnetic sublevel of the clock transition, after the quenching laser transfers the ion to the ground state at each cycle. Using this method, the stability for 40 Ca + optical clock is improved to about . Compared with in previous work, the averaging time is decreased to reach a given level of statistical uncertainty in a clock comparison .
Direct comparison of multi-ion optical clocks based on 40Ca+ and 88Sr+
We report the first direct frequency comparison between two multi-ion optical clocks based on the S1/2 to D5/2 transition in \Ca and \Sr ions. Using linear chains of up to nine \Ca ions and six \Sr ions, we demonstrate improved stability as a function of the number of ions that are contributing to the laser frequency stabilization servo. The measured joint fractional frequency stability of the two clocks reaches 1.37(12)×10−15 at one second, placing an upper bound on the same stability of one of the clocks at 9.6(8)×10−16 in one second. We measured the frequency ratio of the two clocks to be RSr/Ca=1.082076536381896986(18), where the systematic uncertainty is primarily limited by the room temperature blackbody radiation. Our direct measurement represents an order of magnitude improvement compared to existing indirect frequency ratio measurements. Furthermore, by combining our results with recent absolute frequency measurements of the \Sr transition, referenced to a primary frequency standard, we refined the absolute frequency of the \Ca transition to νCa+=411042129776400.21(4) Hz, reducing its uncertainty by a factor of three. This study presents the first direct comparison between two multi-ion optical clocks, highlighting their significant potential for future applications in fundamental physics tests, geodesy, and precision metrology.
Many-particle optical lattice clocks have the potential for unprecedented measurement precision and stability due to their low quantum projection noise. However, this potential has so far never been realized because clock stability has been limited by frequency noise of optical local oscillators. By synchronously probing two 87Sr lattice systems using a laser with a thermal noise floor of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$, we remove classically correlated laser noise from the intercomparison, but this does not demonstrate independent clock performance. With an improved optical oscillator that has a $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ thermal noise floor, we demonstrate an order of magnitude improvement over the best reported stability of any independent clock, achieving a fractional instability of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ in 1000 s of averaging time for synchronous or asynchronous comparisons. This result is within a factor of 2 of the combined quantum projection noise limit for a 160 ms probe time with $\ensuremath{\sim}{10}^{3}$ atoms in each clock. We further demonstrate that even at this high precision, the overall systematic uncertainty of our clock is not limited by atomic interactions. For the second Sr clock, which has a cavity-enhanced lattice, the atomic-density-dependent frequency shift is evaluated to be $\ensuremath{-}3.11\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ with an uncertainty of $8.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$.