A progressive decline in fertility is a well-documented aspect of female aging and is associated with a range of cellular and molecular alterations,including genomic instability and modifications in epigenetic regulat...A progressive decline in fertility is a well-documented aspect of female aging and is associated with a range of cellular and molecular alterations,including genomic instability and modifications in epigenetic regulation.Epigenetic clocks,which estimate biological age based on DNA methylation patterns,have been extensively utilized to evaluate general health status and the risk of various diseases.Despite their broad application,the utility of epigenetic clocks in assessing female reproductive health remains only partially characterized.This minireview consolidates recent advancements in the application of epigenetic clocks to evaluate the functional status of the female reproductive system.The objective is to investigate their potential for quantifying and predicting the biological age of reproductive tissues,thereby establishing a theoretical basis for clinical applications in reproductive medicine.To date,no comprehensive minireview has systematically examined multi-tissue epigenetic clock models in the context of female reproductive aging,positioning this minireview as a novel contribution to the field.展开更多
The phenomenon that a clock at a higher gravitational potential ticks faster than one at a lower potential,also known as gravitational redshift,is one of the classical tests of Einstein’s theory of general relativity...The phenomenon that a clock at a higher gravitational potential ticks faster than one at a lower potential,also known as gravitational redshift,is one of the classical tests of Einstein’s theory of general relativity.Owing to their ultra-high accuracy and stability,state-of-the-art optical lattice clocks have enabled resolving the gravitational redshift with a millimeter-scale height difference.Further reducing the vertical inter-clock separation down to the sub-millimeter level and especially shortening the required measurement time may be achieved by employing spin squeezing.Here,we theoretically investigate the spin-squeezing-enhanced differential frequency comparison between two optical clocks within a lattice-trapped cloud of^(171)Yb atoms.The numerical results illustrate that for a sample of 10^(4)atoms,the atomic-collision-limited resolution of the vertical separation between two clocks can reach 0.48 mm,corresponding to a fractional gravitational redshift at the 10^(-20)level.In addition,the required averaging time may be reduced to less than one hundredth of that of conventional clocks with independent atoms.Our work opens a door to the future spin-squeezing-enhanced test of general relativity.展开更多
We present analog clocks fitted to the Mars solar day.These clocks use the standard Earth-based second of the International System of Units(SI)as their operational unit of time,unlike current practice for Mars timekee...We present analog clocks fitted to the Mars solar day.These clocks use the standard Earth-based second of the International System of Units(SI)as their operational unit of time,unlike current practice for Mars timekeeping.We discuss the importance of preserving the SI second.On this basis,we identify the two analog clocks most suitable for public use by a future Mars population.These are a 20-hour clock with a hand motion similar to that of the standard Earth clock,and a 24-hour clock with a novel“Martian”hand motion which strikes the hour when all 3 hands converge onto that hour mark on the dial.Both clocks have Earth-day equivalents to assist learning.We also present a 24-hour“SpaceClock”,similar to the Martian clock but with no favored reference plane,hence equally readable from any viewing orientation.展开更多
A frequency servo system-on-chip(FS-SoC)featuring output power stabilization technology is introduced in this study for high-precision and miniaturized cesium(Cs)atomic clocks.The proposed power stabilization loop(PSL...A frequency servo system-on-chip(FS-SoC)featuring output power stabilization technology is introduced in this study for high-precision and miniaturized cesium(Cs)atomic clocks.The proposed power stabilization loop(PSL)technique,incorporating an off-chip power detector(PD),ensures that the output power of the FS-SoC remains stable,mitigating the impact of power fluctuations on the atomic clock's stability.Additionally,a one-pulse-per-second(1PPS)is employed to syn-chronize the clock with GPS.Fabricated using 65 nm CMOS technology,the measured phase noise of the FS-SoC stands at-69.5 dBc/Hz@100 Hz offset and-83.9 dBc/Hz@1 kHz offset,accompanied by a power dissipation of 19.7 mW.The Cs atomic clock employing the proposed FS-SoC and PSL obtains an Allan deviation of 1.7×10^(-11) with 1-s averaging time.展开更多
According to General Relativity Theory(GRT),by comparing the frequencies between two precise clocks at two different stations,the gravity potential(geopotential)difference between the two stations can be determined du...According to General Relativity Theory(GRT),by comparing the frequencies between two precise clocks at two different stations,the gravity potential(geopotential)difference between the two stations can be determined due to the gravity frequency shift effect.Here,we conduct a clock-transportation experiment for measuring geopotential differences based on frequency comparisons via satellite links between two remote hydrogen atomic clocks.Based on the net frequency shift between the two clocks in two different periods,the geopotential difference between stations of the Beijing 203 Institute Laboratory(BIL)and Luojiashan Time-frequency Station(LTS)is determined.Comparisons show that the experimental result deviated from the reference of Earth gravity model EGM2008 result by(38.5�45.9)m in Orthometric Height(OH).The results are consistent with the frequency stabilities of the hydrogen clocks(at the level of 1015)used in the experiment.With the rapid development of time and frequency science and technology,the approach discussed in this study for measuring the geopotential is prospective and thus,could have broad applications.展开更多
20th century physics experimentally established beyond doubt the fact that moving clocks read differently from “static” clocks. This fact is typically interpreted as support for special relativity. On the other hand...20th century physics experimentally established beyond doubt the fact that moving clocks read differently from “static” clocks. This fact is typically interpreted as support for special relativity. On the other hand, the same century produced proof that clocks at various locations in the gravitational field also read differently, and this fact is explained by general relativity, which is, in general, not Lorentz transformable. This paper establishes a common framework for the physics of clocks in these different situations.展开更多
The optical atomic clocks have the potential to transform global timekeeping,relying on the state-of-the-art accuracy and stability,and greatly improve the measurement precision for a wide range of scientific and tech...The optical atomic clocks have the potential to transform global timekeeping,relying on the state-of-the-art accuracy and stability,and greatly improve the measurement precision for a wide range of scientific and technological applications.Herein we report on the development of the optical clock based on 171Yb atoms confined in an optical lattice.A minimum width of 1.92-Hz Rabi spectra has been obtained with a new 578-nm clock interrogation laser.The in-loop fractional instability of the 171Yb clock reaches 9.1×10-18 after an averaging over a time of 2.0×104 s.By synchronous comparison between two clocks,we demonstrate that our 171Yb optical lattice clock achieves a fractional instability of 4.60×10-16/√τ.展开更多
As a rule,stability calculation of atomic clock requires observations with equivalent sampling interval.Apart from atomic clocks in laboratory,orbital atomic clock stability calculations are impacted of raw data sampl...As a rule,stability calculation of atomic clock requires observations with equivalent sampling interval.Apart from atomic clocks in laboratory,orbital atomic clock stability calculations are impacted of raw data sampling intervals,noncontinuous time series,non-data segment,frequency drift,and other factors.So,the calculated stability results are not so exact.In this article,the impacts of kinds of error sources on Allan and Hadamard variances are analyzed using global positioning system satellite precise clock offset data.And the laws of variety are summarized.展开更多
We report an experimental demonstration of geopotential difference measurement using a pair of transportable ^(40)Ca^(+) optical clocks(TOC-729-1 and TOC-729-3)in the laboratory,each of them has an uncertainty of 1.3&...We report an experimental demonstration of geopotential difference measurement using a pair of transportable ^(40)Ca^(+) optical clocks(TOC-729-1 and TOC-729-3)in the laboratory,each of them has an uncertainty of 1.3×10^(−17) and an instability of 4.8×10^(−15)/√τ.Referenced to a stationary clock of TOC-729-1,the geopotential difference measurements are realized by moving TOC-729-3 to three different locations and the relevant altitude differences are measured with uncertainties at the level of 20 cm.After correcting the systematic shifts(including gravitational red shift),the two-clock frequency difference is measured to be–0.7(2.2)×10^(−17),considering both the statistic(1.0×10^(−17))and the systematic(1.9×10^(−17))uncertainties.The frequency difference between these two clocks is within their respective uncertainties,verifying the reliability of transportable ^(40)Ca^(+) optical clocks at the low level of 10^(−17).展开更多
The Al^＋ ion optical clock is a very promising optical frequency standard candidate due to its extremely small black-body radiation shift. It has been successfully demonstrated with the indirect cooled, quantum-logic...The Al^＋ ion optical clock is a very promising optical frequency standard candidate due to its extremely small black-body radiation shift. It has been successfully demonstrated with the indirect cooled, quantum-logic-based spectroscopy technique. Its accuracy is limited by second-order Doppler shift, and its stability is limited by the number of ions that can be probed in quantum logic processing. We propose a direct laser cooling scheme of AI＋ ion optical clocks where both the stability and accuracy of the clocks are greatly improved. In the proposed scheme, two Al^＋ traps are utilized. The first trap is used to trap a large number of Al^＋ ions to improve the stability of the clock laser, while the second trap is used to trap a single Al^＋ ion to provide the ultimate accuracy. Both traps are cooled with a continuous wave 167nm laser. The expected clock laser stability can reach 9.0 × 10^-17/√τ. For the second trap, in addition to 167nm laser Doppler cooling, a second stage pulsed 234nm two-photon cooling laser is utilized to further improve the accuracy of the clock laser. The total systematic uncertainty can be reduced to about 1 × 10^-18. The proposed Al^＋ ion optical clock has the potential to become the most accurate and stable optical clock.展开更多
It is commonly believed that most European clocks that reached China before the nineteenth century were sent to the emperor as diplomatic presents from European rulers,or were given to Chinese officials by European me...It is commonly believed that most European clocks that reached China before the nineteenth century were sent to the emperor as diplomatic presents from European rulers,or were given to Chinese officials by European merchants in attempts to improve trading conditions.Although such presents had been given in earlier times,British records show that,by the eighteenth century when the export of clocks to China reached its height,most clocks,including the finest,reached China as private trade goods.Once in Canton(Guangzhou),the best clocks were bought by local Chinese officials for inclusion in their annual tribute to the emperor and senior members of the government in Beijing,where many of these clocks survive in the former imperial collection.展开更多
Digital media offer unique opportunities for museums to bring to life the secrets and stories of their historical collections.To bring insight into the process of developing digital media exhibits,this paper presents ...Digital media offer unique opportunities for museums to bring to life the secrets and stories of their historical collections.To bring insight into the process of developing digital media exhibits,this paper presents the perspective of a creative practitioner in approaching technology-and media-based interpretation for collection objects.It follows the Time,Culture and Identity digital workshop held in Beijing in October 2019,which explored and shared ideas about collaborative research and interdisciplinary practice in digital interpretation between academics,institutions,creative practitioners,and developers.Following the direction of the workshop,the paper takes as its focus the clocks and automatons of the imperial collection at the Palace Museum in Beijing.Observations are based on the author’s practice-led experience in running a design studio,Harmonic Kinetic,developing new media exhibits using digital technology and audiovisual media for museums,galleries,and exhibitions in the UK,including the Science Museum,V&A,Barbican,Tate,and the Tower of London.Taking a broad interaction-design-led outlook,the paper explores a personal design perspective for developing interpretive content and considers the particular opportunities and approaches these historical devices suggest.The paper concludes with a final section that reviews the process and reflects on outcomes from the Time,Culture and Identity digital workshop.This explored possibilities for an interpretive exhibit on the Country Scene clock from the Palace Museum collection.展开更多
Foreign-made clocks and watches began to be exported to China in the 17th century. During the Qing Dynasty (1644-1911), the imperial court imported tens of thousands of clocks and watches. At the Palace Museum in Beij...Foreign-made clocks and watches began to be exported to China in the 17th century. During the Qing Dynasty (1644-1911), the imperial court imported tens of thousands of clocks and watches. At the Palace Museum in Beijing, about 200 timepieces collected by the imperial court are still on display in the clock and watch exhibition hall. They were made in Britain, France, Switzerland and Japan. Many foreign presidents and展开更多
Based on gravity frequency shift effect predicted by general relativity theory, this study discusses an approach for determining the gravity potential(geopotential) difference between arbitrary two points P and Q by r...Based on gravity frequency shift effect predicted by general relativity theory, this study discusses an approach for determining the gravity potential(geopotential) difference between arbitrary two points P and Q by remote comparison of two precise optical clocks via optical fiber frequency transfer. After synchronization, by measuring the signal's frequency shift based upon the comparison of bidirectional frequency signals from P and Q oscillators connected with two optical atomic clocks via remote optical fiber frequency transfer technique, the geopotential difference between the two points could be determined, and its accuracy depends on the stabilities of the optical clocks and the frequency transfer comparison technique. Due to the fact that the present stability of optical clocks achieves 1.6×10-18 and the present frequency transfer comparison via optical fiber provides stabilities as high as 10-19 level, this approach is prospective to determine geopotential difference with an equivalent accuracy of 1.5 cm. In addition, since points P and Q are quite arbitrary, this approach may provide an alternative way to determine the geopotential over a continent, and prospective potential to unify a regional height datum system.展开更多
Research on chip-scale atomic clocks （CSACs） based on coherent population trapping （CPT） is reviewed. The back- ground and the inspiration for the research are described, including the important schemes proposed t...Research on chip-scale atomic clocks （CSACs） based on coherent population trapping （CPT） is reviewed. The back- ground and the inspiration for the research are described, including the important schemes proposed to improve the CPT signal quality, the selection of atoms and buffer gases, and the development of micro-cell fabrication. With regard to the re- liability, stability, and service life of the CSACs, the research regarding the sensitivity of the CPT resonance to temperature and laser power changes is also reviewed, as well as the CPT resonance＇s collision and light of frequency shifts. The first generation CSACs have already been developed but its characters are still far from our expectations. Our conclusion is that miniaturization and power reduction are the most important aspects calling for further research.展开更多
Due to high stable rotations, timing of pulsars provides a natural tool to correct the frequency deviation of spaceborne atomic clocks. Based on processing the observational data about a year of Crab pulsar given by X...Due to high stable rotations, timing of pulsars provides a natural tool to correct the frequency deviation of spaceborne atomic clocks. Based on processing the observational data about a year of Crab pulsar given by XPNAV-1 satellite, we study the possibility of correcting the frequency deviation of spaceborne atomic clocks using pulsar timing. According to the observational data in X-ray band and the timing model parameters from radio observations, the pre-fit timing residuals with a level of 67.66 μs are obtained. By fitting the slope of the timing residuals affected by the faked frequency-biased reference clock, we estimated successfully the relative frequency deviation of the reference clock. For a satellite clock with frequency deviation of the order about 10^(-12), a calibration accuracy with relative error of about 2% can be obtained from the Crab pulsar’s data for one year.The stability of the time scale based on Crab pulsar is about 10^(-12) for an interval of one year.展开更多
Cold atom clocks have made remarkable progresses in the last two decades and played critical roles in precision measurements. Primary Cs fountain frequency standards have achieved a total uncertainty of a few parts in...Cold atom clocks have made remarkable progresses in the last two decades and played critical roles in precision measurements. Primary Cs fountain frequency standards have achieved a total uncertainty of a few parts in 1016, and the best optical clock has reached a type B uncertainty below 10-18. Besides applications in the metrology, navigation, etc.,ultra-stable and ultra-accurate atomic clocks have also become powerful tools in the basic scientific investigations. In this paper, we focus on the recent developments in the high-performance cold atomic clocks which can be used as frequency standards to calibrate atomic time scales. The basic principles, performances, and limitations of fountain clocks and optical clocks based on signal trapped ion or neutral atoms are summarized. Their applications in metrology and other areas are briefly introduced.展开更多
We describe the microfabrication of ^85Rb vapour cells using a glass-silicon anodic bonding technique and in situ chemical reaction between rubidium chloride and barium azide to produce Rb. Under controlled conditions...We describe the microfabrication of ^85Rb vapour cells using a glass-silicon anodic bonding technique and in situ chemical reaction between rubidium chloride and barium azide to produce Rb. Under controlled conditions, the pure metallic Rb drops and buffer gases were obtained in the cells with a few mm^3 internal volumes during the cell sealing process. At an ambient temperature of 90 ℃ the optical absorption resonance of ^85Rb D1 transition with proper broadening and the corresponding coherent population trapping （CPT） resonance, with a signal contrast of 1.5% and linewidth of about 1.7 kHz, have been detected. The sealing quality and the stability of the cells have also been demonstrated experimentally by using the helium leaking detection and the after-9-month optoelectronics measurement which shows a similar CPT signal as its original status. In addition, the physics package of chip-scale atomic clock （CSAC） based on the cell was realized. The measured frequency stability of the physics package can reach to 2.1 × 10^-10 at one second when the cell was heated to 100 ℃ which proved that the cell has the quality to be used in portable and battery-operated devices.展开更多
General relativity theory(GRT)concludes that a precise clock ticks at different running rates if it is under the influence of different geopotentials.Therefore,by comparing the running rates of clocks at arbitrary two...General relativity theory(GRT)concludes that a precise clock ticks at different running rates if it is under the influence of different geopotentials.Therefore,by comparing the running rates of clocks at arbitrary two stations,the geopotential difference between them can be determined.In this study,with the help of two hydrogen atomic clocks(noted as H-masers),using the two-way satellite time and frequency transfer(TWSTFT)technique,we carried out experiments of the geopotential difference determination at the China Aerospace Science&Industry Corporation(CASIC),Beijing.Here the ensemble empirical mode decomposition(EEMD)method is adopted to remove periodic signals included in the original observations.Finally,the clock-comparison-determined geopotential difference in the experiments is determined.Results show that the difference between the geopotential difference determined by GRT and that determined by measuring tape is about 1316.1±931.0 m2s-2,which is equivalent to 134.3±95.0 m in height,and in consistence with the stability of the H-masers applied in the experiments(at the level of10-15/day).With the rapid improvement of atomic clocks’accuracy,the geopotential determination by accurate clocks is prospective,and it is promising to realize the unification of the world vertical height system(WVHS).展开更多
We demonstrate an optical frequency comb(OFC)based on a turnkey mode-locked laser with a figure-9-shape structure and polarization-maintaining fibers,for the comparison of frequency among optical clocks with wavelengt...We demonstrate an optical frequency comb(OFC)based on a turnkey mode-locked laser with a figure-9-shape structure and polarization-maintaining fibers,for the comparison of frequency among optical clocks with wavelengths of 698 nm,729 nm,1068 nm,and 1156 nm.We adopt a multi-branch approach in order to produce high power OFC signals at these specific wavelengths,enabling the signal-to-noise ratio of the beatnotes between the OFC and the clock lasers to exceed30 d B at a resolution bandwidth of 300 k Hz.This approach makes the supercontinuum spectra much easier to be generated than a single branch OFC.However,more out-of-loop fibers degrade the long-term frequency instability due to thermal drift.To minimize the thermal drift effect,we set the fiber lengths of different branches to be similar,and we stabilize the temperature as well.The out-of-loop frequency instability of the OFC due to the incoherence of the multi-branch is about5.5×10^(19) for 4000 s,while the in-loop frequency instability of fceo and that of fbeat are 7.5×10^(18) for 1 s and 8.5×10^(18) for 1 s,respectively.The turnkey OFC meets the requirement for the comparison of frequency between the best optical clocks.展开更多
文摘A progressive decline in fertility is a well-documented aspect of female aging and is associated with a range of cellular and molecular alterations,including genomic instability and modifications in epigenetic regulation.Epigenetic clocks,which estimate biological age based on DNA methylation patterns,have been extensively utilized to evaluate general health status and the risk of various diseases.Despite their broad application,the utility of epigenetic clocks in assessing female reproductive health remains only partially characterized.This minireview consolidates recent advancements in the application of epigenetic clocks to evaluate the functional status of the female reproductive system.The objective is to investigate their potential for quantifying and predicting the biological age of reproductive tissues,thereby establishing a theoretical basis for clinical applications in reproductive medicine.To date,no comprehensive minireview has systematically examined multi-tissue epigenetic clock models in the context of female reproductive aging,positioning this minireview as a novel contribution to the field.
基金supported by CAS Project for Young Scientists in Basic Research(Grant No.YSBR-085)the National Time Service Center(Grant No.E239SC1101)+1 种基金Innovation Program for Quantum Science and Technology(Grant No.2021ZD0303200)China Postdoctoral Science Foundation(Grant No.BX2021020).
文摘The phenomenon that a clock at a higher gravitational potential ticks faster than one at a lower potential,also known as gravitational redshift,is one of the classical tests of Einstein’s theory of general relativity.Owing to their ultra-high accuracy and stability,state-of-the-art optical lattice clocks have enabled resolving the gravitational redshift with a millimeter-scale height difference.Further reducing the vertical inter-clock separation down to the sub-millimeter level and especially shortening the required measurement time may be achieved by employing spin squeezing.Here,we theoretically investigate the spin-squeezing-enhanced differential frequency comparison between two optical clocks within a lattice-trapped cloud of^(171)Yb atoms.The numerical results illustrate that for a sample of 10^(4)atoms,the atomic-collision-limited resolution of the vertical separation between two clocks can reach 0.48 mm,corresponding to a fractional gravitational redshift at the 10^(-20)level.In addition,the required averaging time may be reduced to less than one hundredth of that of conventional clocks with independent atoms.Our work opens a door to the future spin-squeezing-enhanced test of general relativity.
文摘We present analog clocks fitted to the Mars solar day.These clocks use the standard Earth-based second of the International System of Units(SI)as their operational unit of time,unlike current practice for Mars timekeeping.We discuss the importance of preserving the SI second.On this basis,we identify the two analog clocks most suitable for public use by a future Mars population.These are a 20-hour clock with a hand motion similar to that of the standard Earth clock,and a 24-hour clock with a novel“Martian”hand motion which strikes the hour when all 3 hands converge onto that hour mark on the dial.Both clocks have Earth-day equivalents to assist learning.We also present a 24-hour“SpaceClock”,similar to the Martian clock but with no favored reference plane,hence equally readable from any viewing orientation.
基金supported by the National Natural Science Foundation of China under Grant 62034002 and 62374026.
文摘A frequency servo system-on-chip(FS-SoC)featuring output power stabilization technology is introduced in this study for high-precision and miniaturized cesium(Cs)atomic clocks.The proposed power stabilization loop(PSL)technique,incorporating an off-chip power detector(PD),ensures that the output power of the FS-SoC remains stable,mitigating the impact of power fluctuations on the atomic clock's stability.Additionally,a one-pulse-per-second(1PPS)is employed to syn-chronize the clock with GPS.Fabricated using 65 nm CMOS technology,the measured phase noise of the FS-SoC stands at-69.5 dBc/Hz@100 Hz offset and-83.9 dBc/Hz@1 kHz offset,accompanied by a power dissipation of 19.7 mW.The Cs atomic clock employing the proposed FS-SoC and PSL obtains an Allan deviation of 1.7×10^(-11) with 1-s averaging time.
基金supported by the National Natural Science Foundations of China[grant numbers 42030105,41721003,42274011]the Fundamental Research Funds for the Central Universities[grant number 2042022kf0003]the Space Station Project[grant number 2020-228].
文摘According to General Relativity Theory(GRT),by comparing the frequencies between two precise clocks at two different stations,the gravity potential(geopotential)difference between the two stations can be determined due to the gravity frequency shift effect.Here,we conduct a clock-transportation experiment for measuring geopotential differences based on frequency comparisons via satellite links between two remote hydrogen atomic clocks.Based on the net frequency shift between the two clocks in two different periods,the geopotential difference between stations of the Beijing 203 Institute Laboratory(BIL)and Luojiashan Time-frequency Station(LTS)is determined.Comparisons show that the experimental result deviated from the reference of Earth gravity model EGM2008 result by(38.5�45.9)m in Orthometric Height(OH).The results are consistent with the frequency stabilities of the hydrogen clocks(at the level of 1015)used in the experiment.With the rapid development of time and frequency science and technology,the approach discussed in this study for measuring the geopotential is prospective and thus,could have broad applications.
文摘20th century physics experimentally established beyond doubt the fact that moving clocks read differently from “static” clocks. This fact is typically interpreted as support for special relativity. On the other hand, the same century produced proof that clocks at various locations in the gravitational field also read differently, and this fact is explained by general relativity, which is, in general, not Lorentz transformable. This paper establishes a common framework for the physics of clocks in these different situations.
基金Project supported by the National Key Basic Research and Development Program of China(Grant Nos.2016YFA0302103,2017YFF0212003,and 2016YFB0501601)the Municipal Science and Technology Major Project of Shanghai,China(Grant No.2019SHDZX01)+1 种基金the National Natural Science Foundation of China(Grant No.11134003)the Excellent Academic Leaders Program of Shanghai,China(Grant No.12XD1402400).
文摘The optical atomic clocks have the potential to transform global timekeeping,relying on the state-of-the-art accuracy and stability,and greatly improve the measurement precision for a wide range of scientific and technological applications.Herein we report on the development of the optical clock based on 171Yb atoms confined in an optical lattice.A minimum width of 1.92-Hz Rabi spectra has been obtained with a new 578-nm clock interrogation laser.The in-loop fractional instability of the 171Yb clock reaches 9.1×10-18 after an averaging over a time of 2.0×104 s.By synchronous comparison between two clocks,we demonstrate that our 171Yb optical lattice clock achieves a fractional instability of 4.60×10-16/√τ.
基金This work was supported by the National Natural Science Foundation of China(No.41074020).
文摘As a rule,stability calculation of atomic clock requires observations with equivalent sampling interval.Apart from atomic clocks in laboratory,orbital atomic clock stability calculations are impacted of raw data sampling intervals,noncontinuous time series,non-data segment,frequency drift,and other factors.So,the calculated stability results are not so exact.In this article,the impacts of kinds of error sources on Allan and Hadamard variances are analyzed using global positioning system satellite precise clock offset data.And the laws of variety are summarized.
基金Project supported by the Basic Frontier Science Research Program of Chinese Academy of Sciences (Grant No.ZDBS-LY-DQC028)the National Key Research and Development Program of China (Grant No.2017YFA0304404)the National Natural Science Foundation of China (Grant No.11674357)。
文摘We report an experimental demonstration of geopotential difference measurement using a pair of transportable ^(40)Ca^(+) optical clocks(TOC-729-1 and TOC-729-3)in the laboratory,each of them has an uncertainty of 1.3×10^(−17) and an instability of 4.8×10^(−15)/√τ.Referenced to a stationary clock of TOC-729-1,the geopotential difference measurements are realized by moving TOC-729-3 to three different locations and the relevant altitude differences are measured with uncertainties at the level of 20 cm.After correcting the systematic shifts(including gravitational red shift),the two-clock frequency difference is measured to be–0.7(2.2)×10^(−17),considering both the statistic(1.0×10^(−17))and the systematic(1.9×10^(−17))uncertainties.The frequency difference between these two clocks is within their respective uncertainties,verifying the reliability of transportable ^(40)Ca^(+) optical clocks at the low level of 10^(−17).
基金Supported by the National Basic Research Program of China under Grant No 2012CB821300the National Natural Science Foundation of China under Grant Nos 91336213,11304109,91536116 and 11174095the Program for New Century Excellent Talents by the Ministry of Education under Grant No NCET-11-0176
文摘The Al^＋ ion optical clock is a very promising optical frequency standard candidate due to its extremely small black-body radiation shift. It has been successfully demonstrated with the indirect cooled, quantum-logic-based spectroscopy technique. Its accuracy is limited by second-order Doppler shift, and its stability is limited by the number of ions that can be probed in quantum logic processing. We propose a direct laser cooling scheme of AI＋ ion optical clocks where both the stability and accuracy of the clocks are greatly improved. In the proposed scheme, two Al^＋ traps are utilized. The first trap is used to trap a large number of Al^＋ ions to improve the stability of the clock laser, while the second trap is used to trap a single Al^＋ ion to provide the ultimate accuracy. Both traps are cooled with a continuous wave 167nm laser. The expected clock laser stability can reach 9.0 × 10^-17/√τ. For the second trap, in addition to 167nm laser Doppler cooling, a second stage pulsed 234nm two-photon cooling laser is utilized to further improve the accuracy of the clock laser. The total systematic uncertainty can be reduced to about 1 × 10^-18. The proposed Al^＋ ion optical clock has the potential to become the most accurate and stable optical clock.
文摘It is commonly believed that most European clocks that reached China before the nineteenth century were sent to the emperor as diplomatic presents from European rulers,or were given to Chinese officials by European merchants in attempts to improve trading conditions.Although such presents had been given in earlier times,British records show that,by the eighteenth century when the export of clocks to China reached its height,most clocks,including the finest,reached China as private trade goods.Once in Canton(Guangzhou),the best clocks were bought by local Chinese officials for inclusion in their annual tribute to the emperor and senior members of the government in Beijing,where many of these clocks survive in the former imperial collection.
文摘Digital media offer unique opportunities for museums to bring to life the secrets and stories of their historical collections.To bring insight into the process of developing digital media exhibits,this paper presents the perspective of a creative practitioner in approaching technology-and media-based interpretation for collection objects.It follows the Time,Culture and Identity digital workshop held in Beijing in October 2019,which explored and shared ideas about collaborative research and interdisciplinary practice in digital interpretation between academics,institutions,creative practitioners,and developers.Following the direction of the workshop,the paper takes as its focus the clocks and automatons of the imperial collection at the Palace Museum in Beijing.Observations are based on the author’s practice-led experience in running a design studio,Harmonic Kinetic,developing new media exhibits using digital technology and audiovisual media for museums,galleries,and exhibitions in the UK,including the Science Museum,V&A,Barbican,Tate,and the Tower of London.Taking a broad interaction-design-led outlook,the paper explores a personal design perspective for developing interpretive content and considers the particular opportunities and approaches these historical devices suggest.The paper concludes with a final section that reviews the process and reflects on outcomes from the Time,Culture and Identity digital workshop.This explored possibilities for an interpretive exhibit on the Country Scene clock from the Palace Museum collection.
文摘Foreign-made clocks and watches began to be exported to China in the 17th century. During the Qing Dynasty (1644-1911), the imperial court imported tens of thousands of clocks and watches. At the Palace Museum in Beijing, about 200 timepieces collected by the imperial court are still on display in the clock and watch exhibition hall. They were made in Britain, France, Switzerland and Japan. Many foreign presidents and
基金supported by the National Natural Science Foundation of China (Nos. 41631072, 41721003, 41574007, and 41429401)the Discipline Innovative Engineering Plan of Modern Geodesy and Geodynamics (No. B17033)+1 种基金the DAAD Thematic Network Project (No. 57173947)the International Space Science Institute (ISSI) 2017–2019
文摘Based on gravity frequency shift effect predicted by general relativity theory, this study discusses an approach for determining the gravity potential(geopotential) difference between arbitrary two points P and Q by remote comparison of two precise optical clocks via optical fiber frequency transfer. After synchronization, by measuring the signal's frequency shift based upon the comparison of bidirectional frequency signals from P and Q oscillators connected with two optical atomic clocks via remote optical fiber frequency transfer technique, the geopotential difference between the two points could be determined, and its accuracy depends on the stabilities of the optical clocks and the frequency transfer comparison technique. Due to the fact that the present stability of optical clocks achieves 1.6×10-18 and the present frequency transfer comparison via optical fiber provides stabilities as high as 10-19 level, this approach is prospective to determine geopotential difference with an equivalent accuracy of 1.5 cm. In addition, since points P and Q are quite arbitrary, this approach may provide an alternative way to determine the geopotential over a continent, and prospective potential to unify a regional height datum system.
基金Project support by the National Natural Science Foundation of China(Grant No.11074012)
文摘Research on chip-scale atomic clocks （CSACs） based on coherent population trapping （CPT） is reviewed. The back- ground and the inspiration for the research are described, including the important schemes proposed to improve the CPT signal quality, the selection of atoms and buffer gases, and the development of micro-cell fabrication. With regard to the re- liability, stability, and service life of the CSACs, the research regarding the sensitivity of the CPT resonance to temperature and laser power changes is also reviewed, as well as the CPT resonance＇s collision and light of frequency shifts. The first generation CSACs have already been developed but its characters are still far from our expectations. Our conclusion is that miniaturization and power reduction are the most important aspects calling for further research.
基金supported by the National SKA Program of China(No.2020SKA0120103)the National Natural Science Foundation of China(Nos.U1831130 and U1531112).
文摘Due to high stable rotations, timing of pulsars provides a natural tool to correct the frequency deviation of spaceborne atomic clocks. Based on processing the observational data about a year of Crab pulsar given by XPNAV-1 satellite, we study the possibility of correcting the frequency deviation of spaceborne atomic clocks using pulsar timing. According to the observational data in X-ray band and the timing model parameters from radio observations, the pre-fit timing residuals with a level of 67.66 μs are obtained. By fitting the slope of the timing residuals affected by the faked frequency-biased reference clock, we estimated successfully the relative frequency deviation of the reference clock. For a satellite clock with frequency deviation of the order about 10^(-12), a calibration accuracy with relative error of about 2% can be obtained from the Crab pulsar’s data for one year.The stability of the time scale based on Crab pulsar is about 10^(-12) for an interval of one year.
基金Project supported by the National Natural Science Foundation of China(Grant No.11873044)the National Key Research and Development Project of China(Grant No.2016YFF0200202)Consulting Research Project of Chinese Academy of Engineering(Grant No.2018-ZCQ-03)。
文摘Cold atom clocks have made remarkable progresses in the last two decades and played critical roles in precision measurements. Primary Cs fountain frequency standards have achieved a total uncertainty of a few parts in 1016, and the best optical clock has reached a type B uncertainty below 10-18. Besides applications in the metrology, navigation, etc.,ultra-stable and ultra-accurate atomic clocks have also become powerful tools in the basic scientific investigations. In this paper, we focus on the recent developments in the high-performance cold atomic clocks which can be used as frequency standards to calibrate atomic time scales. The basic principles, performances, and limitations of fountain clocks and optical clocks based on signal trapped ion or neutral atoms are summarized. Their applications in metrology and other areas are briefly introduced.
基金Project supported by National 863/973 Plans Projects (Grant Nos. 2006AA04Z361,2006CB932402)NSFC (Grant No. 60971002)
文摘We describe the microfabrication of ^85Rb vapour cells using a glass-silicon anodic bonding technique and in situ chemical reaction between rubidium chloride and barium azide to produce Rb. Under controlled conditions, the pure metallic Rb drops and buffer gases were obtained in the cells with a few mm^3 internal volumes during the cell sealing process. At an ambient temperature of 90 ℃ the optical absorption resonance of ^85Rb D1 transition with proper broadening and the corresponding coherent population trapping （CPT） resonance, with a signal contrast of 1.5% and linewidth of about 1.7 kHz, have been detected. The sealing quality and the stability of the cells have also been demonstrated experimentally by using the helium leaking detection and the after-9-month optoelectronics measurement which shows a similar CPT signal as its original status. In addition, the physics package of chip-scale atomic clock （CSAC） based on the cell was realized. The measured frequency stability of the physics package can reach to 2.1 × 10^-10 at one second when the cell was heated to 100 ℃ which proved that the cell has the quality to be used in portable and battery-operated devices.
基金supported by National Natural Science Foundation of China(NSFC)(grant Nos.41721003,41631072,41874023,41804012,41429401,41574007)Natural Science Foundation of Hubei Province(grant No.2019CFB611)
文摘General relativity theory(GRT)concludes that a precise clock ticks at different running rates if it is under the influence of different geopotentials.Therefore,by comparing the running rates of clocks at arbitrary two stations,the geopotential difference between them can be determined.In this study,with the help of two hydrogen atomic clocks(noted as H-masers),using the two-way satellite time and frequency transfer(TWSTFT)technique,we carried out experiments of the geopotential difference determination at the China Aerospace Science&Industry Corporation(CASIC),Beijing.Here the ensemble empirical mode decomposition(EEMD)method is adopted to remove periodic signals included in the original observations.Finally,the clock-comparison-determined geopotential difference in the experiments is determined.Results show that the difference between the geopotential difference determined by GRT and that determined by measuring tape is about 1316.1±931.0 m2s-2,which is equivalent to 134.3±95.0 m in height,and in consistence with the stability of the H-masers applied in the experiments(at the level of10-15/day).With the rapid improvement of atomic clocks’accuracy,the geopotential determination by accurate clocks is prospective,and it is promising to realize the unification of the world vertical height system(WVHS).
基金Project supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB35030101)the National Natural Science Foundation of China(Grant No.61825505)+1 种基金the Quantum Control and Quantum Information of the National Key Research and Development Program of China(Grant No.2020YFA0309800)the Natural Science Basic Research Program of Shaanxi Province,China(Grant No.2020JQ434)。
文摘We demonstrate an optical frequency comb(OFC)based on a turnkey mode-locked laser with a figure-9-shape structure and polarization-maintaining fibers,for the comparison of frequency among optical clocks with wavelengths of 698 nm,729 nm,1068 nm,and 1156 nm.We adopt a multi-branch approach in order to produce high power OFC signals at these specific wavelengths,enabling the signal-to-noise ratio of the beatnotes between the OFC and the clock lasers to exceed30 d B at a resolution bandwidth of 300 k Hz.This approach makes the supercontinuum spectra much easier to be generated than a single branch OFC.However,more out-of-loop fibers degrade the long-term frequency instability due to thermal drift.To minimize the thermal drift effect,we set the fiber lengths of different branches to be similar,and we stabilize the temperature as well.The out-of-loop frequency instability of the OFC due to the incoherence of the multi-branch is about5.5×10^(19) for 4000 s,while the in-loop frequency instability of fceo and that of fbeat are 7.5×10^(18) for 1 s and 8.5×10^(18) for 1 s,respectively.The turnkey OFC meets the requirement for the comparison of frequency between the best optical clocks.
