NIST的原子鐘準到可以做出更好的地球模型

原文網址：https://www.nist.gov/news-events/news/2018/11/nist-atomic-clocks-now-keep-time-well-enough-improve-models-earth

NIST的原子鐘準到可以做出更好的地球模型

美國國家標準與技術研究院(NIST)的實驗性原子鐘在三項表現上達到了新的紀錄，它運行的精準程度不只能讓它更加準時並提升導航技術，還能偵測重力、早期宇宙、甚至可能是由暗物質產生的微弱訊號

NIST的物理學家Andrew Ludlow和同事對比兩個鐿光晶格鐘而在原子鐘的表現上達成新紀錄。圖中前景可以看見兩個原子鐘用的雷射系統，其中一個的主要儀器在Ludlow後方。(圖片來源：Burrus/NIST)

這些時鐘每一座都以雷射交織而成的網――光學晶格(optical lattice)來抓住一千顆鐿原子。這些原子在兩個能階之間震盪(轉換)的時侯會給出時間。NIST的物理學家透過比較個別兩個原子鐘，在三個重要的測量指標上達成了創紀錄的表現，分別是系統不確定度(systematic uncertainty)、穩定性(stability)和再現性(reproducibility)。

根據今日發表在期刊《自然》(Nature)線上版的論文，NIST的新原子鐘達成的紀錄如下：

• 系統不確定度：指原子鐘可以多準確地測出原子的固有震盪(natural vibration)或固有頻率。NIST的研究人員發現每個原子鐘的運行速率相當吻合原子的固有頻率，其中的可能誤差(possible error)低於10¹⁸分之1.4，大約是一百萬兆分之一。
• 穩定性：指原子鐘的頻率在給定一段時間內的變化情形。測量結果達到了在一天之內僅有10¹⁹之3.2(即0.00000000000000000032)的變化。
• 再現性：指兩座原子鐘依據同一頻率運行時得出的時間有多相近。比較同一對時鐘10次，得到它們之間的頻率差距低於10^-18之一 (再一次，誤差小於一百萬兆分之一)。

「對這些原子鐘來說，在系統不確定度、穩定性和再現性都能達到如此好的表現就像拿到了『同花順』一樣。」計畫主持人Andrew Ludlow表示。「而兩座時鐘之間的一致度，也就是我們所稱的再現性達到前所未見的層級或許是最為重要的一項成果。因為它基本上就包含了另外兩個性質，因此也證實了原子鐘在這兩方面的表現。」

「這點相當重要，因為我們的原子鐘表現出來的再現性，顯示它們的總誤差已經低於通常能用地球重力對當地時間造成的效應來解釋的程度。所以，如果我們想像這樣的時鐘用在美國或世界各地，則它們的相對表現將取決於地球的重力效應，這可是頭一遭。」

愛因斯坦的相對論預測原子鐘的運行步調，也就是原子的振盪頻率在較強的重力下觀測時會降低，因此會往電磁光譜紅光的那端移動。意味著在低海拔地區時間會過得比較慢。

雖然這種「紅移」(redshift)會讓原子鐘越來越不準時；但反過來說，如此敏銳的特質也可以用來精準測出重力。超級靈敏的原子鐘能夠以前所未有的精準程度來繪製出重力扭曲的時空分布。這可以運用在相對論大地測量(relativistic geodesy)，也就是測量地球重力場的形狀；另外還能偵測到許多訊號，像是早期宇宙傳來的重力波，說不定還能測到迄今仍無法解釋的暗物質。

一般用來測量大地水準面(geoid)，也就是地球形狀的方法是根據驗潮儀(tidal gauge)測繪出來的海平面。NIST的鐿原子鐘在這方面具有更強的能力。對比相隔甚遠，比方說不同大陸上的原子鐘所得出的大地水準面解析度可以小於一公分，比目前最先進的技術所能達到的數公分還要更好。

在過去十年由NIST和世上其他實驗室發表新原子鐘表現的論文裡，研究人員說這篇最新的論文展現出水準極高的再現性。而比較兩原子鐘即為傳統用來評量原子鐘表現的方法。

NIST對於最新的鐿原子鐘做出的改良之一是裝入可以屏蔽熱和電的物質，它可以包覆原子來防止雜散電場(stray electric field)的影響，也能讓研究人員更好找出並修正由熱輻射造成的頻率變化。

鐿原子的光頻是未來重新定義時間的國際單位――「秒」的候選人之一。NIST新的原子鐘達到的紀錄符合國際上重新定義單位的流程之一：經過驗證出來的準確度必須比以現今標準做出來最好的原子鐘還要準100倍。目前的標準是銫原子，其震盪頻率屬於較低頻的微波。

NIST目前正在建造可以攜帶且品質絕佳的鐿晶格鐘，這可以運到世上其他實驗室進行原子鐘之間的對比，也可以送往其他地點來探討相對論大地測量技術。

此研究的經費來自NIST、美國國家航空暨太空總署和國防高等研究計劃署。

NIST atomic clocks now keep time well enough to improve models of earth

Experimental atomic clocks at the National Institute of Standards and Technology (NIST) have achieved three new performance records, now ticking precisely enough to not only improve timekeeping and navigation, but also detect faint signals from gravity, the early universe and perhaps even dark matter.

The clocks each trap a thousand ytterbium atoms in optical lattices, grids made of laser beams. The atoms tick by vibrating or switching between two energy levels. By comparing two independent clocks, NIST physicists achieved record performance in three important measures: systematic uncertainty, stability and reproducibility. 

Published online today in the journal Nature, the new NIST clock records are:

• Systematic uncertainty: How well the clock represents the natural vibrations, or frequency, of the atoms. NIST researchers found that each clock ticked at a rate matching the natural frequency to within a possible error of just 1.4 parts in 10¹⁸—about one billionth of a billionth.
• Stability: How much the clock’s frequency changes over a specified time interval, measured to a level of 3.2 parts in 10¹⁹ (or 0.00000000000000000032) over a day.
• Reproducibility: How closely the two clocks tick at the same frequency, shown by 10 comparisons of the clock pair, yielding a frequency difference below the 10^-18 level (again, less than one billionth of a billionth).

“Systematic uncertainty, stability, and reproducibility can be considered the ‘royal flush’ of performance for these clocks,” project leader Andrew Ludlow said. “The agreement of the two clocks at this unprecedented level, which we call reproducibility, is perhaps the single most important result, because it essentially requires and substantiates the other two results.”

“This is especially true because the demonstrated reproducibility shows that the clocks’ total error drops below our general ability to account for gravity’s effect on time here on Earth. Hence, as we envision clocks like these being used around the country or world, their relative performance would be, for the first time, limited by Earth's gravitational effects.”

Einstein’s theory of relativity predicts that an atomic clock’s ticking, that is, the frequency of the atoms’ vibrations, is reduced—shifted toward the red end of the electromagnetic spectrum—when observed in stronger gravity. That is, time passes more slowly at lower elevations.

While these so-called redshifts degrade a clock’s timekeeping, this same sensitivity can be turned on its head to exquisitely measure gravity. Super-sensitive clocks can map the gravitational distortion of space-time more precisely than ever. Applications include relativistic geodesy, which measures the Earth’s gravitational shape, and detecting signals from the early universe such as gravitational waves and perhaps even as-yet-unexplained dark matter.

NIST’s ytterbium clocks now exceed the conventional capability to measure the geoid, or the shape of the Earth based on tidal gauge surveys of sea level. Comparisons of such clocks located far apart such as on different continents could resolve geodetic measurements to within 1 centimeter, better than the current state of the art of several centimeters.

In the past decade of new clock performance records announced by NIST and other labs around the world, this latest paper showcases reproducibility at a high level, the researchers say. Furthermore, the comparison of two clocks is the traditional method of evaluating performance. 

Among the improvements in NIST’s latest ytterbium clocks was the inclusion of thermal and electric shielding, which surround the atoms to protect them from stray electric fields and enable researchers to better characterize and correct for frequency shifts caused by heat radiation.

The ytterbium atom is among potential candidates for the future redefinition of the second—the international unit of time—in terms of optical frequencies. NIST’s new clock records meet one of the international redefinition roadmap's requirements, a 100-fold improvement in validated accuracy over the best clocks based on the current standard, the cesium atom, which vibrates at lower microwave frequencies.

NIST is building a portable ytterbium lattice clock with state-of-the-art performance that could be transported to other labs around the world for clock comparisons and to other locations to explore relativistic geodesy techniques.

The work is supported by NIST, the National Aeronautics and Space Administration and the Defense Advanced Research Projects Agency.

原始論文：W. F. McGrew, X. Zhang, R. J. Fasano, S. A. Schäffer, K. Beloy, D. Nicolodi, R. C. Brown, N. Hinkley, G. Milani, M. Schioppo, T. H. Yoon, A. D. Ludlow. Atomic clock performance enabling geodesy below the centimetre level. Nature, 2018; DOI: 10.1038/s41586-018-0738-2

引用自：National Institute of Standards and Technology (NIST). "Atomic clocks now keep time well enough to improve models of Earth."