Einstein Lecture – Passion for precision

Optical frequency combs from mode‐locked femtosecond lasers have link optical and microwave frequencies in a single step, and they provide the long missing clockwork for optical atomic clocks. By extending the limits of time and frequency metrology, they enable new tests of fundamental physics laws. Precise comparisons of optical resonance frequencies of atomic hydrogen and other atoms with the microwave frequency of a cesium atomic clock are establishing sensitive limits for possible slow variations of fundamental constants. Optical high harmonic generation is extending frequency comb techniques into the extreme ultraviolet, opening a new spectral territory to precision laser spectroscopy. Frequency comb techniques are also providing a key to attosecond science by offering control of the electric field of ultrafast laser pulses. In our laboratories at Stanford and Garching, the development of new instruments and techniques for precision laser spectroscopy has long been motivated by the goal of ever higher resolution and measurement accuracy in optical spectroscopy of the simple hydrogen atom which permits unique confrontations between experiment and fundamental theory. This lecture recounts these adventures and the evolution of laser frequency comb techniques from my personal perspective.     

Trapping of neutral mercury atoms and prospects for optical lattice clocks

We report vapor-cell magneto-optical trapping of Hg isotopes on the (1)S(0)-(3)P(1) intercombination transition. Six abundant isotopes, including four bosons and two fermions, were trapped. Hg is the heaviest nonradioactive atom trapped so far, which enables sensitive atomic searches for "new physics" beyond the standard model. We propose an accurate optical lattice clock based on Hg and evaluate its systematic accuracy to be better than 10;{-18}. Highly accurate and stable Hg-based clocks will provide a new avenue for the research of optical lattice clocks and the time variation of the fine-structure constant.     

Atomic clocks: A brief history and current status of research in India

Frequency corresponding to the energy difference between designated levels of an atom provides precise reference for making a universally accurate clock. Since the middle of the 20th century till now, there have been tremendous efforts in the field of atomic clocks making time the most accurately measured physical quantity. National Physical Laboratory India (NPLI) is the nation’s timekeeper and is developing an atomic fountain clock which will be a primary frequency standard. The fountain is currently operational and is at the stage of complete frequency evaluation. In this paper, a brief review on atomic time along with some of the recent results from the fountain clock will be discussed.     

Atomic fountains and optical clocks at SYRTE: Status and perspectives

In this article, we report on the work done with the LNE–SYRTE atomic clock ensemble during the last 10 years. We cover the progress made in atomic fountains and in their application to timekeeping. We also cover the development of optical lattice clocks based on strontium and on mercury. We report on tests of fundamental physical laws made with these highly accurate atomic clocks. We also report on work relevant to a future possible redefinition of the SI second.     

从引力波探测中的压缩光到光原子钟

2020 年度“墨子量子奖”授予量子精密测量领域, 获奖科学家是 Carlton Caves, 香取秀俊和叶军. 香取秀俊和叶军又获得2021 年基础物理学突破奖. 对于引力波探测中的量子噪声,Caves 分析了海森堡不确定关系所带来的测量精度极限, 并且提出用压缩光来克服这个极限. 这个方法已经被探测引力波的激光干涉仪实际采用. 原子钟基于原子中电子改变能量状态时, 发射或吸收的电磁波, 提供了最精确的时间和频率标准. 与基于微波的原子钟相比. 光原子钟, 特别是光晶格上的大量原子, 可以达到更好的精度. 叶军的研究组将约1 万个锶原子放在3 维光晶格中, 实现光原子钟, 相对精度达到2.5 × 10-19 . 香取秀俊的研究组搭建的两个可移动光原子钟, 精度达到了5 ×10-18 , 并用来测量了引力红移, 达到地面测量的最好精度.     

光晶格原子钟研究进展

钟跃迁频率在光学频段的光晶格原子钟已经实现了10-19 量级的频率稳定度和10-18 量级的频率不确定度, 在量子频标、 量子模拟和精密测量等领域有着重要的应用. 本文综述了光晶格原子钟的发展历史、 工作原理、 性能评估和应用与展望.     

Cold atom clocks and their applications in precision measurements

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.     

Recent atomic clock comparisons at NIST

The record of atomic clock frequency comparisons at NIST over the past half-decade provides one of the tightest constraints of any present-day temporal variations of the fundamental constants. Notably, the 6-year record of increasingly precise measurements of the absolute frequency of the Hg⁺ single-ion optical clock (using the cesium primary frequency standard NIST-F1) constrains the temporal variation of the fine structure constant α to less than 2 · 10⁻⁶yr⁻¹ and offers a Local Position Invariance test in the framework of General Relativity. The most recent measurement of the frequency ratio of the Al⁺ and Hg⁺ optical clocks is reported with a fractional frequency uncertainty of ±5.2 · 10⁻¹⁷. The record of such measurements over the last year sensitively tests for a temporal variation of α and constrains , consistent with zero.     

Space clocks and fundamental tests: The ACES experiment

In this article we give an overview of the applications of ultrastable clocks in space. We focus on the case of the ESA space mission ACES, which is scheduled for flight onboard the international space station in 2013. With a laser cooled cesium clock, PHARAO, a space hydrogen maser, SHM, and a precise time and frequency transfer system, MWL, several precision tests in fundamental physics can be performed such as a measurement of Einstein’s gravitational frequency shift with 2 ppm sensitivity and a search for time variations of the fundamental physical constants at 10^-17/year. We present the advancement of the various mission instruments and present briefly applications in geodesy and Global Navigation Satellite Systems (GNSS).     

Precision atomic spectroscopy for improved limits on variation of the fine structure constant and local position invariance

We report tests of local position invariance and the variation of fundamental constants from measurements of the frequency ratio of the 282-nm 199Hg+ optical clock transition to the ground state hyperfine splitting in 133Cs. Analysis of the frequency ratio of the two clocks, extending over 6 yr at NIST, is used to place a limit on its fractional variation of <5.8x10(-6) per change in normalized solar gravitational potential. The same frequency ratio is also used to obtain 20-fold improvement over previous limits on the fractional variation of the fine structure constant of |alpha/alpha|<1.3x10(-16) yr-1, assuming invariance of other fundamental constants. Comparisons of our results with those previously reported for the absolute optical frequency measurements in H and 171Yb+ vs other 133Cs standards yield a coupled constraint of -1.5x10(-15)相似文献   

Precision test of mass-ratio variations with lattice-confined ultracold molecules

We propose a precision measurement of time variations of the proton-electron mass ratio using ultracold molecules in an optical lattice. Vibrational energy intervals are sensitive to changes of the mass ratio. In contrast to measurements that use hyperfine-interval-based atomic clocks, the scheme discussed here is model independent and does not require separation of time variations of different physical constants. The possibility of applying the zero-differential-Stark-shift optical lattice technique is explored to measure vibrational transitions at high accuracy.     

Clocks in Weyl space-time

This paper deals with the usual methods for the construction of clocks as a first step in the direction of measurements in Weyl space-time. The atomic clocks constructed via atomic transitions, and the gravitational clocks constructed by the method of Marzke and Wheeler measure quantities with Weyl weights zero, whereas the Kundt and Hoffmann method for the construction of a clock was not found to be suitable. The atomic clocks are suitable for the chronometry and as a result of the chronometry it is shown that the Weyl space-time should be integrable.     

Frequency ratios of Sr,Yb, and Hg based optical lattice clocks and their applications

This article describes the recent progress of optical lattice clocks with neutral strontium (⁸⁷Sr), ytterbium (¹⁷¹Yb) and mercury (¹⁹⁹Hg) atoms. In particular, we present frequency comparison between the clocks locally via an optical frequency comb and between two Sr clocks at remote sites using a phase-stabilized fibre link. We first review cryogenic Sr optical lattice clocks that reduce the room-temperature blackbody radiation shift by two orders of magnitude and serve as a reference in the following clock comparisons. Similar physical properties of Sr and Yb atoms, such as transition wavelengths and vapour pressure, have allowed our development of a compatible clock for both species. A cryogenic Yb clock is evaluated by referencing a Sr clock. We also report on an Hg clock, which shows one order of magnitude less sensitivity to blackbody radiation, while its large nuclear charge makes the clock sensitive to the variation of fine-structure constant. Connecting all three types of clocks by an optical frequency comb, the ratios of the clock frequencies are determined with uncertainties smaller than possible through absolute frequency measurements. Finally, we describe a synchronous frequency comparison between two Sr-based remote clocks over a distance of 15 km between RIKEN and the University of Tokyo, as a step towards relativistic geodesy.     

Femtosecond optical frequency combs

A laser frequency comb allows the conversion of the very rapid oscillations of visible light of some 100’s of THz down to frequencies that can be handled with conventional electronics. This capability has enabled the most precise laser spectroscopy experiments yet that allowed to test quantum electrodynamics, to determine fundamental constants and to search for possible slow changes of these constants. Using an optical frequency reference in combination with a laser frequency comb has made it possible to construct all optical atomic clocks, that are now outperforming even the best cesium atomic clocks. In future direct frequency comb spectroscopy might enable high resolution laser spectroscopy in the extreme ultraviolet for the first time. Frequency combs are also used to calibrate astronomical spectrographs and might reach an accuracy that is sufficient to observe the expansion of the universe in real time.     

Atomic-based stabilization for laser-pumped atomic clocks

We describe a novel technique for stabilizing frequency shifts in laser-interrogated vapor-cell atomic clocks. The method suppresses frequency shifts due to changes in the laser frequency, intensity, and modulation index as well as atomic vapor density. The clock operating parameters are monitored by using the atoms themselves, rather than by using conventional schemes for laser frequency and cell temperature control. The experiment is realized using a chip-scale atomic clock. The novel atomic-based stabilization approach results in a simpler setup and improved long-term performance.     

冷原子气体的时频测量——光晶格原子钟

基于冷原子气体的时频测量在近20年里快速发展,引起了人们的广泛关注,其典型代表是基于大量中性原子的光晶格原子钟。利用超稳钟激光同时探测囚禁在光晶格里成千上万个冷原子的钟跃迁信号,光晶格原子钟已实现10^-18量级的频率准确度和10^-17量级的秒级稳定度,大幅度提高了时频测量的精度。文章概述了光晶格原子钟的发展历史、工作原理、性能评估及应用前景。     

冷原子气体的时频测量——光晶格原子钟

基于冷原子气体的时频测量在近20年里快速发展,引起了人们的广泛关注,其典型代表是基于大量中性原子的光晶格原子钟。利用超稳钟激光同时探测囚禁在光晶格里成千上万个冷原子的钟跃迁信号,光晶格原子钟已实现10^-18量级的频率准确度和10^-17量级的秒级稳定度,大幅度提高了时频测量的精度。文章概述了光晶格原子钟的发展历史、工作原理、性能评估及应用前景。     

AC Stark-shift in CPT-based Cs miniature atomic clocks

We report on studies on the light-shift in caesium miniature atomic clocks based on coherent population trapping (CPT) using a micro-fabricated buffer-gas cell (MEMS cell). The CPT signal is observed on the Cs D1-line by coupling the two hyperfine ground-state Zeeman sublevels involved in the clock transition to a common excited state, using two coherent electromagnetic fields. These light fields are created with a distributed feedback laser and an electro-optical modulator. We study the light-shift phenomena at different cell temperatures and laser wavelengths around 894.6?nm. By adjusting the cell temperature, conditions are identified where a miniature CPT atomic clock can be operated with simultaneously low temperature coefficient and suppressed light-shift. The impact of the light-shift on the clock frequency stability is evaluated. These results are relevant for improving the long-term frequency stability of CPT-based Cs vapour-cell clocks.     

Sub-shot-noise magnetometry with a correlated spin-relaxation dominated alkali-metal vapor

Spin noise sets fundamental limits to the precision of measurements using spin-polarized atomic vapors, such as performed with sensitive atomic magnetometers. Spin squeezing offers the possibility to extend the measurement precision beyond the standard quantum limit of uncorrelated atoms. Contrary to current understanding, we show that, even in the presence of spin relaxation, spin squeezing can lead to a significant reduction of spin noise, and hence an increase in magnetometric sensitivity, for a long measurement time. This is the case when correlated spin relaxation due to binary alkali-atom collisions dominates independently acting decoherence processes, a situation realized in thermal high atom-density magnetometers and clocks.     

拉曼喷泉原子钟

利用托曼光场代替喷泉原子钟的微波腔实现拉曼喷泉原子钟.将分离托曼光场技术与冷原子喷泉技术相结合.避免了存真空腔内放置微波腔,简化了真空系统.同时还保持了很高的准确度.采用半经典理论研究了冷原子喷泉与托曼光场的相互作用过程.得到了冉赛(Ramsey)条纹.比较了托曼喷泉原子钟与热铯束拉曼原子钟,前者有更小的体积和功耗,其精度可能达到或超过商用小铯钟.还比较了拉曼喷泉原子钟与微波喷泉原子钟的差别,分析了光子反冲的影响,提出利用同向传播和相向传播的两台拉曼原子钟测最精细结构常数.