 报告人：  David A. Howe   the National Institute of Standards and Technology (NIST)  报告题目：  Phase Stability in Nextgeneration Atomic Frequency Standards  报告时间：  2018年4月4日（周三）9:30  报告地点：  西区·溢智厅 
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CV of David A. Howe  David A. Howe is Senior Advisor to the Time and Frequency Metrology Group of the National Institute of Standards and Technology (NIST), Boulder, CO, and the Physics Laboratory’s Time and Frequency Division, Boulder, CO. In 1971, he completed undergraduate and graduate studies in physics and math under Neil Ashby at Colorado University in Boulder where he is a faculty member in its Physics Department. His expertise includes statistical phasenoise analysis, digital servo design, automated accuracy evaluation of primary cesium standards, atomicsystems analysis, reduction of oscillator acceleration sensitivity for special applications, communication theory, clockensemble algorithms, and spectral estimation using digital processing techniques. From 1970 to 1973, he was with the Dissemination Research Section at NIST (then the National Bureau of Standards) where he coordinated the first lunar ranging and spacecraft atomicclock timesynchronization experiments as well as TV time experiments, from which evolved closed captioning. He worked in NIST’s Atomic Standards Section with David Wineland from 1973 to 1984 doing advanced research on NIST’s primary cesium standard and compact rubidium, hydrogen, and ammonia standards. He developed and built the first six operating compact hydrogen masers in 1979 and later returned to the Dissemination Research Section in 1984 to lead and implement several global highaccuracy satellitebased timesynchronization experiments with other national laboratories in the maintenance of Universal Coordinated Time (UTC). For this contribution, he was awarded the Commerce Department’s highest commendation, the Gold Medal, in 1990 for implementing twoway satellite time networks resulting in new global synchronization standards. From 1994 to 1999, he succeeded David Allan (of Allan variance fame) as statistical analyst for the Time Scale Group which maintains UTC(NIST) from an ensemble of laboratory atomic frequency standards. David Howe developed the Total and TheoH variances which attain highaccuracy frequencystability estimation for longerterm than the sample Allan variance and are recommended statistics in the ITU Time and Frequency Working Group. He won a NIST Bronze Medal and a second Bronze in 2012 for Achievements in Time and Frequency Metrology. He received the 2013 IEEE Cady Award and was corecipient of the 2013 IEEE UFFC Outstanding Paper and 2015 NIST Astin Measurement Sciences awards. He is an IEEE Fellow and has over 160 publications and three patents in subjects related to precise frequency and phasenoise standards, timing, and synchronization. He is an avid pianist and hamradio enthusiast.  Abstract of the Lecture  Atomic clocks (or oscillators) form the basis of standard, everyday timekeeping. Separated, hiaccuracy clocks can maintain nanosecondlevel autonomous synchronization for many days. The world’s best Cs time standards are atomic fountains that use a RF quantum transition at 9,192,631,770 Hz and reach total frequency uncertainties of 2.7 – 4 × 1016 with many days of averaging time. But the days of averaging prohibit realtime use of this accuracy, and even the accuracy of today’s commercial Cs of a few × 1013. A new class of optical atomic standards with quantum transitions having +1 × 1015 uncertainty at ~200 THz, which is inconvenient for applications, drives an optical frequencycomb divider (OFD), thus providing exceptional phase stability, or ultralow phase noise (ULPN), at convenient RF frequencies. Most importantly, this scheme produces exquisite realtime accuracy at RF, as in the previous example of a few × 1013 accuracy, as quickly as fractions of a second. This single property elevate their usage to a vast array of applications that extend far beyond everyday timekeeping. “Accuracy” is the agreement with a standard realization of a reference, carrier, or local oscillator (LO) frequency. “Phase stability” quantifies the precision with which we can determine frequency as a function of averaging time in the time domain or phase noise in the frequency domain, a singlesideband (SSB) measurement of noise denoted as L(f). The L(f) measurement is used in virtually all technology sectors because it fully decomposes and describes phase instability, or phase noise, into all of its components at an offsetfrequency from the carrier on a frequencybyfrequency basis. I show how accurate oscillators with lowphase noise dramatically improves: (1) position, navigation, and timing; (2) highspeed communications, (3) private messaging and cryptology, and (4) spectrum sharing. This talk outlines gamechanging possibilities in these four areas, given nextgeneration, nearly phasenoise free, quantumbased (or atomic) frequency generators with +1 x 1015 accuracy whose properties are sustained across an application’s environmental range. I show how the combination of high atomic accuracy and lowphase noise coupled with reduced size, weight, and power usage pushes certain limits of physics to unlock a new paradigm – creating networks of separated oscillators that maintain extended phase coherence, or a virtual lock, with no means of synchronization whatsoever except at the start. “Phase coherence” means that separate oscillators maintain at least 0.1 rad phase difference at a common, or normalized, carrier frequency for long periods after synchronization. Quantumbased fractionalfrequency accuracy within +1 × 1015 when combined with equally lowphase noise synchronization at 1 × 1015 (1 fs in 1 s), means the relative phase difference increases only as √τ ? 1015 ? carrier frequency (ωо). In terms of time, this means that a 1 ns time difference wouldn’t occur in a network for 15 days! I will show a summary of several ongoing U.S. programs in which the commercial availability of such lowphase noise, atomic oscillators is now a real possibility. 
