| ||报告人： ||David A. Howe |
| ||the National Institute of Standards and Technology (NIST) |
|报告题目： ||Phase Stability in Next-generation 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 |
|报告人简介 ||报告简介 ||本期论坛海报 ||本期论坛照片 ||本期论坛简讯 |
|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 phase-noise analysis, digital servo design, automated accuracy evaluation of primary cesium standards, atomic-systems analysis, reduction of oscillator acceleration sensitivity for special applications, communication theory, clock-ensemble 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 atomic-clock time-synchronization 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 high-accuracy satellite-based time-synchronization 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 two-way 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 high-accuracy frequency-stability estimation for longer-term 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 co-recipient 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 phase-noise standards, timing, and synchronization. He is an avid pianist and ham-radio enthusiast.
|Abstract of the Lecture |
Atomic clocks (or oscillators) form the basis of standard, everyday timekeeping. Separated, hi-accuracy clocks can maintain nanosecond-level 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 × 10-16 with many days of averaging time. But the days of averaging prohibit real-time use of this accuracy, and even the accuracy of today’s commercial Cs of a few × 10-13. A new class of optical atomic standards with quantum transitions having +1 × 10-15 uncertainty at ~200 THz, which is inconvenient for applications, drives an optical frequency-comb divider (OFD), thus providing exceptional phase stability, or ultra-low phase noise (ULPN), at convenient RF frequencies. Most importantly, this scheme produces exquisite real-time accuracy at RF, as in the previous example of a few × 10-13 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 single-sideband (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 offset-frequency from the carrier on a frequency-by-frequency basis. I show how accurate oscillators with low-phase noise dramatically improves: (1) position, navigation, and timing; (2) high-speed communications, (3) private messaging and cryptology, and (4) spectrum sharing. This talk outlines game-changing possibilities in these four areas, given next-generation, nearly phase-noise free, quantum-based (or atomic) frequency generators with +1 x 10-15 accuracy whose properties are sustained across an application’s environmental range. I show how the combination of high atomic accuracy and low-phase 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. Quantum-based fractional-frequency accuracy within +1 × 10-15 when combined with equally low-phase noise synchronization at 1 × 10-15 (1 fs in 1 s), means the relative phase difference increases only as √τ ? 10-15 ? 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 low-phase noise, atomic oscillators is now a real possibility.