A Century of Time Measurement: From Pendulum to Optical Clocks

Transcription

1 A Century of Time Measurement: From Pendulum to Optical Clocks Michael Lombardi NIST Time and Frequency Division NCSLI 2011

2 Introduction For most of recorded history, the most accurate measurements of time involved dividing the period of a day into smaller parts. For example, the solar second was measured by dividing the solar day in 86,400 parts. The rotational rate of the Earth was known to be the best clock of all, the ultimate reference for timekeeping. This began to change when mechanical clocks first appeared in the 14 th century. Mechanical clocks measured time by counting the oscillations of a repetitive event, such as the swings of a pendulum or balance wheel. This was a fundamental change in timekeeping, because instead of dividing days to get seconds, they multiplied seconds to get days. Early mechanical clocks were not very accurate, and for about 600 years, mechanical clocks were calibrated with astronomical clocks. About a century ago, technology finally improved to the point where man made clocks were known to be more stable and accurate than the Earth s rotation. This presentation begins at that point, and takes a very brief look at developments in clock technology during the past 100 years.

3 Outline Pendulum Clocks Quartz Clocks Atomic Clocks and the Redefinition of the Second Optical Clocks Radio Controlled Clocks

4 Riefler Pendulum, 1904 Manufactured by the Clemens Riefler Company of Munich, Germany. Served as the U. S. national standard for time interval from 1904 to Accurate to tens of milliseconds per day (parts in 10 7 ).

5 Shortt Pendulum, 1921 A mainstay of astronomical observatories in the 1920s and 1930s, the Shortt pendulum was advertised as The Perfect Clock. Designed by the British railroad engineer William H. Shortt. Used two pendulums. The master pendulum was disturbed as little as possible. Accurate to 1 s per year (a few parts in 10 8 ). It was so accurate that it suggested for the first time that the Earth was not a perfect timekeeper. Briefly used as the U. S. national standard for time interval, during part of 1929.

6 Quartz Clocks The Curie Brothers demonstrated the piezoelectric effect in quartz and other crystals in It remained a scientific curiosity for years. The first application of piezoelectricity was detecting enemy submarines in World War I, with independent work conducted in France and the U. S. Walter Cady, an American physicist, worked on submarine detection systems during the war. After the war he focused on building a standard for radio frequency, and patented the first quartz oscillator circuit in 1920.

7 General Radio Type 275, 1924 The first commercially available quartz oscillator, it sold for $145. Cady and George Pierce (who improved upon Cady s basic circuits) were involved in the design. Used by radio engineers to calibrate transmitters, and was soon followed by more accurate quartz standards.

8 U. S. National Frequency Standard, 1929 A group of four 100- khz quartz oscillators manufactured by Bell Telephone Laboratories. Accurate to about Even though this was no better than the pendulum clocks it replaced, it could serve as a standard for both radio frequency and time interval, something that the pendulum could not do.

9 First Quartz Clock, 1927 Designed by William Marrison and Joseph Horton of Bell Telephone Laboratories Used a 100 khz oscillator and perhaps the first frequency divider, which reduced the frequency to 1 khz The 1 khz frequency controlled the speed of a synchronous motor that moved the hands Accurate to about 2 parts per million, slightly better than a typical quartz watch of today

10 Rohde & Schwarz Model CFQ, 1938

11 Hamilton Electric 500, 1957 The first battery powered watch, it had the unique styling of a 1950s automobile. Ran at the same frequency as a mechanical watch, 5 Hz, but it derived its frequency from tiny electrical contacts that opened and closed five times per second. Was not particularly reliable or accurate, but was very popular, showing the large demand for a watch that never needed to be wound.

12 Bulova Accutron Spaceview, 1962 Designed by the Swiss engineer Max Hetzel, the Bulova Accutron was first introduced in 1960 Its tuning fork oscillator ran at 360 Hz, as opposed to 5 Hz for mechanical watches Accurate to 2 seconds per day ( ), a factor of 10 improvement over the best mechanical watches Hummed instead of Ticked

13 First Quartz Watch Oscillator Circuit, 1966 Designed by Armin Frei of the Centre Electronique Horloger (CEH) in Switzerland in response to the threat posed to the Swiss watch industry by the Bulova Accutron. Ran at 8192 Hz. Thirteen binary flip-flops divided the frequency to 1 Hz. Was included in the first quartz watch prototype (July 1967), but was never used in a commercially available watch.

14 Seiko Astron, 1969 Introduced on Christmas day in 1969, beating the Swiss watchmakers to the market. Sold for $1250, about the same price as an economy car. It had nearly 200 analog parts that had to be hand soldered (the Swiss had designed ICs). Ran at 8192 Hz with 13 binary divider stages. By 1972, nearly all quartz watches used 15 divider stages and ran at Hz, which became the standard.

15 Atomic Clocks The Scottish physicist James Clerk Maxwell suggested that atoms could be used to keep time as early as The first atomic clock experiments took place some 60 years later at Columbia University in New York, conducted by a team led by Isidor Rabi. Rabi publicly discussed his plans for an atomic clock during a lecture at Columbia in 1945, and the New York Times (left) covered the story. The concept was actually simple. Because all atoms of a specific element are identical, they should produce the exact same frequency when they absorb or release energy. An atom, then, could potentially serve as a perfect oscillator.

16 First Atomic Clock, 1949 Based on the ammonia molecule, it was unveiled in January 1949, designed by a team led by Harold Lyons at the National Bureau of Standards. It never worked well enough to be used as a standard or reference. Its best reported uncertainty was about 2 x 10-8, less accurate than the quartz oscillators then used as the national frequency standard. But it provided a glimpse of what the future would bring

17 NBS-1, Cesium Prototype, 1952 The NBS team, led by Harold Lyons and Jesse Sherwood, had a large early lead in the race to build the first cesium clock. They began work in 1950 and reported their first results in NBS interrupted the program in 1953, for budgetary and other reasons. By 1955, both Lyons and Sherwood had left NBS. The clock was taken apart and moved to the new NBS labs in Boulder, Colorado where it was reassembled. It finally became the national frequency standard in 1959, but by then NPL in England had operated a cesium standard for several years.

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19 First Cesium Time Standard, NPL, 1955

20 The Atomic Second, 1967 In 1956, the second was defined as 1/31,556, of the tropical year The ephemeris second was nearly impossible to use as a time reference and of little use to metrologists or engineers. Ephemeris time was determined by measuring the position of the Moon with respect to several surrounding stars. The best Moon observations had been recorded at the United States Naval Observatory (USNO) in Washington, DC by the astronomer William Markowitz. Louis Essen and Jack Parry of NPL compared their new cesium clock to a quartz clock steered to ephemeris time at the USNO. Because the two clocks were located across the Atlantic from each other, they simultaneously compared each clock to radio signals that could be received at both laboratories, a measurement technique now known as common-view time transfer. Four different solutions were made to determine the effects of using different data. The final result was the average of the four solutions, and was published as cycles/s in August In 1967, the second was finally redefined as: the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

21 NBS/NIST constructed seven Cesium Beam Primary Frequency Standards from 1959 to NBS-6 NIST-7 NBS-5 NBS-4 NBS-1 NBS-2 NBS-3

22 NIST-F1 Atomic Fountain Clock A cesium fountain frequency standard that provides the best possible realization of the SI second. NIST-F1 laser-cooled fountain standard atomic clock Current accuracy (uncertainty): 3 x trillionths of a second per day. 1 second in 105 million years. Equivalent to measuring distance from earth to sun (1.5 x m or 93 million miles) to uncertainty of about 45 mm (less than thickness of human hair).

23 Frequency Uncertainty Improvements in Primary Frequency Standards at NBS/NIST NBS-1 NBS-2 More than 50 Years of Progress in Atomic Clocks NBS-3 NBS NBS NBS-6 NIST NIST-F1 Initial NIST-F1 Today Year

24 Optical Clocks: The Next Generation of Primary Standards Optical clocks tick faster than microwave clocks. For example, the mercury ion clock resonates at a frequency more than 100,000 times higher than a cesium clock. This is comparable to using a second, rather than a day, as the base unit of time. In principle, faster ticks means better accuracy and stability. f 0 Al + Hg + Yb Ca Cs 15 f0 optical microwave THz (1124 x Hz) 1064 THz 520 THz 456 THz THz 5 Optical Microwave

25 Improvements in Primary Frequency Standards: Optical Clocks Optical clocks have the potential for accuracy at the level, >100 times better than NIST-F1. Likely to take many years to realize that potential. Laser-cooled calcium atoms. Ytterbium atoms in optical lattice. Single mercury ion. Single mercury ion trap.

26 Frequency Uncertainty Improvements in Primary Frequency Standards: Optical Clocks NBS NIST-F NIST-F2 Optical Standards Year

27 Junghans MEGA 1, 1990 The first radio controlled wristwatch, manufactured in Germany. The antenna was hidden inside the wrist band. Synchronized to time signals broadcast by radio station DCF77 on 77.5 khz. This station was synchronized to the German time standard maintained by PTB.

28 Low Frequency (LF) Radio Controlled Clocks Low frequency time signal stations operate at frequencies ranging from about 40 to 80 khz. The pictured watch can synchronize to transmitters in the United States, England, Germany, Japan, or China. The U. S. transmitter is radio station WWVB, operated by NIST. More than 50 million WWVB radio controlled clocks are believed to be in operation.

29 GOES Satellite Clock, 1976 The first clocks controlled by satellites received NBS time signals from the GOES geostationary satellites. These clocks appeared about three years before the launch of the first GPS satellite. Designed by a team led by Dick Davis, these clocks could remove most of the path delay between the clock and the satellite and were accurate to less than 100 microseconds. The picture shows a GOES clock built to commemorate the U. S. bicentennial.

30 GPS Clocks Best known as a positioning and navigation system, GPS is also the main system used to distribute accurate time and frequency worldwide. A constellation of as many as 32 satellites can deliver time accurate to less than 1 microsecond (typically 100 ns) anywhere on Earth. This has revolutionized timekeeping, and made many new technologies possible. Many metrology labs use GPS disciplined oscillators as their standard for frequency.

31 Mobile Phones are Radio Controlled Clocks The clocks on mobile phones are usually very accurate. Many phones synchronize to GPS clocks located at cellular base stations, with only a few microseconds of additional delay added. Unlike LF radio controlled clocks, mobile phones automatically correct when you change time zones. A recent study indicates that 1 out of 7 people have stopped wearing watches, mostly because of mobile phones. That figure is twice as high among 15 to 24 year olds.

32 Summary The uncertainty of time measurements has improved by about 10 orders of magnitude during the past century, from parts in 10 6 to parts in Optical clocks should further reduce uncertainties by at least two more orders of magnitude. In everyday life, time-of-day clocks that are accurate to within 1 second will become more common, and should eventually be the norm rather than the exception. Many technologies could contribute to this trend, including LF radio signals, satellite signals, mobile phone signals, Internet time codes, and miniature atomic clocks. If you are interested in reading more about the history of time measurements, see the five-part series now being published in IEEE Instrumentation and Measurement Magazine (the first installment is in the August 2011 issue).