USNO’s Rubidium Fountain Clocks


One of USNO's Rubidium Fountain Clocks.  Dr. William Phillips, one of the 1997 Nobel Laureates whose work led to the development of the clock, stands at right.
 
 

Since its establishment as the U.S. Navy Depot of Charts & Instruments in 1830, the U.S. Naval Observatory has been involved in precise timekeeping for the Navy and the Department of Defense.  Beginning with making astronomical observations to define reference time-scales to calibrate marine chronometers, the Observatory has expanded its role to the extent that it is now considered to be the world authority on Precise Time and Time Interval (PTTI).
 
To meet the ever-evolving requirements of more precise time-scales USNO has continually embraced the newest technology in timing and frequency standards.  As clock technology evolved from pendulums to quartz crystals and today’s atomic frequency devices, we have continually sought to find ways to embody our “Master Clock” time-scale with the highest possible precision.
 
Atomic frequency standards were first developed experimentally in the late 1950s, and by the early 1960s began to be incorporated into USNO’s clock systems.  During this time a USNO astronomer, Dr. William Markowitz, used observations of the path of the Moon against the stars to establish a relationship between a long-term astronomical time-scale, Ephemeris Time, and the second as defined by a resonant frequency of the cesium atom, specifically “the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.”  This frequency became the formal definition of the SI (Système Internationale) second in 1967.
 
By the early 1970’s the USNO’s Master Clock consisted of dozens of cesium-beam standards and hydrogen masers, which, when averaged together and sampled every 100 seconds, provided a uniform time-scale with a precision of about one nanosecond (10-9 s) per day, averaged over a year.
 
As miniscule as this seems, though, requirements of both the DoD and the private sector demanded even more precision with the advent of the Internet and global digital communication.  To that end, in 1996 USNO began development of a new clock system.
 
Conventional atomic clocks sample the frequencies of hot atoms sprayed into a microwave cavity where they spend less than one ten-thousandth of a second in the sampling region.  If the atoms could be slowed down to the point where they pass through the cavity at very slow velocities, they can be sampled for a much longer time interval.  However, in order to slow atoms down you have to make them extremely cold, effectively cooling them to near absolute zero.
 
The solution to this problem was to use lasers to cool and trap atoms in a high vacuum, a technique which garnered the Nobel Prize in Physics in 1997.  Devices were quickly developed to exploit this new technology; these new frequency standards became known as atomic fountains.
 
Over the next decade USNO’s Clock Development Division successfully built and tested a device based on the Nobel prize-winning principle, the USNO’s Rubidium Fountain Clock. 
 
Unlike most fountain devices, which have a limited duty cycle, our fountains must run continuously.  By 2013 we built and deployed six of these clocks which now form the basis of our Master Clock system.  These devices are so precise that their day-to-day precision is measured at the femtosecond (10-15 s) level.  They are the most precise operational clocks in the world today.
 
Below is a cutaway diagram of a Rubidium Fountain and a brief description of how it works.

 
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