Meeting Information

Primary Frequency References at NIST: Atomic Clocks

November 15, 2017
NIST- Time and Frequency Division
Boulder, CO

Date: November 15, 2017

Speaker: Steven R. Jefferts
Time and Frequency Division NIST
Boulder, CO 80305 USA

Topic: Primary Frequency References at NIST: Atomic Clocks

Location: 1:00 p.m., with Q&A to follow in a 1st floor conference room at the American Center for Physics (, 1 Physics Ellipse, College Park, MD — off River Rd., between Kenilworth Ave. and Paint Branch Parkway.

Abstract: In the SI system of units a second is defined as 9,192,631,770 cycles (exactly) of the ground state hyperfine transition frequency of an unperturbed cesium atom. We take the atom to be at rest on the reference geoid (~mean sea level) of the Earth. Primary frequency standards (aka atomic clocks) such as NIST-F1 & F2 in Boulder, Colorado attempt to realize this definition with the highest possible fidelity. Atomic clocks have progressed steadily from fractional inaccuracies of δ f / f ~ 10-9 fifty years ago to the best microwave clocks (NIST-F1) giving inaccuracies at the δ f / f < 2 x 10-16 level, with optical clocks exhibiting even more phenomenal performance at the 10-17 level and beyond. This level of performance requires an excruciating attention to detail when attempting to correct for frequency biases. For example, an uncertainty of 1 meter in the altitude of the device with respect to the reference geoid causes a frequency uncertainty of more than δ f / f = 1 x 10-16 while an uncertainty in the temperature of the radiation field to which the atom is exposed of 1K yields frequency shifts of several times this much. In this talk I will discuss some history of these devices, the current state of the art in laser-cooled microwave clocks and some fundamental limits to their attainable accuracy and briefly examine some of the current uses of this level of accuracy. New and exciting laser-cooled microwave clocks for use in commercial applications and in space will also be examined.

Biography: Steven Jefferts joined NIST, Boulder, Colorado in 1994 and since 1998 has been designing, building and operating the NIST primary frequency references. Steve received a BS in physics from the University of Washington in 1984, and a PhD in physics from JILA - University of Colorado Boulder in 1992. He served as an NRC postdoc under Dr. David Wineland 1992 - 1994 and has been a member of the NIST Technical Staff in the Time and Frequency Division since 1994. He has won a Flemming Award, two Department of Commerce Gold Medals, and a Condon Award.