For time-nuts in the Washington/Baltimore area... > > http://www.aps.org/units/maspg/ > November 2017 Event > > Date: November 15, 2017 > Speaker: Steven R. Jefferts > Topic: Primary Frequency References at NIST: Atomic Clocks > Time and Location: 1:00 p.m., with Q&A to follow in a 1st floor conference > room at the American Center for Physics (www.acp.org <http://www.acp.org/>), > 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 df/f ~ 10-9 fifty years > ago to the best microwave clocks (NIST-F1) giving inaccuracies at the df / 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 df / f < 1 = 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. > > >
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