Physics & Astronomy Colloquium

Speaker: Steven Jefferts  (NIST)

Title: Primary Frequency References at NIST

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, CO attempt to realize this definition with the highest possible fidelity. Atomic clocks have progressed steadily from fractional inaccuracies of fifty years ago to the best microwave clocks (NIST-F1) giving inaccuracies at the level, with optical clocks exhibiting even more phenomenal performance at the 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 while un 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 we 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. We will briefly examine some of the current uses of this level of accuracy. We also examine new and exciting laser-cooled microwave clocks for use in commercial applications and in space.