Steven R. Jefferts

Testing the stability of fundamental constants with the 199Hg+ single-ion optical clock

Over a two-year duration, we have compared the frequency of the 199Hg+ 5d(10)6s (2)S(1/2)(F=0)<-->5d(9)6s(2) (2)D(5/2)(F=2) electric-quadrupole transition at 282 nm with the frequency of the ground-state hyperfine splitting in neutral 133Cs. These measurements show that any fractional time variation of the ratio nu(Cs)/nu(Hg) between the two frequencies is smaller than +/-7 x 10(-15) yr(-1) (1sigma uncertainty). According to recent atomic structure calculations, this sets an upper limit to a possible fractional time variation of g(Cs)(m(e)/m(p))alpha(6.0) at the same level.

Accuracy evaluation of NIST-F1

The evaluation procedure of a new laser-cooled caesium fountain primary frequency standard developed at the National Institute of Standards and Technology (NIST) is described. The new standard, NIST-F1, is described in some detail and typical operational parameters are discussed. Systematic frequency biases for which corrections are made - second-order Zeeman shift, black-body radiation shift, gravitational red shift and spin-exchange shift - are discussed in detail. Numerous other frequency shifts are evaluated, but are so small in this type of standard that corrections are not made for their effects. We also discuss comparisons of this standard both with local frequency standards and with standards at other national laboratories.

NIST-F1: recent improvements and accuracy evaluations

In the last several years we have made many improvements to NIST-F1 (a laser-cooled Cs fountain primary frequency standard at the National Institute of Standards and Technology (NIST) in Boulder, Colorado) resulting in a reduction by over a factor of 2 in the uncertainty of the realization of the SI second. The two most recent accuracy evaluations of NIST-F1 had combined standard fractional uncertainties of 0.61 × 10 −15 (June 2004) and 0.53 × 10 −15 (January 2005), which were submitted to the Bureau International des Poids et Mesures with total fractional uncertainties (including time-transfer contributions) of, respectively, 0.88 × 10 −15 and 0.97 × 10 −15 . Here we discuss the improvements and evaluation methods and present an updated uncertainty budget.

Double-pass acousto-optic modulator system

A practical problem that arises when using acousto-optic modulators (AOMs) to scan the laser frequency is the dependence of the beam diffraction angle on the modulation frequency. Alignment problems with AOM-modulated laser beams can be effectively eliminated by using the AOM in the double-pass configuration, which compensates for beam deflections. On a second pass through the AOM, the beam with its polarization rotated by 90° is deflected back such that it counterpropagates the incident laser beam and it can be separated from the input beam with a polarizing beam splitter. Here we present our design for a compact, stable, double-pass AOM with 75% double-pass diffraction efficiency and a tuning bandwidth of 68 MHz full width at half maximum for light transmitted through a single-mode fiber. The overall efficiency of the system (defined as the optical power out of the single-mode fiber divided by the optical power into the apparatus) is 60%.

The absolute frequency of the 87 Sr optical clock transition

The absolute frequency of the 1 S0– 3 P0 clock transition of 87 Sr has been measured to be 429 228 004 229 873.65 (37) Hz using lattice-confined atoms, where the fractional uncertainty of 8.6 × 10 −16 represents one of the most accurate measurements of an atomic transition frequency to date. After a detailed study of systematic effects, which reduced the total systematic uncertainty of the Sr lattice clock to 1.5 × 10 −16 , the clock frequency is measured against a hydrogen maser which is simultaneously calibrated to the US primary frequency standard, the NIST Cs fountain clock, NIST-F1. The comparison is made possible using a femtosecond laser based optical frequency comb to phase coherently connect the optical and microwave spectral regions and by a 3.5 km fibre transfer scheme to compare the remotely located clock signals. (Some figures in this article are in colour only in the electronic version)

Origin of 1/f PM and AM noise in bipolar junction transistor amplifiers

In this paper we report the results of extensive research on phase modulation (PM) and amplitude modulation (AM) noise in linear bipolar junction transistor (BJT) amplifiers. BJT amplifiers exhibit 1/f PM and AM noise about a carrier signal that is much larger than the amplifiers thermal noise at those frequencies in the absence of the carrier signal. Our work shows that the 1/f PM noise of a BJT based amplifier is accompanied by 1/f AM noise which can be higher, lower, or nearly equal, depending on the circuit implementation. The 1/f AM and PM noise in BJTs is primarily the result of 1/f fluctuations in transistor current, transistor capacitance, circuit supply voltages, circuit impedances, and circuit configuration. We discuss the theory and present experimental data in reference to common emitter amplifiers, but the analysis can be applied to other configurations as well. This study provides the functional dependence of 1/f AM and PM noise on transistor parameters, circuit parameters, and signal frequency, thereby laying the groundwork for a comprehensive theory of 1/f AM and PM noise in BJT amplifiers. We show that in many cases the 1/f PM and AM noise can be reduced below the thermal noise of the amplifier.

First accuracy evaluation of NIST-F2

We report the first accuracy evaluation of NIST-F2, a second-generation laser-cooled caesium fountain primary standard developed at the National Institute of Standards and Technology (NIST) with a cryogenic (liquid nitrogen) microwave cavity and flight region. The 80?K atom interrogation environment reduces the uncertainty due to the blackbody radiation shift by more than a factor of 50. Also, the Ramsey microwave cavity exhibits a high quality factor (>50?000) at this low temperature, resulting in a reduced distributed cavity phase shift. NIST-F2 has undergone many tests and improvements since we first began operation in 2008. In the last few years NIST-F2 has been compared against a NIST maser time scale and NIST-F1 (the US primary frequency standard) as part of in-house accuracy evaluations. We report the results of nine in-house comparisons since 2010 with a focus on the most recent accuracy evaluation. This paper discusses the design of the physics package, the laser and optics systems and the accuracy evaluation methods. The type B fractional uncertainty of NIST-F2 is shown to be 0.11???10?15 and is dominated by microwave amplitude dependent effects. The most recent evaluation (August 2013) had a statistical (type A) fractional uncertainty of 0.44???10?15.

Frequency evaluation of the doubly forbidden 1S0–3P0 transition in bosonic 174Yb

We report an uncertainty evaluation of an optical lattice clock based on the

First comparison of remote cesium fountains

^{1}{S}_{0}\ensuremath{\leftrightarrow}^{3}{P}_{0}

Broadband Rydberg Atom-Based Electric-Field Probe for SI-Traceable, Self-Calibrated Measurements

transition in the bosonic isotope