Eric A. Burt

Lattice-Induced Frequency Shifts in Sr Optical Lattice Clocks at the 10{sup -17} Level

We present a comprehensive study of the frequency shifts associated with the lattice potential in a Sr lattice clock by comparing two such clocks with a frequency stability reaching 5×10(-17) after a 1 h integration time. We put the first experimental upper bound on the multipolar M1 and E2 interactions, significantly smaller than the recently predicted theoretical upper limit, and give a 30-fold improved upper limit on the effect of hyperpolarizability. Finally, we report on the first observation of the vector and tensor shifts in a Sr lattice clock. Combining these measurements, we show that all known lattice related perturbations will not affect the clock accuracy down to the 10(-17) level, even for lattices as deep as 150 recoil energies.

A compensated multi-pole linear ion trap mercury frequency standard for ultra-stable timekeeping

The multi-pole linear ion trap frequency standard (LITS) being developed at the Jet Propulsion Laboratory (JPL) has demonstrated excellent short- and long-term stability. The technology has now demonstrated long-term field operation providing a new capability for timekeeping standards. Recently implemented enhancements have resulted in a record line Q of 5 times 1012 for a room temperature microwave atomic transition and a short-term fractional frequency stability of 5 times 10-14/tau1/2. A scheme for compensating the second order Doppler shift has led to a reduction of the combined sensitivity to the primary LITS systematic effects below 5 times 10-17 fractional frequency. Initial comparisons to JPLs cesium fountain clock show a systematic floor of less than 2 times 10-16. The compensated multi-pole LITS at JPL was operated continuously and unattended for a 9-mo period from October 2006 to July 2007. During that time it was used as the frequency reference for the JPL geodetic receiver known as JPLT, enabling comparisons to any clock used as a reference for an International GNSS Service (IGS) site. Comparisons with the laser-cooled primary frequency standards that reported to the Bureau International des Poids et Mesures (BIPM) over this period show a frequency deviation less than 2.7 times 10-17/day. In the capacity of a stand-alone ultra-stable flywheel, such a standard could be invaluable for long-term timekeeping applications in metrology labs while its methodology and robustness make it ideal for space applications as well.

Stability measurements between Hg/sup +/ lite 12-pole clocks

We describe frequency stability measurements of two Hg-ion clocks based upon linear 12-pole shuttle ion traps. The inter-comparison was carried out over several multi-day intervals with the short-term stability of each clock better than 2/spl times/10/sup -13/ at 1 second. Longer-term stability as good as 3/spl times/10/sup -16/ was demonstrated for these clocks.

Characterization and reduction of number dependent sensitivity in multi-pole linear ion trap standards

The multipole linear ion trap standard developed at the Jet propulsion laboratory has demonstrated excellent short and long-term stability and improved immunity from two of its remaining systematic effects, the second order Doppler shift and second order Zeeman shift. The technology has also demonstrated long-term operation in the field. In this paper, we discuss the LITS systematic effects and present the characterization and reduction of the primary shifts and their dependence on ion number

Probing magnetic field effects in 12-pole linear ion trap frequency standards

The second-order Zeeman shift in a 12-pole buffer-gas-cooled linear ion trap frequency standard is characterized. Results for magnetic shielding effectiveness and long term stability against magnetic field perturbations are presented. The clock frequency is found to be stable against typical ambient magnetic field fluctuations to less than 2/spl times/10/sup -16/. The frequency shift as a function of ion number is also studied and a plausibility argument is given relating this to magnetic field inhomogeneity.

Characterization of the USNO cesium fountain

We have performed an initial characterization of the stability of the U.S. Naval Observatory (USNO) cesium fountain atomic clock. This device has a short-term fractional frequency stability of up to 1.5/spl times/10/sup -13/ /spl tau/-/sup 1/2/. This short-term performance enables us to measure hydrogen maser behavior over the short to medium term. We have recently implemented real time steering of a hydrogen maser with the fountain. Over a period of roughly 9 days of continuous operation, we have steered out the drift of a cavity tuned maser.

Estimating the stability of N clocks with correlations

Estimation of an atomic clocks frequency stability, separate from its reference, is often done using a three-cornered hat procedure. A major requirement for the success of this method is that clocks be uncorrelated. If this requirement is not satisfied, the three-cornered hat procedure can lead to misleading or even negative variance estimates. Others have considered this problem and developed an analysis that allows for the possibility of cross correlation between clocks. We have extended and applied these ideas to obtain mathematically consistent frequency stability estimates on atomic clock data from the U.S. Naval Observatory. In addition, we derived an expression for the clock weights that produce a minimum variance combination of clocks in the presence of correlations.

Mercury Ion Clock for a NASA Technology Demonstration Mission

There are many different atomic frequency standard technologies but only few meet the demanding performance, reliability, size, mass, and power constraints required for space operation. The Jet Propulsion Laboratory is developing a linear ion-trap-based mercury ion clock, referred to as DSAC (DeepSpace Atomic Clock) under NASAs Technology Demonstration Mission program. This clock is expected to provide a new capability with broad application to space-based navigation and science. A one-year flight demonstration is planned as a hosted payload following an early 2017 launch. This first-generation mercury ion clock for space demonstration has a volume, mass, and power of 17 L, 16 kg, and 47 W, respectively, with further reductions planned for follow-on applications. Clock performance with a signal-to-noise ratio (SNR)*Q limited stability of 1.5E - 13/τ1/2 has been observed and a fractional frequency stability of 2E-15 at one day measured (no drift removed). Such a space-based stability enables autonomous timekeeping of Δt <; 0.2 ns/day with a technology capable of even higher stability, if desired. To date, the demonstration clock has been successfully subjected to mechanical vibration testing at the 14 grms level, thermal-vacuum operation over a range of 42 °C, and electromagnetic susceptibility tests.

Improvements to JPL's Compensated Multi-Pole Linear Ion Trap Standard and Long-Term Measurements at the 10-16 Level

The Multi-pole Linear Ion Trap Standard (LITS) being developed at the Jet Propulsion Laboratory (JPL) has demonstrated excellent short and long-term stability. The technology has now demonstrated long-term field operation providing a new capability for timekeeping standards. Recently implemented enhancements have resulted in a record room temperature microwave line Q of 5times1012, a short-term fractional frequency stability of 5times10-14/tau1/2 and reduction of the combined sensitivity to the primary LITS systematic effects below 5times10-17 fractional frequency. Initial comparisons to JPLs cesium fountain clock show a systematic floor of less than 2times10-16. The multi-pole LITS at JPL has been operating continuously and unattended since October, 2006 and is used as the frequency reference for the JPL geodetic receiver known as JPLT, enabling comparisons to any clock used as a reference for an IGS site. Initial comparisons with UTC over a 6-month period show a frequency deviation equivalent to less than 2.5times10-17/day. In the capacity of a stand-alone ultra-stable flywheel, such a standard could be invaluable for long-term timekeeping applications in metrology labs while its simplicity and robustness make it ideal for space applications as well.

Cesium fountain development at USNO

In this paper we discuss progress made at the U.S. Naval Observatory (USNO) towards building a cesium fountain atomic clock. In particular we will address the efficacy of a 4-beam optical lattice as an atom collection and launch mechanism. To date we have measured temperatures in a 4-beam lattice of 1.4(0.3) /spl mu/K and have launched atoms from this lattice to a height of just under a meter with a temperature of 1.7(0.1) /spl mu/K. We are able to collect 2.4/spl times/10/sup 6/ atoms using only the lattice beams and no magnetic fields. We have completed the design for and are in the process of fabricating all aspects of the fountain device including the collection region, the drift region, the microwave cavity and the magnetic shields. We present our progress to date including a discussion of our launch results and the design and testing of our magnetic shields.