John Prestage

Nitrogen buffer gas experiments in mercury trapped ion frequency standards

Practical, continuous operating mercury trapped ion frequency standards have traditionally used a helium buffer gas to increase loading efficiency and cool ions to near room temperature. The fractional frequency shift of the 40,507,347.9968x Hz clock transition due to collisions with helium is measured to be (df/dP/sub He/)(1/f)=+1.2/spl times/10/sup -10//Pa. The use of a nitrogen buffer gas is considered for low power and mass applications where unattended operational life must be greater than 10 years. The nitrogen pressure shift is measured to be (df/dP/sub N2/)(1/f)=-8.7/spl times/10/sup -9//Pa. Nitrogen would allow long operation with a only small ion pump but require increased pressure regulation to achieve the ultra-high stability obtained using helium in multipole Hg+ standards.

A highly miniaturized vacuum package for a trapped ion atomic clock

We report on the development of a highly miniaturized vacuum package for use in an atomic clock utilizing trapped ytterbium-171 ions. The vacuum package is approximately 1 cm(3) in size and contains a linear quadrupole RF Paul ion trap, miniature neutral Yb sources, and a non-evaporable getter pump. We describe the fabrication process for making the Yb sources and assembling the vacuum package. To prepare the vacuum package for ion trapping, it was evacuated, baked at a high temperature, and then back filled with a helium buffer gas. Once appropriate vacuum conditions were achieved in the package, it was sealed with a copper pinch-off and was subsequently pumped only by the non-evaporable getter. We demonstrated ion trapping in this vacuum package and the operation of an atomic clock, stabilizing a local oscillator to the 12.6 GHz hyperfine transition of (171)Y b(+). The fractional frequency stability of the clock was measured to be 2 × 10(-11)/τ(1/2).

Micro ion frequency standard

We are developing a highly miniaturized trapped ion clock to probe the 12.6 GHz hyperfine transition in the 171Yb+ ion. The clock development is being funded by the Integrated Micro Primary Atomic Clock Technology (IMPACT) program from DARPA where the stated goals are to develop a clock that consumes 50 mW of power, has a size of 5 cm3, and has a long-term frequency stability of 10-14 at one month. One of the significant challenges will be to develop miniature single-frequency lasers at 369 nm and 935 nm and the optical systems to deliver light to the ions and to collect ion fluorescence on a detector.

One-liter Hg ion clock for space and ground applications

We describe the development of a small Hg/sup +/ ion clock suitable for space use. A small clock occupying 1-2 liters volume and producing stability of 10/sup -12///spl radic//spl tau/ would significantly advance the state of space-qualified atomic clocks. Based on recent measurements, this technology should produce long-term stability as good as 10/sup -15/.

Miniature microwave frequency standard with trapped 171 Yb

We report the development of a low-power, miniature Yb-171 ion clock at Sandia National Laboratories. This work is funded by the DARPA micro Position, Navigation, and Timing program under the Integrated Micro Primary Atomic Clock Technology (IMPACT) project. The ultimate goal is to develop a frequency standard that has frequency stability comparable to a commercial Cs beam standard, but with 100 to 1000 times smaller size and power consumption. The Yb-171 ion has a microwave clock traItem MIN : YXB4-01335-A112nsition at 12.6 GHz, and the natural linewidth of the clock resonance is expected to be less than 10-3 Hz, which leads to a very high-Q clock resonance. An atomic clock using trapped ions is an excellent candidate for miniaturization because ions are well isolated from the environment independently of the size of the trap. Compared to optical traps for neutral atoms, the trapping depth of RF traps for ions is usually several orders of magnitude deeper. Therefore, the requirement for the vacuum level is more forgiving. We have successfully developed miniature ion-trap vacuum packages as shown in Fig. 1. Usually, a few microTorr of He buffer gas is introduced into each of our miniature ion-trap vacuum packages, which are sealed and passively pumped by non-evaporable getters. Using a sealed 3 cc ion-trap package, we were able to demonstrate long-term clock operation, with the stability reaching the 10-14 range after a few days of integration [1].

Hg ion atomic clock for deep space navigation and science

We have recently completed a breadboard ion-clock physics package based on Hg ions shuttled between a quadrupole and a 16-pole rf trap. With this architecture we have demonstrated short-term stability ~1-2x10-13 at 1 second, averaging to 10-15 at 1 day. This development shows that H-maser quality stabilities can be produced in a small clock package, comparable in size to an ultra-stable quartz oscillator required for holding 1-2x10-13 at 1 second. This performance was obtained in a sealed vacuum configuration where only a getter pump was used to maintain vacuum. The vacuum tube containing the traps has now been under sealed vacuum conditions for nearly two years with no measurable degradation of ion trapping lifetimes or clock short-term performance. We have fabricated the vacuum tube, ion trap and UV windows from materials that will allow a ~ 400°C tube bake-out to prepare for tube seal-off. This approach to the vacuum follows the methods used in flight vacuum tube electronics, such as flight TWTAs where tube operation lifetime and shelf life of up to 15 years is achieved. We use neon as a buffer gas with 2-3 times less pressure induced frequency pulling than helium and, being heavier, negligible diffusion losses will occur over the operation lifetime.