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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Progress Toward a 10¿15 Stable Ion Clock for Deep Space Applications

John D. Prestage, S. Chung, T. Le, Lawrence Lim, Lute Maleki

Published Jun 1, 2006• 2 citations
Publisher

Abstract

The authors 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 the authors have demonstrated short-term stability ~2-3times10-13at 1 second, averaging to 10-15at 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 1-2times10-13at 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 a year 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 TWTA's where tube operation lifetime and shelf life of up to 15 years is achieved. They 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

DOI
10.1109/freq.2006.275474

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The authors 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 the authors have demonstrated short-term stability ~2-3times10-13at 1 second, averaging to 10-15at 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 1-2times10-13at 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 a year 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 TWTA's where tube operation lifetime and shelf life of up to 15 years is achieved. They 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
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 TWTA's 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.
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 ~1-2times10-13at 1 second, averaging to 10-15at 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 1-2times10-13second. 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 ~400degC 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 TWTA's 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.
In this paper, we report our recent effort on developing a micro mercury trapped ion atomic clock. In particular, we present preliminary results of a mercury-ion clock using a 15-cc vacuum trap tube. The mercury-ion clock can achieve a short-term fractional frequency instability of 1.5 ×10!12τ!1/2and reach a fractional frequency instability of 3 ×10!14after averaging for an hour. The clock can remain at the 10!14fractional frequency instability level up to several hours. Furthermore, the sealed 15-cc vacuum tube has demonstrated a very long ion lifetime of over two hundred days. We also investigated the mercury vapor density inside the vacuum trap tube by measuring the coherence time of the microwave clock transition.
Miniature atomic clocks based on the interrogation of the ground state hyperfine splitting of buffer gas cooled ions confined in radio frequency Paul traps have shown great promise as high precision prototype clocks. We report on the performance of two miniature ion trap vacuum packages after being sealed for as much as 10 years. We find the lifetime of the ions within the trap has increased over time for both traps and can be as long as 50 days. We form two clocks using the two traps and compare their relative frequency instability one with another to demonstrate a short-term instability of 5×10−13τ−1/2 integrating down to 1×10−14 after 2 ks of integration. The trapped ion lifetime and clock instability demonstrated by these miniature devices despite only being passively pumped for many years represents a critical advance toward their proliferation in the clock community.
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).
This dissertation reports on the development of a low-power, high-stability miniature atomic frequency standard based on 171Yb+ ions. The ions are buffer-gas cooled and held in a linear quadrupole trap that is integrated into a sealed, getter-pumped vacuum package, and interrogated on the 12.6 GHz hyperfine transition. We hope to achieve a long-term fractional frequency stability of 10^-14 with a miniature clock that consumes only 50 mW of power and occupies a volume of 5 cm^3. I discuss our progress over several years of work on this project. We began by building a conventional tabletop clock to use as a test bed while developing several designs of miniature ion-trap vacuum packages, while also developing techniques for various aspects of the clock operation, including ion loading, laser and magnetic field stabilization, and a low power ion trap drive. The ion traps were modeled using boundary element software to assist with the design and parameter optimization of new trap geometries. We expect a novel trap geometry that uses a material new to ion traps to lead to an exceptionally small ion trap vacuum package in the next phase of the project. To achieve the long-term stability required, we have also considered the sensitivity of the clock frequency to magnetic fields. A study of the motion of the individual ions in a room-temperature cloud in the trap was performed with the purpose of understanding the effect of both spatially varying and constant magnetic fields on the clock resonance and therefore the operation of the clock. These effects were studied experimentally and theoretically for several traps. In summary, this dissertation is a contribution to the design, development, and testing of a 171Yb+ ion cloud frequency standard and related techniques, including analyses of trap geometries and parameters, modeling of the ion motion, and the practical operation of the clock.