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Joint Conference of the IEEE International Frequency Control Symposium and International Symposium on Applications of Ferroelectrics (IFCS-ISAF)

Stability Demonstration of a 15-cc Mercury-Ion Vacuum Trap Tube

Thai M. Hoang, Sang Kun Chung, Thanh Le, John D. Prestage, Lin Yi, Robert L. Tjoelker, Nan Yu

Published Jul 1, 2020• 4 citations
Publisher

Abstract

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.

DOI
10.1109/ifcs-isaf41089.2020.9234925

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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.
Trapped mercury ion is a promising candidate for portable and operational atomic clocks. It operates at the 40.5GHz ground state hyperfine transition of 199Hg ions. Mercury ions are confined in a linear RF trap, buffer-gas cooled, with a long ion storage lifetime in an ultra-high vacuum enclosure without an active pump. The clock transition is optically-pumped and detected with a Hg discharge lamp without need of a laser light source. This approach has proven to provide simplicity in design and implementation and robustness in operation. In this paper, we report our recently effort on developing a micro and low-power mercury trapped ion clock in the DARPA ACES program. In a 5 mm x 10 mm linear trap (0.25 cc), we have achieved the clock fractional frequency stability of .7 × 10-12τ-1/2and reached down to 7 × 10-14after 1000 seconds of averaging. We will discuss the clock performance under different conditions and implications toward a micro mercury trapped ion clock with low power and high stability.
In this work, we report a prototype of micro mercury trapped ion atomic clock. All the clock components have been packaged and demonstrated in a magnetic shield box of 4 liters. The prototype clock shows promising performance with a short-term fractional frequency instability below 10−11τ−12and can reach the 10−14-instability level after a day of averaging. The prototype clock is capable of reaching the10−15-fractional frequency instability range. Our demonstration of the integrated clock prototype paves the way for further SWaP reduction and maturation.
Mercury trapped ion clocks have demonstrated great long-term frequency stability and robustness. In this paper, we report a demonstration of an integrated 100-cc physics package in an effort to develop a micromercury trapped ion clock with high frequency stability. The physics package consists of a sealed 30-cc vacuum tube with one layer of magnetic shielding, light source, and detector assembly. A field emitter array and a 194-nm microplasma lamp were employed together with a microtrap tube to reduce the size and power consumption for a mercury trapped ion clock. We show that the 100-cc physics package is capable of providing a fractional frequency stability of 1×10−11τ−1/2 down to 5×10−14 after a few hours of integration. We also show a set of environmental sensitivity evaluations as well as the clock frequency retrace.
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.
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
Abstract Optical clocks have well surpassed the frequency stability performance of microwave clocks. They have been mainly limited to tabletop setups and large rack-mount systems. Miniaturizing optical clocks will make the new technology and capability more ubiquitously applied to applications from navigation to fundamental science. Here we aim to develop a miniature space optical clock (mSOC) with the size of a compact microwave clock and capable of achieving a short-term stability of 10−14 τ −1/2 and noise floor of 10−16. In this paper, we will describe the mSOC concept, discuss the experimental setup, and present recent results. We have demonstrated trapping single ions and characterized the ion lifetime in a 16-cc trap tube. The trap tube is completely sealed off without any active pump. We also constructed a clock laser referenced to a small ultra-low expansion reference cavity at a low 10−14 level and observed the clock transition.