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

Recent progress on the NRC 88Sr+ single-ion optical frequency standard

P. Dubé, Alan A. Madej, J. E. Bernard, G. Humphrey, Markku Vainio, J. Jiang, David J. Jones

Published Jun 1, 2010• 2 citations
Publisher

Abstract

We report our recent progress made with the NRC88Sr+single-ion optical frequency standard. The long-term operation of the standard was improved by actively stabilizing the cooling, repump and clearout laser sources, and by using a femtosecond fiber laser frequency comb to link the probe laser frequency to the microwave time standards. With the femtosecond fiber comb, we have demonstrated continuous operation for a period of eight days and continuous measurement of the probe laser source for three days. Micromotion shifts have been the dominant source of uncertainty in our rf Paul trap because laser beam access is only possible along one axis in the current design. With the aim of reducing to a minimum these shifts, we have constructed an endcap trap designed for minimization of micromotion along three orthogonal axes. We report on the successful trapping of single ions with this endcap trap. Since electrode contamination during trap loading can also induce micromotion shifts as a result of evolving patch potentials, we have increased the trap loading efficiency with photo-ionization.

DOI
10.1109/freq.2010.5556371

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We report our recent progress made with the NRC88Sr+single-ion optical frequency standard. The long-term operation of the standard was improved by actively stabilizing the cooling, repump and clearout laser sources, and by using a femtosecond fiber laser frequency comb to link the probe laser frequency to the microwave time standards. With the femtosecond fiber comb, we have demonstrated continuous operation for a period of eight days and continuous measurement of the probe laser source for three days. Micromotion shifts have been the dominant source of uncertainty in our rf Paul trap because laser beam access is only possible along one axis in the current design. With the aim of reducing to a minimum these shifts, we have constructed an endcap trap designed for minimization of micromotion along three orthogonal axes. We report on the successful trapping of single ions with this endcap trap. Since electrode contamination during trap loading can also induce micromotion shifts as a result of evolving patch potentials, we have increased the trap loading efficiency with photo-ionization.
The present decade has seen great advances in optical frequency standards following the development of femtosecond laser frequency combs. With this technology, combined with a stable and narrow probe laser source (50-100 Hz linewidth), we made a systematic study of the 5s 2S1/2-4d 2D5/2 (S-D) transition frequency of the 88Sr+ ion as a function of the quantization axis in the trap. This study revealed the presence of systematic shifts caused by residual micromotion in our rf Paul trap and led to an evaluation of those shifts. Also, a method was developed based on the measurement of the Zeeman spectrum of the clock transition to cancel the electric quadrupole shift caused by patch potentials on the trap electrodes. From these results, we also determined the absolute frequency of the S-D clock transition frequency at 674 nm with a fractional frequency uncertainty of 3.4 × 10-14. We have since made improvements to our probe laser system and have observed 5 Hz Fourier transform limited linewidths on a Zeeman component of the S-D transition of the 88Sr+ ion. To observe such a narrow transition it is essential to have a known and well behaved drift of the laser frequency during the few minutes it takes to measure the ion resonance using the quantum jump signal. Our approach to minimize the drift rate and its sensitivity to temperature fluctuations has been to stabilize the cavity at the temperature where its coefficient of thermal expansion is zero. We will present an overview of our recent progress made on the frequency standard based on the laser-cooled and trapped single ion of 88Sr+.
We describe the recent advances of the NRC single ion based optical frequency standard using the 445-THz S-D transition in88Sr+. Laser cooled confinement and probing of the clock transition has been obtained for periods exceeding 24 hours showing high levels of stability. Spectral resolutions better than 5 Hz at 445 THz have been obtained. Work is described on the development of an improved ion trap system and the continuous linkage of the optical frequency to the RF domain using a fiber based frequency comb.
As part of the ongoing world-wide effort in improving time and frequency references, a high accuracy optical frequency standard was developed using the electric quadrupole allowed clock transition at 445 THz (674 nm) in a trapped and laser cooled 88Sr+ion. An ion trap system of the endcap design has been recently evaluated with a fractional frequency uncertainty which surpasses the accuracy of the current realization of the SI second. This thesis seeks to further evaluate the limiting systematic shifts of the device by optimizing a second ion trap reference based on a rf Paul trap design, which was then compared with the endcap trap reference frequency. The comparison of the two ion traps' reference frequencies confirmed an overall offset of -0.36 pm 0.08 Hz at the 445 THz reference frequency corresponding to a fractional frequency offset of 8 x 10^-16.
We report on the progress on a single-ion optical clock that is being developed at the FEMTO-ST institute. Our objective is to reach a fractional frequency stability of the order of 10-14 at one second for a total volume well below 500 L. The experimental setup is based on a surface electrode single-ion trap. A new trapping chip was designed and produced at the local cleanroom facility, using deep reactive-ion etching (DRIE) on a silicon on insulator- (SOI) wafer. Paul traps require a source of highly-stable and low noise voltage in the radio-frequency (RF) domain. A resonator of our own design has been used to supply high voltage to the trap. We have developed and implemented techniques for 3D micromotion reduction in order to minimize the 2nd order Doppler shift. Indeed, for Yb+ optical spectroscopy, second order Doppler shift and Stark shift due to the ac fields can contribute significantly to the uncertainty budget. We will present the latest results of the trap characterization, which should include lifetime and heating rate measurements of the trap.
Optical frequency standards based on narrow absorptions in laser-cooled single trapped ions have recently begun to demonstrate stabilities that are competitive with cold atom fountain microwave standards. This paper presents a short review of the wider state-of-the-art development of these single cold trapped ion frequency standards, coupled with a more detailed account of recent results achieved at National Physical Laboratory in respect of single ion systems based on 88Sr+, 87Sr+ and 171Yb+. Narrow linewidth data for the optical clock quadrupole and octupole transitions respectively at 674 nm in 88Sr+ and 467 nm in 171Yb+, are presented, together with a discussion of current systematics and future projections. The potential for optical clock operation is outlined.
We describe recent experiments and results which show the effective cancellation of micromotion induced systematic shifts for the 445-THz reference transition in a trapped and laser cooled88Sr+single ion reference. Using two different ion trap systems based on a Paul and endcap design, the critical or magic frequency at which micromotion effects cancel has been effectively measured at 14.4 MHz. This work also allows an improved determination of the polarizability difference of the ground and excited states and an improvement in the evaluated blackbody shifts to the low 10-17level.