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Microfabricated ion frequency standard

Peter D. D. Schwindt

Published Jan 23, 2023• 0 citations
OpenAlex

Abstract

A microfabricated ion frequency standard (i.e. an ion clock) is disclosed with a permanently-sealed vacuum package containing a source of ytterbium (Yb) ions and an octupole ion trap. The source of Yb ions is a micro-hotplate which generates Yb atoms which are then ionized by a ultraviolet light-emitting diode or a field-emission electron source. The octupole ion trap, which confines the Yb ions, is formed from suspended electrodes on a number of stacked-up substrates. A microwave source excites a ground-state transition frequency of the Yb ions, with a frequency-doubled vertical-external-cavity laser (VECSEL) then exciting the Yb ions up to an excited state to produce fluorescent light which is used to tune the microwave source to the ground-state transition frequency, with the microwave source providing a precise frequency output for the ion clock.

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We are developing a highly miniaturized171Yb+ion clock that operates in a continuous microwave-optical double-resonance mode by continuously probing the 12.6 GHz hyperfine transition in the171Yb+ion. In the continuous mode, the clock will not require optical shutters and electrical switches, minimizing the components of the clock. Here, we demonstrate operating the ion clock with ions trapped in a permanently sealed, passively pumped vacuum package 3 cm3in volume. With resonant 369-nm light continuously illuminating the ions, the light shift is a substantial systematic shift of the hyperfine ground state that must to be well characterized and controlled. We present measurements of the liaht shift and demonstrate clock frequency instabilities below 1×10-12at 100 s of averaging.

Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)•May 1, 2018•3 citations
A microfabricated ion frequency standard (i.e. an ion clock) is disclosed with a permanently-sealed vacuum package containing a source of ytterbium (Yb) ions and an octupole ion trap. The source of Yb ions is a micro-hotplate which generates Yb atoms which are then ionized by a ultraviolet light-emitting diode or a field-emission electron source. The octupole ion trap, which confines the Yb ions, is formed from suspended electrodes on a number of stacked-up substrates. A microwave source excites a ground-state transition frequency of the Yb ions, with a frequency-doubled vertical-external-cavity laser (VECSEL) then exciting the Yb ions up to an excited state to produce fluorescent light which is used to tune the microwave source to the ground-state transition frequency, with the microwave source providing a precise frequency output for the ion clock.
Outline • Introduction• Why trapped ions for miniaturization?• Miniaturized vacuum package: 1 cm 3 • Pulsed-mode ion clock performance • F-state trapping • Current focus: microwave optical double resonance continuous operation • Alternative ion clock: Alkali Mediated Atomic Clock • Conclusion Miniaturization with Long-Term and Environmental Stability • Achieve Cs Beam Clock performance in a mass and power constrained package • DARPA programs: • IMPACT: improved long-term stability • ACES: improved environmental stability Applications--Excellent timing for: • Rapid GPS acquisition, and GPS denied navigation and timing • Nano/pico (cube) satellites • Pulsed radio and spread spectrum communications Microsemi 5071A Miniature primary frequency standard Approved for Public Release, Distribution Unlimited Critical Elements of the 171 Yb + Clock Trap Electrodes Ions Yb Source Vacuum Package Control and Power Electronics Local Oscillator Magnetic Shield and Coil Repump (935 nm) Optical Pumping/ State Detection Laser (369 nm) Beam Combiner Fluorescence Collection Lens Fluorescence Detector Photo Multiplier Tube (PMT) Ionization UV Light
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 10-3Hz, 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 10-14range after a few days of integration [1].
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).
The171Yb+ion possesses two transitions that are applicable as references for optical frequency standards and several general features make this ion attractive: All relevant radiations for cooling and excitation can be generated from diode lasers. A nuclear spin of 1/2 ensures a simple sublevel structure. Long storage times of Yb+in ion traps are easily obtained. The2S1/2(F = 0) →2D3/2(F = 2, mF= 0) electric quadrupole transition at 436 nm with a natural linewidth of 3.1 Hz provides a quantum noise limited instability of a single-ion frequency standard of about 2 × 10-15in 1 s. The electric octupole transition2S1/2(F = 0)→2F7/2(F = 3, mF= 0) at 467 nm with a natural linewidth in the nHz range will allow to surpass that instability level. It also possesses a lower sensitivity to field-induced frequency shifts. In this system the Fourier-limited interrogation linewidth can be adapted to that of the best available laser sources, which are presently at the level below 1 Hz. Both transitions have been recommended as secondary representations of the SI second.
We present the design of a compact optical clock based on the 2 S 1/2 → 2 D 3/2 435.5 nm transition in 171 Yb + . The ion trap will be based on a micro-fabricated circuit, with surface electrodes generating a trapping potential to localize a single Yb ion a few hundred μ m from the electrodes. We present our trap design as well as simulations of the resulting trapping pseudo-potential. We also present a compact, multi-channel wavelength meter that will permit the frequency stabilization of the cooling, repumping and clear-out lasers at 369.5 nm, 935.2 nm and 638.6 nm needed to cool the ion. We use this wavelength meter to characterize and stabilize the frequency of extended cavity diode lasers at 369.5 nm and 638.6 nm.
We are developing a highly miniaturized171Yb+ion clock that operates in a continuous microwave-optical double-resonance mode by continuously probing the 12.6 GHz hyperfine transition in the171Yb+ion. In the continuous mode, the clock will not require optical shutters and electrical switches, minimizing the components of the clock. Here, we demonstrate operating the ion clock with ions trapped in a permanently sealed, passively pumped vacuum package 3 cm3in volume. With resonant 369-nm light continuously illuminating the ions, the light shift is a substantial systematic shift of the hyperfine ground state that must to be well characterized and controlled. We present measurements of the liaht shift and demonstrate clock frequency instabilities below 1×10-12at 100 s of averaging.