[Paper Review] An $^{27}$Al$^{+}$ quantum-logic clock with systematic uncertainty below $10^{-18}$

Samuel M. Brewer, Jwo-Sy Chen+6 more|arXiv (Cornell University)|Feb 20, 2019

Advanced Frequency and Time StandardsPhysics and Astronomy42 citations

TL;DR

Demonstrates a 27Al+ quantum-logic clock with total systematic uncertainty of 9.4e-19 and a stability of 1.2e-15/√τ, using sympathetic cooling with 25Mg+ and improved trap design to reach near 3D ground-state motion.

ABSTRACT

We describe an optical atomic clock based on quantum-logic spectroscopy of the $^1$S$_0$ $\leftrightarrow$ $^3$P$_0$ transition in $^{27}$Al$^{+}$ with a systematic uncertainty of ${9.4 imes 10^{-19}}$ and a frequency stability of ${1.2 imes10^{-15}/\sqrtτ}$. A $^{25}$Mg$^{+}$ ion is simultaneously trapped with the $^{27}$Al$^{+}$ ion and used for sympathetic cooling and state readout. Improvements in a new trap have led to reduced secular motion heating, compared to previous $^{27}$Al$^{+}$ clocks, enabling clock operation with ion secular motion near the three-dimensional ground state. Operating the clock with a lower trap drive frequency has reduced excess micromotion compared to previous $^{27}$Al$^{+}$ clocks. Both of these improvements have led to a reduced time-dilation shift uncertainty. Other systematic uncertainties including those due to blackbody radiation and the second-order Zeeman effect have also been reduced.

Motivation & Objective

• Showcase a 27Al+ quantum-logic clock with sub-1e-18 systematic uncertainty.
• Quantify and mitigate dominant systematic shifts such as excess micromotion and time dilation.
• Demonstrate improved trap design enabling near ground-state 3D motional cooling.
• Characterize clock stability against a reference Yb lattice clock.
• Discuss implications for fundamental constants and potential SI second redefiniton roadmap.

Proposed method

• Quantum-logic spectroscopy of the 1S0 <-> 3P0 transition in 27Al+.
• Sympathetic cooling and readout using a co-trapped 25Mg+ ion.
• Ground-state cooling sequences and reduced trap drive to lower EMM.
• Two opposing clock beams with a Doppler-tracking servo to cancel first-order Doppler.
• Monte-Carlo analysis to propagate EMM measurement uncertainties into the shift budget.

Experimental results

Research questions

• RQ1 What is the total fractional frequency uncertainty achievable with a 27Al+ quantum-logic clock using the described trap and cooling enhancements?
• RQ2 What are the dominant systematic shifts limiting the uncertainty and how can they be quantified and reduced?
• RQ3 How does real-time micromotion compensation influence the time-dilation and Doppler-related shifts?
• RQ4 How does the clock stability compare to a reference Yb lattice clock over long averaging times?
• RQ5 Can the setup support clock operation near 3D motional ground state to minimize Doppler shifts?

Key findings

• Total fractional frequency shift is Δν/ν = -9336.0(9.4)×10^-19, with a total systematic uncertainty of 9.4×10^-19.
• Excess micromotion contributes -45.8(5.9)×10^-19 to the shift.
• Blackbody radiation contributes -30.5(4.2)×10^-19.
• Quadratic Zeeman shift contributes -9241.8(3.7)×10^-19.
• Secular motion contributes -17.3(2.9)×10^-19.
• First-order Doppler and clock-laser Stark shifts are constrained to near zero within uncertainties of 2.2×10^-19 and 2.0×10^-19 respectively.