Optical atomic clock comparison through turbulent air

Open Access

Optical atomic clock comparison through turbulent air

Martha I. Bodine et al.

Phys. Rev. Research 2, 033395 – Published 11 September 2020

Abstract

We use frequency-comb-based optical two-way time-frequency transfer (O-TWTFT) to measure the optical frequency ratio of state-of-the-art ytterbium and strontium optical atomic clocks separated by a 1.5-km open-air link. Our free-space measurement is compared to a simultaneous measurement acquired via a noise-cancelled fiber link. Despite nonstationary, ps-level time-of-flight variations in the free-space link, ratio measurements obtained from the two links, averaged over 30.5 hours across six days, agree to $6 \times 10^{- 19}$ , showing that O-TWTFT can support free-space atomic clock comparisons below the $10^{- 18}$ level.

Received 1 June 2020
Accepted 4 August 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.033395

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic, optical & lattice clocks Optics & lasers

Atomic, Molecular & Optical

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Issue

Vol. 2, Iss. 3 — September - November 2020

Subject Areas

Atomic and Molecular Physics

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Images

Figure 1
(a) In the Yb Clock Lab, frequency-doubled light from a 1157-nm cavity stabilized laser (CSL) is steered into resonance with the ${}^{171}{Yb}$ atomic transition via frequency shifts from an acousto-optic modulator (AOM). This steered 1157-nm light is sent to the Transfer Lab, where a fiber comb [55] transfers the frequency stability and accuracy of this 1157-nm light to a 1535-nm laser. This laser light is then sent to a rooftop laboratory where it serves as the reference for comb A within the O-TWTFT transceiver. In the Sr Clock Lab, the frequency of a 698-nm CSL is steered into resonance with the ${}^{87}{Sr}$ atomic transition. The 698-nm light is steered by adjusting the frequency of a low-noise 1542-nm CSL [56]. A frequency comb then acts to transfer the frequency adjustments of the 1542-nm light to the 698-nm light. The steered 1542-nm light also serves as the reference for comb B within the O-TWTFT transceiver. On both sides of the free-space link, the heterodyne beat between local and remote comb pulses is detected and processed within the O-TWTFT transceivers. Additionally, the atomic clock frequencies are compared via a noise-cancelled fiber link. BD: Balanced detector. (b) Photo from O-TWTFT transceiver at CU, looking toward NIST. A free-space optical (FSO) terminal with active beam steering maintains the bidirectional link between sites (photo edited to remove window frame).
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Figure 2
(a) Time stamps $k_{A}$ recorded at NIST and time stamps $k_{B}$ recorded at CU, as well as their sum $k_{sum}$ . Inset: Expanded 10-s data segment with linear fits removed. Fluctuations in $k_{A}$ and $k_{B}$ are caused by atmospheric turbulence and building sway. When scaled from ADC clock cycles to approximate time (scale bar), these can easily reach 100 fs in 1 s, but they are cancelled in $k_{sum}$ . (b) Ratios of comb A and comb B repetition frequencies, offset from their mean. Each point is extracted from the slope fit to a single 10-s segment of $k_{sum}$ .
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Figure 3
Measured atomic clock frequency ratios as a function of the local time in Boulder, Colorado. Ratios are plotted as fractional offsets: (measured $-$ expected)/(expected). The fractional offset from the expected value for the atomic frequency ratio $R_{BIPM}$ appears significantly displaced from zero but is well within the $\pm 6.4 \times 1 0^{- 16}$ uncertainty of $R_{BIPM}$ . The expected value of the loopback test is 1, since this test compares the frequency of the 1542-nm CSL to itself. Uncertainty bars show 1- $σ$ statistical uncertainty.
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Figure 4
Fractional instabilities (overlapping Allan deviations) of atomic frequency ratio measurements from March 6, of the differences between O-TWTFT and fiber link measurements from March 6, and of the network loopback test. Also shown is the previously measured fractional instability (modified Allan deviation) of O-TWTFT itself, as well as that of carrier-phase O-TWTFT [42].
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Figure 5
Differences between O-TWTFT and fiber measurements, along with their 1- $σ$ statistical uncertainties (error bars), their weighted average (dashed line), and its uncertainty (gray shading). Also shown is the offset of the network loopback test from its expected value of 1.
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