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European Frequency and Time Forum

Towards an Yb+ optical clock with a BBR uncertainty below 10^(-18)

Jonathan M. Jones, R. M. Godun, S. A. King, Peter B. R. Nisbet-Jones, Charles F. A. Baynham, Thomas Fordell, Tuomas Hieta, Thomas Lindvall, Kai Bongs, P. E. G. Baird, Patrick Robert Gill

Published Jan 1, 2016• 0 citations
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A liquid nitrogen-cooled Ca+ optical clock with systematic uncertainty of 3×10-18

Yao Huang, Baolin Zhang, Mengyan Zeng +6 more

Abstract Here we present a liquid nitrogen-cooled Ca+ optical clock with an overall systematic uncertainty of 3×10-18. In contrast with the room-temperature Ca+ optical clock that we have reported previously, the temperature of the blackbody radiation (BBR) shield in vacuum has been reduced to 82(5) K using liquid nitrogen. An ion trap with a lower heating rate and improved cooling lasers were also introduced. This allows cooling the ion temperature to the Doppler cooling limit during the clock operation, and the systematic uncertainty due to the ion’s secular (thermal) motion is reduced to < 1×10-18. The uncertainty due to the probe laser light shift and the servo error are also reduced to < 1×10-19 and 4×10-19 with the hyper-Ramsey method and the higher-order servo algorithm, respectively. By comparing the output frequency of the cryogenic clock to that of a room-temperature clock, the differential BBR shift between the two was measured with a fractional statistical uncertainty of 7×10-18. The differential BBR shift was used to calculate the static differential polarizability, and it was found in excellent agreement with our previous measurement with a different method. This work suggests that the BBR shift of optical clocks can be well suppressed in a liquid nitrogen environment. This is advantageous because conventional liquid-helium cryogenic systems for optical clocks are more expensive and complicated. Moreover, the proposed system can be used to suppress the BBR shift significantly in other types of optical clocks such as Yb+, Sr+, Yb, Sr, etc.

arXiv:2103.08913•Mar 16, 2021•10 citations

Cryogenic Optical Lattice Clock with Low 10-20 Blackbody Radiation Stark Uncertainty

Hassan, Youssef Salahuddin Sayed

Controlling the Stark perturbation from ambient thermal radiation is key to advancing the performance of many atomic frequency standards. This thermal radiation perturbs the atomic energy levels, inducing a Stark shift to the clock frequency conventionally known as the BBR shift. For over four decades, the BBR shift has been a persistent challenge in the pursuit of better atomic clock performance. For room-temperature optical lattice clocks (OLCs) based on Yb and Sr, the BBR shift represents the largest uncanceled systematic frequency shift while typically also constituting the largest source of uncertainty in these state-of-the-art clocks at the ≈ 1 × 10−18 level. In this thesis, I report on the design, assembly and integration of a BBR enclosure, or “shield,” that achieves an unprecedented BBR shift uncertainty of 1.7 × 10−20 by furnishing the interrogated atoms with a near-ideal BBR environment at a cryogenic temperature, while still allowing all the critical quantum control functions required for clock operation. Also, I present a novel lattice loading technique, known as ratchet loading, which enables programmable control over the spatial distribution of ultra-cold atoms confined in an optical lattice. This method allows for the loading of a large number of atoms, effectively reducing quantum projection noise (QPN) while also mitigating frequency shifts caused by atomic interactions. Additionally, ratchet loading facilitates the creation of spatially resolved atomic ensembles along the lattice, making it particularly useful for lattice light shift measurements and potentially advantageous for emerging techniques that probe atomic coherence beyond the local oscillator (LO) coherence time. With the near-elimination of blackbody radiation (BBR) uncertainty and improved clock stability for systematic evaluations, the realization of a mid- to low-10−19 uncertainty Yb optical lattice clock is now more attainable than ever.

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Progress toward an Yb Optical Lattice Clock

Zeb W. Barber, J. E. Stalnaker, Chris W. Oates +1 more

Bulletin of the American Physical Society•Jun 7, 2007•0 citations
Abstract Here we present a liquid nitrogen-cooled Ca+ optical clock with an overall systematic uncertainty of 3×10-18. In contrast with the room-temperature Ca+ optical clock that we have reported previously, the temperature of the blackbody radiation (BBR) shield in vacuum has been reduced to 82(5) K using liquid nitrogen. An ion trap with a lower heating rate and improved cooling lasers were also introduced. This allows cooling the ion temperature to the Doppler cooling limit during the clock operation, and the systematic uncertainty due to the ion’s secular (thermal) motion is reduced to < 1×10-18. The uncertainty due to the probe laser light shift and the servo error are also reduced to < 1×10-19 and 4×10-19 with the hyper-Ramsey method and the higher-order servo algorithm, respectively. By comparing the output frequency of the cryogenic clock to that of a room-temperature clock, the differential BBR shift between the two was measured with a fractional statistical uncertainty of 7×10-18. The differential BBR shift was used to calculate the static differential polarizability, and it was found in excellent agreement with our previous measurement with a different method. This work suggests that the BBR shift of optical clocks can be well suppressed in a liquid nitrogen environment. This is advantageous because conventional liquid-helium cryogenic systems for optical clocks are more expensive and complicated. Moreover, the proposed system can be used to suppress the BBR shift significantly in other types of optical clocks such as Yb+, Sr+, Yb, Sr, etc.
Controlling the Stark perturbation from ambient thermal radiation is key to advancing the performance of many atomic frequency standards. This thermal radiation perturbs the atomic energy levels, inducing a Stark shift to the clock frequency conventionally known as the BBR shift. For over four decades, the BBR shift has been a persistent challenge in the pursuit of better atomic clock performance. For room-temperature optical lattice clocks (OLCs) based on Yb and Sr, the BBR shift represents the largest uncanceled systematic frequency shift while typically also constituting the largest source of uncertainty in these state-of-the-art clocks at the ≈ 1 × 10−18 level. In this thesis, I report on the design, assembly and integration of a BBR enclosure, or “shield,” that achieves an unprecedented BBR shift uncertainty of 1.7 × 10−20 by furnishing the interrogated atoms with a near-ideal BBR environment at a cryogenic temperature, while still allowing all the critical quantum control functions required for clock operation. Also, I present a novel lattice loading technique, known as ratchet loading, which enables programmable control over the spatial distribution of ultra-cold atoms confined in an optical lattice. This method allows for the loading of a large number of atoms, effectively reducing quantum projection noise (QPN) while also mitigating frequency shifts caused by atomic interactions. Additionally, ratchet loading facilitates the creation of spatially resolved atomic ensembles along the lattice, making it particularly useful for lattice light shift measurements and potentially advantageous for emerging techniques that probe atomic coherence beyond the local oscillator (LO) coherence time. With the near-elimination of blackbody radiation (BBR) uncertainty and improved clock stability for systematic evaluations, the realization of a mid- to low-10−19 uncertainty Yb optical lattice clock is now more attainable than ever.