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

Auto-Compensation of Collisional Shift in Atomic Clocks Based on Bosonic Atoms in an Optical Lattice

Valeriy I. Yudin, А. В. Тайченачев, M. Yu. Basalaev, Прудников Олег Николаевич, T. Zanon-Willette, Sergei N Bagayev

Published Jul 7, 2021• 0 citations
Publisher

Abstract

We propose a new approach to suppress the influence of collisional shift and its fluctuations on the long-term stability of optical clocks based on bosonic atoms (even isotopes) in an 1D or 2D lattice. The successful implementation of our auto-compensation method, in combination with the hyper-Ramsey spectroscopic schemes, will allow considering bosons as a real alternative to fermions while achieving the highest metrological characteristics of atomic clocks.

DOI
10.1109/eftf/ifcs52194.2021.9604346

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We propose a new approach to suppress the influence of collisional shift and its fluctuations on the long-term stability of optical clocks based on bosonic atoms (even isotopes) in an 1D or 2D lattice. The successful implementation of our auto-compensation method, in combination with the hyper-Ramsey spectroscopic schemes, will allow considering bosons as a real alternative to fermions while achieving the highest metrological characteristics of atomic clocks.
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The effects of elastic collisions in optical-lattice atomic clocks are analyzed. Calculations are presented using a separated oscillatory fields clock arrangement. The interactions of atoms in multiply occupied lattice sites cause a linear frequency shift, and also generate asymmetric Ramsey fringe patterns, both complicate the determination of the resonance frequency and reduce the fringe visibility due to interparticle entanglement. A method of reducing these collisional effects in an optical lattice clock containing bosonic atoms by introducing a phase difference of π between the Ramsey driving fields in adjacent sites is developed. This configuration suppresses site-to-site hopping due to the interference of two tunneling pathways, without degrading the fringe visibility. The probability of double occupancy is, thereby, reduced and collisional shifts are, thus, ameliorated.
We theoretically study the effects of elastic collisions on the determination of frequency standards via Ramsey-fringe spectroscopy in optical-lattice atom clocks. Interparticle interactions of bosonic atoms in multiply occupied lattice sites can cause a linear frequency shift, as well as generate asymmetric Ramsey-fringe patterns and reduce fringe visibility due to interparticle entanglement. We propose a method of reducing these collisional effects in an optical lattice by introducing a phase difference of $\ensuremath{\pi}$ between the Ramsey driving fields in adjacent sites. This configuration suppresses site-to-site hopping due to interference of two tunneling pathways, without degrading fringe visibility. Consequently, the probability of double occupancy is reduced, leading to cancellation of collisional shifts.
Optical lattice clocks with uncertainty and instability in the 10􀀀17-range and below have so far been demonstrated exclusively using fermions. Here, we demonstrate a bosonic optical lattice clock with 3 10􀀀18 instability and 2:0 10􀀀17 accuracy, both values improving on previous work by a factor 30. This was enabled by probing the clock transition with an ultra-long interrogation time of 4 s, using the long coherence time provided by a cryogenic silicon resonator, by careful stabilization of relevant operating parameters, and by operating at low atom density. This work demonstrates that bosonic clocks, in combination with highly coherent interrogation lasers, are suitable for highaccuracy applications with particular requirements, such as high reliability, transportability, operation in space, or suitability for particular fundamental physics topics. As an example, we determine the 88Sr { 87Sr isotope shift with 12 mHz uncertainty.
We develop a method of modified hyper-Ramsey spectroscopy in optical clocks, achieving complete immunity to the frequency shifts induced by the probing fields themselves. Using particular pulse sequences with tailored phases, frequencies, and durations, we can derive an error signal centered exactly at the unperturbed atomic resonance with a steep discriminant which is robust against variations in the probe shift. We experimentally investigate the scheme using the magnetically induced ${}^{1}{S}_{0}\text{\ensuremath{-}}{}^{3}{P}_{0}$ transition in 88Sr^{88}\mathrm{Sr}88Sr, demonstrating automatic suppression of a sizable $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ probe Stark shift to below $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ even with very large errors in shift compensation.
Optical lattice clocks demonstrate advantages in metrology and frontier physics because of their high stability.Here, we present approaches to enhancing the stability by decoupling the noise related to the short-term and long-term stability.For the short-term stability, we optimize the clock laser by decoupling the frequency noise, and optimize each noise contribution individually until it is below the thermal noise limit.For the long-term stability, we introduce a method to decouple the instability caused by systematic effects.Having identified that the collision frequency shift was the main limiting factor in our systems, we thus optimized the atom number fluctuations in optical lattices.Through targeted optimization, we achieve a synchronous comparison of two clocks with an average stability of 3.2 × 10 -16 = τ p and a long-term stability of 2.4 × 10 -18 at 8000 s.This work provides an analytical framework for enhancing optical clock stability.