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1. Prospects of a thousand-ion Sn2+ Coulomb-crystal clock with sub-10^(−19) inaccuracy

   https://doi.org/10.48550/arXiv.2205.15484  

   Leibrandt, D.R.; Porsev, S.G.; Cheung, C.; Safronova, M.S. (July 2022, ArXivorg)

   We propose a many-ion optical atomic clock based on three-dimensional Coulomb crystals of order one thousand Sn2+ ions confined in a linear RF Paul trap. Sn2+ has a unique combination of features that is not available in previously considered ions: a 1S0 ↔ 3P0 clock transition between two states with zero electronic and nuclear angular momentum (I = J = F = 0) making it immune to nonscalar perturbations, a negative differential polarizability making it possible to operate the trap in a manner such that the two dominant shifts for three-dimensional ion crystals cancel each other, and a laser-accessible transition suitable for direct laser cooling and state readout. We present analytical calculations of the differential polarizability and other relevant atomic properties, as well as numerical calculations of the motion of ions in large Coulomb crystals, to estimate the achievable accuracy and precision of Sn2+ Coulomb-crystal clocks. 

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2. Vernier microcombs for integrated optical atomic clocks

   https://doi.org/10.1038/s41566-025-01617-0  

   Wu, Kaiyi; O’Malley, Nathan P; Fatema, Saleha; Wang, Cong; Girardi, Marcello; Alshaykh, Mohammed S; Ye, Zhichao; Leaird, Daniel E; Qi, Minghao; Torres-Company, Victor; et al (April 2025, Nature Photonics)

   Abstract Kerr microcombs have drawn substantial interest as mass-manufacturable, compact alternatives to bulk frequency combs. This could enable the deployment of many comb-reliant applications previously confined to laboratories. Particularly enticing is the prospect of microcombs performing optical frequency division in compact optical atomic clocks. Unfortunately, it is difficult to meet the self-referencing requirement of microcombs in these systems owing to the approximately terahertz repetition rates typically required for octave-spanning comb generation. In addition, it is challenging to spectrally engineer a microcomb system to align a comb mode with an atomic clock transition with a sufficient signal-to-noise ratio. Here we adopt a Vernier dual-microcomb scheme for optical frequency division of a stabilized ultranarrow-linewidth continuous-wave laser at 871 nm to an ~235 MHz output frequency. This scheme enables shifting an ultrahigh-frequency (~100 GHz) carrier-envelope offset beat down to frequencies where detection is possible and simultaneously placing a comb line close to the 871 nm laser—tuned so that, if frequency doubled, it would fall close to the clock transition in¹⁷¹Yb⁺. Our dual-comb system can potentially combine with an integrated ion trap towards future chip-scale optical atomic clocks. 

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3. Optical coherence between atomic species at the second scale: improved clock comparisons via differential spectroscopy

   https://doi.org/10.48550/arXiv.2109.09540  

   Kim, M.E.; McGrew, W.F.; Nardelli, N.V.; Clements, E.R.; Hassan, Y.S.; Zhang, X.; Valencia, J.L.; Leopardi, H.; Hume, D.B.; Fortier, T.M.; et al (April 2022, ArXivorg)

   Comparisons of high-accuracy optical atomic clocks \cite{Ludlow2015} are essential for precision tests of fundamental physics \cite{Safronova2018}, relativistic geodesy \cite{McGrew2018, Grotti2018, Delva2019}, and the anticipated redefinition of the SI second \cite{Riehle2018}. The scientific reach of these applications is restricted by the statistical precision of interspecies comparison measurements. The instability of individual clocks is limited by the finite coherence time of the optical local oscillator (OLO), which bounds the maximum atomic interrogation time. In this letter, we experimentally demonstrate differential spectroscopy \cite{Hume2016}, a comparison protocol that enables interrogating beyond the OLO coherence time. By phase-coherently linking a zero-dead-time (ZDT) \cite{Schioppo2017} Yb optical lattice clock with an Al+ single-ion clock via an optical frequency comb and performing synchronised Ramsey spectroscopy, we show an improvement in comparison instability relative to our previous result \cite{network2020frequency} of nearly an order of magnitude. To our knowledge, this result represents the most stable interspecies clock comparison to date. 

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4. Seconds-scale coherence on an optical clock transition in a tweezer array

   https://doi.org/10.1126/science.aay0644  

   Norcia, Matthew A.; Young, Aaron W.; Eckner, William J.; Oelker, Eric; Ye, Jun; Kaufman, Adam M. (October 2019, Science)

   Coherent control of high–quality factor optical transitions in atoms has revolutionized precision frequency metrology. Leading optical atomic clocks rely on the interrogation of such transitions in either single ions or ensembles of neutral atoms to stabilize a laser frequency at high precision and accuracy. We demonstrate a platform that combines the key strengths of these two approaches, based on arrays of individual strontium atoms held within optical tweezers. We report coherence times of 3.4 seconds, single-ensemble duty cycles up to 96% through repeated interrogation, and frequency stability of 4.7 × 10 −16 (τ/s) –1/2 . These results establish optical tweezer arrays as a powerful tool for coherent control of optical transitions for metrology and quantum information science. 

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5. Quantum Metrology with a Molecular Lattice Clock and State-Selected Photodissociation of Ultracold Molecules

   Lee, C.-H. (January 2020, Columbia University thesis)

   Over the past few decades, rapid development of laser cooling techniques and narrow-linewidth lasers have allowed atom-based quantum clocks to achieve unprecedented precision. Techniques originally developed for atomic clocks can be extended to ultracold molecules, with applications ranging from quantum-state-controlled ultracold chemistry to searches for new physics. Because of the richness of molecular structure, quantum metrology based on molecules provides possibilities for testing physics that is beyond the scope of traditional atomic clocks. This thesis presents the work performed to establish a state-of-the-art quantum clock based on ultracold molecules. The molecular clock is based on a frequency difference between two vibrational levels in the electronic ground state of 88Sr2 diatomic molecules. Such a clock allows us test molecular QED, improve constraints on nanometer-scale gravity, and potentially provide a model-independent test of temporal variations of the proton-electron mass ratio. Trap-insensitive spectroscopy is crucial for extending coherent molecule-light interactions and achieving a high quality factor Q. We have demonstrated a magic wavelength technique for molecules by manipulating the optical lattice frequency near narrow polarizability resonances. This general technique allows us to increase the coherence time to tens of ms, an improvement of a factor of several thousand, and to narrow the linewidth of a 25 THz vibrational transition initially to 30 Hz. This width corresponds to the quality factor Q = 8 × 10^11. Besides the molecular quantum metrology, investigations of novel phenomena in state-selected photodissociation are also described in this thesis, including magnetic-field control of photodissociation and observation of the crossover from ultracold to quasiclassical chemistry. 

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