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1. High-precision optical time and frequency transfer

   https://doi.org/10.1364/AOP.545290  

   Caldwell, Emily D; Triano, Theodora M; Sinclair, Laura C (January 2025, Advances in Optics and Photonics)

   High-precision optical time and frequency transfer is accomplished by a collection of laser-based techniques that achieve time dissemination with subpicosecond instabilities and frequency dissemination with instabilities below one part in 10¹⁶. The ability to distribute and compare time and frequency at these precisions enables current optical timing networks such as interconnected optical atomic clocks for the redefinition of the second, relativistic geodesy, and fundamental physics tests as well as time and frequency dissemination systems for large-scale scientific instruments. Future optical timing networks promise to expand these applications and enable new advances in distributed coherent sensing, precise navigation, and more. The field of high-precision optical time and frequency transfer has made significant advances over the last 20 years and has begun to transition from technique development to deployment in applications. Here, we present a review of approaches to high-precision optical time and frequency transfer. We first present a brief overview of the metrics used to assess time and frequency transfer. We then provide a discussion of the difference between time transfer and frequency transfer and review the various technical noise sources. We also provide a background on the optical frequency comb and its role in optical time and frequency transfer for additional context. The next sections of the paper cover specific time–frequency transfer techniques and demonstrations beginning with time and frequency transfer over fiberoptic links including continuous-wave (CW) laser-based frequency transfer, CW-laser-based time transfer, and frequency-comb-based time transfer. We then discuss approaches for time and frequency transfer over free-space including pulsed-source time transfer, CW-laser-based frequency transfer, and frequency-comb-based time transfer. Since no known existing review article covers frequency-comb time transfer over free-space, we provide additional details on the technique. Finally, we provide an outlook that outlines outstanding challenges in the field as well as possible future applications. 

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2. Ratchet loading and multi-ensemble operation in an optical lattice clock

   https://doi.org/10.1088/2058-9565/ad6286  

   Hassan, Youssef S; Kobayashi, Takumi; Bothwell, Tobias; Seigel, Jacob L; Hunt, Benjamin D; Beloy, Kyle; Gibble, Kurt; Grogan, Tanner; Ludlow, Andrew D (August 2024, Quantum Science and Technology)

   Abstract We demonstrate programmable control over the spatial distribution of ultra-cold atoms confined in an optical lattice. The control is facilitated through a combination of spatial manipulation of the magneto-optical trap and atomic population shelving to a metastable state. We first employ the technique to load an extended (5 mm) atomic sample with uniform density in an optical lattice clock (OLC), reducing atomic interactions and realizing remarkable frequency homogeneity across the atomic cloud. We also prepare multiple spatially separated atomic ensembles, and realize multi-ensemble clock operation within the standard one-dimensional (1D) OLC architecture. Leveraging this technique, we prepare two oppositely spin-polarized ensembles that are independently addressable, offering a platform for implementing spectroscopic protocols for enhanced tracking of local oscillator phase. Finally, we demonstrate a relative fractional frequency instability at one second of $2.4\left(1\right)\times {10}^{-17}$between two ensembles, useful for characterization of intra-lattice differential systematics. 

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3. Accuracy evaluation of primary frequency standard NIST-F4

   https://doi.org/10.1088/1681-7575/adc7bd  

   Gerginov, Vladislav; Hoth, Gregory W; Heavner, Thomas P; Parker, Thomas E; Gibble, Kurt; Sherman, Jeff A (April 2025, Metrologia)

   Abstract This work describes the apparatus for NIST-F4, an updated cesium atomic fountain at the National Institute of Standards and Technology (NIST), and presents an accuracy evaluation of the fountain as a primary frequency standard. The fountain uses optical molasses to laser cool a cloud of cesium atoms and launch it vertically in a fountain geometry. In high-density mode, the fractional frequency stability of NIST-F4 is ${\sigma }_y\left(\tau \right)=1.5\times {10}^{-13}/\sqrt{\tau }$, whereτis the measurement time in seconds. The short-term stability is limited by quantum projection noise and by phase noise from the local oscillator, an oven-controlled crystal oscillator operating at 5 MHz. Systematic frequency shifts and their uncertainties have been evaluated, resulting in a systematic (type B) fractional frequency uncertainty ${\sigma }_{\mathrm{B}}=2.2\times {10}^{-16}$. 

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4. A mathematical model of plasmin-mediated fibrinolysis of single fibrin fibers

   https://doi.org/10.1371/journal.pcbi.1012684  

   Ouedraogo, Roukayatou R; Sowers, Hannah K; Lynch, Spencer R; Hudson, Nathan E; Bannish, Brittany E (December 2024, PLOS Computational Biology)

   Fibrinolysis, the plasmin-mediated degradation of the fibrin mesh that stabilizes blood clots, is an important physiological process, and understanding mechanisms underlying lysis is critical for improved stroke treatment. Experimentalists are now able to study lysis on the scale of single fibrin fibers, but mathematical models of lysis continue to focus mostly on fibrin network degradation. Experiments have shown that while some degradation occurs along the length of a fiber, ultimately the fiber is cleaved at a single location. We built a 2-dimensional stochastic model of a fibrin fiber cross-section that uses the Gillespie algorithm to study single fiber lysis initiated by plasmin. We simulated the model over a range of parameter values to learn about patterns and rates of single fiber lysis in various physiological conditions. We also used epifluorescent microscopy to measure the cleavage times of fibrin fibers with different apparent diameters. By comparing our model results to the laboratory experiments, we were able to: 1) suggest value ranges for unknown rate constants(namely that the degradation rate of fibrin by plasmin should be ≤ 10 s⁻¹and that if plasmin crawls, the rate of crawling should be between 10 s⁻¹and 60 s⁻¹); 2) estimate the fraction of fibrin within a fiber cross-section that must be degraded for the fiber to cleave in two; and 3) propose that that fraction is higher in thinner fibers and lower in thicker fibers. Collectively, this information provides more details about how fibrin fibers degrade, which can be leveraged in the future for a better understanding of why fibrinolysis is impaired in certain disease states, and could inform intervention strategies. 

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5. Application of quantum-limited optical time transfer to space-based optical clock comparisons and coherent networks

   https://doi.org/10.1063/5.0170107  

   Caldwell, Emily D; Sinclair, Laura C; Deschenes, Jean-Daniel; Giorgetta, Fabrizio; Newbury, Nathan R (January 2024, APL Photonics)

   With the demonstration of quantum-limited optical time transfer capable of tolerating the losses associated with long ground-to-space links, two future applications of free-space time transfer have emerged: intercontinental clock comparisons for time dissemination and coherence transfer for future distributed sensing in the mm-wave region. In this paper, we estimated the projected performance of these two applications using quantum-limited optical time transfer and assuming existing low-size, low-weight, and low-power hardware. In both cases, we limit the discussion to the simplest case of a single geosynchronous satellite linked to either one or two ground stations. One important consideration for such future space-based operations is the choice of reference oscillator onboard the satellite. We find that with a modestly performing optical reference oscillator and low-power fiber-based frequency combs, quantum-limited time transfer could support intercontinental clock comparisons through a common-view node in geostationary orbit with a modified Allan deviation at the 10−16 level at 10-s averaging time, limited primarily by residual turbulence piston noise. In the second application of coherence transfer from ground-to-geosynchronous orbit, we find the system should support high short-term coherence with ∼10 millirad phase noise on a 300 GHz carrier at essentially unlimited integration times. 

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