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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. 

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.