Fundamental Physics with a State-of-the-Art Optical Clock in Space

Time is an omnipresent concept in modern society and plays a central role in the foundations of physics and our understanding of the cosmos. Atomic clocks keep international time and their quantum measurements are arguably, by orders of magnitude, the most accurate measurements of any physical observable. Figure 1 shows the dramatic improvement of the accuracy of atomic clocks over the last 20 years, far faster than in the last half of the 20th century. This extremely rapid improvement of clocks at optical frequencies resulted from advancements in the manipulation of atomic quantum systems, as well as advanced techniques in laser cooling/trapping and the development of fs-laser frequency combs in 2000, which enabled the reliable counting of the ticks of the cycles of laser light at 1015 cycles per second (Ludlow et al. 2015). The most accurate versions of these optical clocks use trapped quantum absorbers (either ions or lattice-confined neutral atoms) and now have fractional frequency uncertainties approaching or exceeding 1 part in 1018, which enable transformational advances in areas ranging from navigation and ultra-precise ranging, searches for dark matter, observation of gravity waves, and stringent tests of general relativity. The accuracy of optical clocks had been advancing at the breakneck rate of the last decade. Indeed, with no foreseeable barriers to continued steady improvement as projected in Figure 1, clock accuracies would reach 1019 in 2027, 1020 by 2034, and 1021 in two decades, at which point gravity waves could be directly observed in the tick rates of clocks. While specific large advances are not possible to predict, dramatic improvements using quantum entanglement are anticipated and there is no known fundamental limitation in sight to the ultimately achievable accuracy of atomic clocks.