Amongst the beautiful mountainous scenery of the Rocky Mountains and the iconic Flatirons of Boulder, Colorado, sits a device that counts seconds better than almost any device on earth. It is the NIST-F2, the clock that was unveiled by the National Institute for Standards and Technology (NIST) in 2014, which does not gain or lose a second in 300 million years (i.e., with an inaccuracy of approximately 10) [1]. To give some perspective, 300 million years ago the earth had one continent, the Pangea, and reptiles were just rising into dominance. The NIST-F2, alongside its predecessor NIST-F1, serves as the primary standard for civilian time in the US [2]. The International Atomic Time (TAI) is based on the readings combined from many of these high precision clocks worldwide. It is the basis of civilian time and Coordinated Universal Time (UTC) [3].

It is hard to overstate the importance of clocks in our world. Contrary to the readers' current thoughts, clocks with an inaccuracy of 10 serve various purposes in our everyday lives. Accurate clocks are used to synchronise GPS systems, utilise radars for both commercial and military purposes, improve geodesy and metrology, timestamp financial transactions, and can even be used to confirm Einstein's theory of general relativity [4-6]. All of the above applications require a precise measurement of time to operate, and new applications of accurate clocks are realised frequently. At the launch of the NIST-F2, Steven Jefferts, the lead designer said, "If we've learned anything in the last 60 years of building atomic clocks, we've learnt that every time we build a better clock, somebody comes up with a use for it that you couldn't have foreseen" [1].

The most common clocks used for accurate timekeeping are known as atomic clocks, as they take advantage of the stable energy level structures of specific atoms. Atomic clocks arose several decades after the rapid establishment of quantum mechanics in the early 20th century when scientists started to understand the detailed structure of atoms. Lord Kelvin first suggested such a device in 1879¹, but the technology to realise them only came into prominence in the mid-twentieth century [7]. Much of the foundational work on atomic oscillations was laid out by Isador Rabi — a Nobel laureate in physics — in the 1930s and 40s [8,9]. Several laboratories in the UK and US started working on creating an atomic clock a decade later [7].

Atomic clocks work by exploiting the energy levels of atoms. The electrons that surround an atom have discrete energy levels and require a particular amount of energy for it to move to the next energy state. The amount of energy required to transition between energy levels is unique and consistent with each atom² [7,10-12]. This property plays an essential role in mitigating any manufacturing errors as every element is identical to one another. The basic principle behind atomic clocks is as follows.³ An ensemble of atoms is cooled to several milli-Kelvin (near absolute zero) in order to access its ground state energy level. The ensemble is then exposed to radiation at a frequency close to their resonant frequency — the frequency that excites the atom and transitions it to the next energy state (in this case, the first excited state). It is for this reason that we must ensure the initially prepared ensemble is in its ground state. A magnet filters out those atoms that are not excited, and the remaining atoms are fed into a detector. The detector then counts the number of excited atoms, and uses this information to adjust the frequency of the radiation until the maximum number of excited atoms is detected. This feedback control and these self-adjustments are what make atomic clocks so precise. The adjusted frequency is then counted by a separate device to keep track of the time elapsed.⁴ Figure 1 illustrates the general idea.

The most common element used in modern atomic clocks is the Cesium-133 atom [7]. Its heavy mass makes it slow and easier to confine, and its comparatively high resonant frequency makes for a more accurate measurement. In fact, the definition of the SI unit "second" is the duration of 91,926,317,70 periods of the radiation corresponding to the transition between the hyperfine levels of the unperturbed ground state of the Cesium-133 atom [13]. Other common elements used are Hydrogen and Rubidium, though both weigh less and have lower resonant frequencies in their ground states.

The significance of the measurement of time can not be understated when almost all measurements we make are in some form compared to time. Until the mid-1990s, Cesium based atomic clocks reigned supreme in accuracy. Though most clocks used nowadays for government, commercial, and military purposes are still Cesium based (owing to its well-known stability and reliability), various research groups worldwide have realised clocks that have higher accuracies. Here we will explore two promising avenues in next-generation high-precision clocks, the first of which is the optical lattice clock.