# Optical clock

An optical clock keeps time by counting oscillations of electromagnetic waves. Oscillations of light are counted using a frequency comb, and stabilized to atoms. Optical clocks are a subset of atomic clocks which count oscillations of visible light, which oscillates at a much higher frequency than microwaves. Optical frequencies are advantageous because the stability of a clock is inversely proportional to the oscillator's frequency. For this reason, optical clocks are expected to replace microwave caesium clocks as the definition of the second. Several elements have been used in optical clocks, including magnesium, aluminum, potassium, calcium, rubidium, strontium, indium, ytterbium, mercury, and radium. John L. Hall and Theodor W. Hansch shared the 2005 Nobel Prize in Physics for their contributions to optical clock development.

## Overview

The development of femtosecond frequency combs and optical lattices has led to a new generation of atomic clocks. These clocks are based on atomic transitions that are resonant with visible light instead of microwave radiation.
The major obstacle in operating an optical clock was the difficulty of directly measuring optical frequencies. Before the demonstration of the frequency comb in 2000, terahertz techniques were needed to bridge the gap between radio and optical frequencies, and the systems for doing so were cumbersome and complicated. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs. The frequency comb has dramatically increased accessibility and numerous optical clock systems are in development.

### Optical clock configurations

Optical clocks are operated in several experimental systems. For example, many neutral strontium atoms trapped in an optical lattice as well as single aluminum and mercury ions trapped together in an ion trap using the quantum logic technique both form the bases for state-of-the-art optical clocks. The atoms or ions in these systems are highly isolated from external perturbations. Lasers and magneto-optical traps are used to cool the atoms for improved precision.

### Atoms used

An ideal atom for use in an optical clock has a narrow electronic transition, often a quadrupole transition referred to as a clock transition, that is accessible by conventional lasers. The atom often also has an electronic structure that is amenable to laser cooling. Additional desired features include properties that suppress external perturbations, for example stray electric fields, and availability of the atom for long-term use in a sealed system.
Atomic systems under consideration include Al$^+$, Hg$^{+/2+}$, Hg, Sr, Sr$^{+/2+}$, In$^{+/3+}$, Mg, Ca, Ca$^+$, Yb$^{+/2+/3+}$, Yb and Th$^{+/3+}$. The color of a clock's electromagnetic radiation depends on the element that is stimulated. For example, calcium optical clocks resonate when red light is produced, and ytterbium clocks resonate in the presence of violet light.
The rare-earth element ytterbium (Yb) is valued not so much for its mechanical properties but for its complement of internal energy levels. "A particular transition in Yb atoms, at a wavelength of 578 nm, currently provides one of the world's most accurate optical atomic frequency standards," said Marianna Safronova. The estimated uncertainty achieved corresponds to about one second over the lifetime of the universe so far, 15 billion years, according to scientists at the Joint Quantum Institute (JQI) and the University of Delaware in December 2012.

## History
### 2000s

The theoretical move from microwaves as the atomic "escapement" for clocks to light in the optical range, harder to measure but offering better performance, earned John L. Hall and Theodor W. Hänsch the Nobel Prize in Physics in 2005. One of 2012's Physics Nobelists, David J. Wineland, is a pioneer in exploiting the properties of a single ion held in a trap to develop clocks of the highest stability. The first optical lattice clock was completed by Hidetoshi Katori at the University of Tokyo in 2003, who had proposed the concept two years earlier. Another version of the optical clock was completed at the National Institute of Standards and Technology in 2006, as a result of a research project that had started in 2000.

### 2010s

In 2013 optical lattice clocks (OLCs) were shown to be as good as or better than caesium fountain clocks. Two optical lattice clocks containing about 10000 atoms of strontium-87 were able to stay in synchrony with each other at a precision of at least 1.5×10$^{−16}$, which is as accurate as the experiment could measure. These clocks have been shown to keep pace with all three of the caesium fountain clocks at the Paris Observatory. There are two reasons for the possibly better precision. Firstly, the frequency is measured using light, which has a much higher frequency than microwaves, and secondly, by using many atoms, any errors are averaged.
Using ytterbium-171 atoms, a new record for stability with a precision of 1.6×10$^{−18}$ over a 7-hour period was published on 22 August 2013. At this stability, the two optical lattice clocks working independently from each other used by the NIST research team would differ less than a second over the age of the universe (13.8×10$^9$ years); this was 10 times better than previous experiments. The clocks rely on 10 000 ytterbium atoms cooled to 10 microkelvin and trapped in an optical lattice. A laser at 578 nm excites the atoms between two of their energy levels. Having established the stability of the clocks, the researchers are studying external influences and evaluating the remaining systematic uncertainties, in the hope that they can bring the clock's accuracy down to the level of its stability. An improved optical lattice clock was described in a 2014 Nature paper.
In 2015, JILA evaluated the absolute frequency uncertainty of a strontium-87 optical lattice clock at 2.1×10$^{−18}$, which corresponds to a measurable gravitational time dilation for an elevation change of 2 cm (0.79 in) on planet Earth that according to JILA/NIST Fellow Jun Ye is "getting really close to being useful for relativistic geodesy". At this frequency uncertainty, this JILA optical lattice clock is expected to neither gain nor lose a second in more than 15 billion years.
In 2017 JILA reported an experimental 3D quantum gas strontium optical lattice clock in which strontium-87 atoms are packed into a tiny three-dimensional (3-D) cube at 1,000 times the density of previous one-dimensional (1-D) clocks, such as the 2015 JILA clock. A comparison between two regions of the same 3D lattice yielded a residual precision of 5×10$^{−19}$ in 1 hour of averaging time. This precision value does not represent the absolute accuracy or precision of the clock, which remain above 1×10$^{−18}$ and 1×10$^{−17}$ respectively. The 3D quantum gas strontium optical lattice clock's centerpiece is an unusual state of matter called a degenerate Fermi gas (a quantum gas for Fermi particles). The experimental data shows the 3D quantum gas clock achieved a residual precision of 3.5×10$^{−19}$ in about two hours. According to Jun Ye, "this represents a significant improvement over any previous demonstrations". Ye further commented "the most important potential of the 3D quantum gas clock is the ability to scale up the atom numbers, which will lead to a huge gain in stability" and "the ability to scale up both the atom number and coherence time will make this new-generation clock qualitatively different from the previous generation".
In 2018, JILA reported the 3D quantum gas clock reached a residual frequency precision of 2.5×10$^{−19}$ over 6 hours. Recently it has been proved that the quantum entanglement can help to further enhance the clock stability.

### 2020s

In 2020 optical clocks were researched for space applications like future generations of global navigation satellite systems (GNSSs) as replacements for microwave based clocks. Ye's strontium-87 clock has not surpassed the aluminum-27 or ytterbium-171 optical clocks in terms of frequency accuracy.
See for a review up to 2020.
In February 2022, scientists at the University of Wisconsin-Madison reported a "multiplexed" optical atomic clock, where individual clocks deviated from each other with an accuracy equivalent to losing a second in 300 billion years. The reported minor deviation is explainable as the concerned clock oscillators are in slightly different environments. These are causing differing reactions to gravity, magnetic fields, or other conditions. This miniaturized clock network approach is novel in that it uses an optical lattice of strontium atoms and a configuration of six clocks that can be used to demonstrate relative stability, fractional uncertainty between clocks and methods for ultra-high-precision comparisons between optical atomic clock ensembles that are located close together in a metrology facility.
In June 2022, National Institute of Information and Communications Technology (NICT) of Japan began using a strontium optical lattice clock to keep Japan Standard Time (JST) by incorporating it into the existing cesium atom clock system and using it to adjust the time signal.
As of 2022, optical clocks are primarily research projects, and less mature than rubidium and caesium microwave standards, which regularly deliver time to the International Bureau of Weights and Measures (BIPM) for establishing International Atomic Time (TAI). As the optical experimental clocks move beyond their microwave counterparts in terms of accuracy and stability performance, this puts them in a position to replace the current standard for time, the caesium fountain clock. In the future this might lead to redefining the caesium microwave-based SI second, and other new dissemination techniques at the highest level of accuracy to transfer clock signals will be required that can be used in both shorter-range and longer-range (frequency) comparisons between better clocks and to explore their fundamental limitations without significantly compromising their performance. The BIPM reported in December 2021 based on the progress of optical standards contributing to TAI the Consultative Committee for Time and Frequency (CCTF) initiated work towards a redefinition of the second expected during the 2030s.
In July 2022, atomic optical clocks based on iodine molecules were demonstrated at-sea on a naval vessel and operated continuously in the Pacific Ocean for 20 days in the Exercise RIMPAC 2022. These technologies originally funded by the U.S. Department of Defense have led to the world's first commercial rackmount optical clock in November 2023.