Optical clock. An optical clock is a clock that uses light to track time. It differs from an atomic clock in that it uses visible light, rather than microwaves. Several chemical elements have been studied for possible use in optical clocks. These include aluminum, mercury, strontium, indium, magnesium, calcium, ytterbium, and thorium. The concept of an optical clock originated with John L. Hall and Theodor W. Hansch, who together won the 2005 Nobel Prize in Physics.

Overview 
The development of femtosecond frequency combs, optical lattices has led to a new generation of atomic clocks. These clocks are based on atomic transitions that emit visible light instead of microwaves. A major obstacle to developing an optical clock is the difficulty of directly measuring optical frequencies. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs. 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. With the refinement of the frequency comb, these measurements have become much more accessible and numerous optical clock systems are now being developed around the world.

Spectroscopy 
As in the radio range, absorption spectroscopy is used to stabilize an oscillator—in this case, a laser. When the optical frequency is divided down into a countable radio frequency using a femtosecond comb, the bandwidth of the phase noise is also divided by that factor. Although the bandwidth of laser phase noise is generally greater than stable microwave sources, after division it is less.

Systems used 
The primary systems under consideration for use in optical frequency standards are:
 single ions isolated in an ion trap;
 neutral atoms trapped in an optical lattice and
 atoms packed in a three-dimensional quantum gas optical lattice.

These techniques allow the atoms or ions to be highly isolated from external perturbations, thus producing an extremely stable frequency reference. Lasers and magneto-optical traps are used to cool the atoms for improved precision.

Atoms used 

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

20th century 

The idea of trapping atoms in an optical lattice using lasers was proposed by Russian physicist Vladilen Letokhov in the 1960s.

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 development of the first optical clock was started at the National Institute of Standards and Technology in 2000 and finished in 2006.

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  of strontium-87 were able to stay in synchrony with each other at a precision of at least , 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  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 (); this was  better than previous experiments. The clocks rely on  atoms cooled to  and trapped in an optical lattice. A laser at  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 , which corresponds to a measurable gravitational time dilation for an elevation change of  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  in 1 hour of averaging time. This precision value does not represent the absolute accuracy or precision of the clock, which remain above  and  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  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  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.

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.

References 

Clocks