optical clocks (original) (raw)

Definition: time measurement devices based on optical frequency standards

Alternative term: optical atomic clock

Categories: article belongs to category photonic devices photonic devices, article belongs to category optical metrology optical metrology

optical metrology
- frequency metrology
  * optical frequency standards
  * optical clocks
  * optical clockworks

Page views in 12 months: 1234

DOI: 10.61835/174 Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

📦 For purchasing optical clocks, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

What are Optical Clocks?

An optical clock is a clock the output of which is derived from an optical frequency standard. As explained in the article on optical frequency standards, such a reference is based on atoms or ions which are kept in an optical trap and subject to laser cooling to suppress Doppler broadening. Their transition frequency is probed with a frequency-stabilized laser, the emission frequency of which is precisely locked to the atomic transition. That ultrastable optical frequency is far too high for electronic counting of oscillation cycles. However, it can be precisely related to lower (microwave) frequencies via some kind of optical clockwork, which is nowadays normally based on a frequency comb, as explained below. The obtained relation between the optical and microwave frequencies is highly accurate, normally not allowing for any phase slips.

An optical clock can offer an extremely high frequency precision and stability, far exceeding the performance of the best cesium atomic clocks. As its output, one may use the stabilized microwave frequency, the stable laser frequency or any of the spectral lines of the generated frequency comb. All those frequencies are highly stable, but the optical frequencies are more useful in the sense that precise frequency comparisons can be done within much shorter measurement times for such high frequencies.

Modern Optical Clockworks

In the early years of optical clocks, a severe challenge was to relate the stable optical frequency to a microwave frequency standard such as a cesium atomic clock: the required optical clockworks, at that time realized as frequency chains, were very difficult to make, and were applicable only to certain isolated optical frequencies. From 1999 on, however, very much simpler and much more versatile while equally precise optical clockworks have been realized on the basis of frequency combs from femtosecond mode-locked lasers [2].

setup of optical clock

Figure 1: Schematic setup of an optical clock.

A time signal is generated by a cesium clock. An optical frequency standard is used for reducing the long-term drift of the cesium clock. The frequency comparison is done using an optical clockwork. That clockwork may also provide an optical output, e.g. in the form of a frequency comb, allowing frequency comparison with other optical standards.

Comparison with Microwave Frequency Standards

Optical clocks are of interest not only for measuring optical frequencies, but also for general timekeeping where ultimate precision is required. Compared with microwave standards such as cesium atomic clocks, they have the following key advantages:

There are certain atoms and ions with extremely well-defined clock transitions (forbidden transitions) which promise higher accuracy and stability than the best microwave atomic clocks. The anticipated (but not yet demonstrated) relative frequency uncertainty of atomic optical clocks using electronic transitions is of the order of 10−18 for long enough averaging times (possibly a few days). It may even become possible to use certain low-energy nuclear transitions to get to 10−19 level [11].
The high optical frequencies themselves are of high importance because these allow precise clock comparisons within much shorter times. For example, a 10−15 precision can be achieved in a few seconds if the compared frequencies are in the optical range (hundreds of terahertz), whereas the order of a full day would be required with microwave clocks.
Optical signals can be easily transported over long distances using optical fibers. It is even possible to obtain a very precise time comparison between different clocks at different positions, using optical-frequency transfer over fiber-optic links [12]. Compared with fibers, microwave cables are more expensive and have much higher losses, and are therefore technically much more limited.

Redefining the Second

Because of the advantages of optical clocks in terms of accuracy, speed of comparison and remote synchronization, it is to be expected that in the not too far future the cesium clock as the fundamental timing reference will be replaced by an optical clock. This will effectively mean that the second as a fundamental SI unit will be redefined based on such a clock, then referring to an optical frequency rather than to a microwave frequency.

However, it is so far not clear which type of optical clock would be used as such a standard. A lattice clock [8] appears to be a good candidate, but the best choice of a particular clock atom is not obvious. Many criteria need to be considered. One of those is that the accuracy should be improved substantially (probably by about two orders of magnitude), but others are related to various practical aspects.

Even after that profound change, cesium clocks (and other non-optical atomic clocks, such as rubidium clocks) will continue to play an important role in technological applications as they can be simpler and more compact than optical clocks.

Your browser does not support the video tag. However, you can download the video file.

Video 1: This video on optical clocks has been provided by Menlo Systems.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is an optical clock?

An optical clock is a clock whose timekeeping is derived from an optical frequency standard. This standard is based on the extremely stable transition frequency of atoms or ions, which is probed by a frequency-stabilized laser.

How does an optical clock work?

It uses a laser whose frequency is precisely locked to a stable atomic or ionic transition. Since this optical frequency is too high to be counted electronically, an 'optical clockwork', usually a frequency comb, is used to precisely link it to a countable microwave frequency.

What are the main advantages of optical clocks over traditional atomic clocks?

Optical clocks offer higher potential accuracy and stability. Their high frequencies also allow for much faster high-precision comparisons, and their optical signals can be easily transmitted over long distances using fiber-optic links.

Will optical clocks lead to a redefinition of the second?

Yes, it is expected that due to their superior accuracy, the SI unit of the second will eventually be redefined based on an optical clock, referencing a specific optical frequency rather than the current microwave frequency of cesium atoms.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains two suppliers for optical clocks. Among them:

⚙ hardware

Mechanical clocks were the most accurate timekeepers for centuries achieving typical accuracies of seconds per day (10−4) and record values of seconds per year (10−7). Modern optical atomic clocks are accurate to seconds in the age of the universe (3 · 10−18) and can be compared using TOPTICA’s Difference Frequency Comb (DFC).

The complete stabilized laser system including the DFC CORE, any desired wavelength extension, beat unit, stabilization electronics, wavelength meter, counter, and lasers is now available from one source. Any of TOPTICA’s tunable diode lasers with a wavelength between 190 nm and 2200 nm can be locked to the DFC, lasers with shorter wavelengths can be stabilized using the fundamental of their SHG unit. The complete laser system is controlled from a single GUI.

⚙ hardware

Menlo Systems offers a complete CW laser system with an optically referenced frequency comb, the FC1500-Quantum, for experiments with cold atoms or ions, such as optical (lattice) clocks or quantum simulation or quantum computing. The rack-mounted system consists of a cavity-stabilized laser for sub-Hz linewidth, an optical frequency comb in the visible and infrared, and several customizable cw lasers. The comb transfers the narrow linewidth and stability throughout the entire spectrum and to the cw lasers, which are locked to the comb for all required electronic transitions: cooling, repumping, and clock transitions.

Bibliography

[1]	N. V. Goldovskaya et al., “Possibility of establishment of a quantum frequency standard for the visible range using an intercombination spectral transition in the ytterbium atom”, Sov. J. Quantum Electron. 12 (12), 1659 (1982); doi:10.1070/QE1982v012n12ABEH006318
[2]	S. A. Diddams et al., “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb”, Phys. Rev. Lett. 82 (18), 3568 (1999); doi:10.1103/PhysRevLett.82.3568
[3]	S. A. Diddams et al., “An optical clock based on a single trapped 199Hg+ ion”, Science 293, 825 (2001); doi:10.1126/science.1061171
[4]	R. Holzwarth et al., “Optical clockworks and the measurement of laser frequencies with a mode-locked frequency comb”, IEEE J. Quantum Electron. 37 (12), 1493 (2001); doi:10.1109/3.970894
[5]	T. Udem et al., “Optical frequency metrology”, Nature 416, 233 (2002); doi:10.1038/416233a
[6]	L.-S. Ma et al., “Optical frequency synthesis and comparison with uncertainty at the 10−19 level”, Science 303, 1843 (2004); doi:10.1126/science.1095092
[7]	S. A. Diddams et al., “Standards of time and frequency at the outset of the 21st century”, Science 306, 1318 (2004); doi:10.1126/science.1102330
[8]	M. Takamoto et al., “An optical lattice clock”, Nature 435, 321 (2005); doi:10.1038/nature03541
[9]	A. D. Ludlow et al., “Sr lattice clock at 1 × 10−16 fractional uncertainty by remote optical evaluation with a Ca clock”, Science Express Feb. 14, 2008; doi:10.1126/science.1153341
[10]	C. W. Chou et al., “Frequency comparison of two high-accuracy Al+ optical clocks”, Phys. Rev. Lett. 104 (7), 070802 (2010); doi:10.1103/PhysRevLett.104.070802
[11]	C. J. Campbell et al., “Single-ion nuclear clock for metrology at the 19th decimal place”, Phys. Rev. Lett. 108 (12), 120802 (2012); doi:10.1103/PhysRevLett.108.120802
[12]	S. Droste et al., “Optical-frequency transfer over a singe-span 1840 km fiber link”, Phys. Rev. Lett. 111 (11), 110801 (2013); doi:10.1103/PhysRevLett.111.110801
[13]	B. J. Bloom et al., “A new generation of atomic clocks: accuracy and stability at the 10−18 level”, http://arxiv.org/abs/1309.1137 (2013)
[14]	A. D. Ludlow et al., “Optical atomic clocks”, arXiv:1407.3493v2
[15]	F. Riehle, “Optical clock networks”, Nature Photon. 11, 25 (2017); doi:10.1038/nphoton.2016.235
[16]	W. F. McGrew et al., “Towards the optical second: verifying optical clocks at the SI limit”, Optica 6 (4), 448 (2019); doi:10.1364/OPTICA.6.000448
[17]	S. Herbers et al., “Transportable clock laser system with an instability of 1.6 × 10−16”, Opt. Lett. 47 (20), 5441 (2022); doi:10.1364/OL.470984

(Suggest additional literature!)

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.