Toward an optical atomic clock: Physicists develop atomic frequency standard for one of world’s most precise clocks

The construction of a Polish optical atomic clock, a device of precision rarely matched in the world, will soon enter its final phase. A group led by Prof. Wojciech Gawlik at the Department of Photonics, Institute of Physics, Jagiellonian University has just constructed the last of the key components required for the construction of the clock: an atomic standard based on cold strontium atoms. After the conclusion of a series of tests, the device will be transported to the National Laboratory for Atomic, Molecular and Optical Physics (KL FAMO) in Toruń, where physicists will combine it with the two remaining components: an optical comb developed by a group led by Prof. Czesław Radzewicz from the University of Warsaw (UW) and an ultra-precise laser built under the supervision of Roman Ciuryłło, PhD, at the Nicolaus Copernicus University (UMK).

Precise time measurements are of key importance to the effective functioning of our civilization. Without them, modern telecommunication and navigation systems, particularly satellite systems, would cease to function. Ultra-precise clocks are also indispensable for research on the fundamental properties of reality, among others, for investigating whether the values of physical constants are truly constant and investigating the extent to which the general theory of relativity provides an accurate description of the Universe.

The Polish optical atomic clock construction project, financed solely by the Ministry of Science and Higher Education, was started two years ago and is conducted by groups of physicists from all over Poland, currently collaborating within the framework of KL FAMO which is a part of the National Laboratory for Quantum Technologies. The theoretical precision of time measurements of the clock under construction will be two orders of magnitude greater than that of the most precise cesium clocks, which are currently used, among others, to define the standard second. "The Polish clock will have an accuracy of one second in a few dozen billion years, which is a period several times longer than that which elapsed from the Big Bang. Such precise timekeeping devices can nowadays be found only in a handful of research centres in the world," says Prof. Wojciech Gawlik, head of the team of physicists from Cracow.

All clocks make use of a certain frequency standard, a periodic physical phenomenon. In the case of a wristwatch, the standard is a quartz resonator with an oscillating quartz crystal. Commonly used atomic clocks make use of an electronic transition between energy levels in cesium atoms. The physicists from Cracow, on the other hand, have constructed a standard based on strontium atoms, in which electronic transitions between atomic energy levels require absorption and emission of electromagnetic radiation of a much higher frequency than in cesium. The frequency lies in the optical range (hence the adjective "optical" in the name of the clock). Confined in a laser trap, strontium atoms are isolated from the surroundings and cooled with a laser to an extremely low temperature of the order of microkelvins. Under these conditions, the probability of atomic collisions is low, which greatly reduces the possibility of disturbance. The new standard is currently undergoing the first tests.

The frequency standard based on strontium atoms is one of the components of the optical atomic clock. It will be used to stabilize the frequency of the ultra-precise laser built in Toruń. It is precisely the vibrations of the electric field of a light beam emitted by the laser that will be counted as elementary units of time, recurring with great precision. Yet the laser operates with such a high frequency that counting the individual oscillations is beyond the capabilities of electronic systems. What is needed is a device which acts as a toothed gear. The device in question is a frequency comb -- a set of numerous light waves of narrow, equidistant frequencies. The comb, generated by a laser emitting ultra-short light pulses, allows for a synchronic and error-free transfer of atomic standard oscillations into radio wave frequency range -- radio waves can be electronically counted. The frequency comb has already been operated by scientists from the Ultrafast Phenomena Laboratory of the Institute of Experimental Physics, University of Warsaw and preliminarily combined with a commercial reference laser of a stabilized frequency of light. Works are currently underway to combine it with the ultra-precise laser built at the National Laboratory FAMO in Toruń.

"Our atomic standard based on strontium atoms is the third, final piece of the puzzle. In several months' time, after the tests and after transporting it to Toruń, we will be able to start putting the clock together," observes Prof. Gawlik.

The National Laboratory for Quantum Technologies (nltk.fuw.edu.pl) is a consortium of leading Polish scientific institutions conducting research in the field of quantum technologies, including quantum computing, quantum engineering and related fields. The NLTK consists of: the University of Warsaw, the Wrocław University of Technology, the Institute of Physics of the Polish Academy of Sciences, the Nicolaus Copernicus University in Toruń, the Jagiellonian University, the University of Gdańsk, the University of Łódź, and the Center for Theoretical Physics of the Polish Academy of Sciences. A project of the same name is carried out in five of the eight institutions of the NLTK consortium. The aim of the project is to create and equip the member scientific institutions with equipment necessary for conducting world-class joint scientific research as well as research and development. The National Laboratory for Quantum Technologies Project is co-financed by the European Regional Development Fund under the Operational Programme Innovative Economy 2007-2013, Priority 2, Infrastructure of the R&D sphere, Action 2.2 "Support for the creation of joint research infrastructure of scientific institutions."