Atomic Clock: from Navigation to Search for Dark Matter. Interview with Dr. Sc. (Physics and Mathematics) Ksenia Khabarova

The atomic clock that the modern satellite navigation can hardly manage without has a lot of non-obvious applications: from a search for dark matter and a drift of fundamental constants to Earth’s gravitational potential mapping. Head of the Complex Quantum Systems Optics Laboratory of P.N. Lebedev Physical Institute of the Russian Academy of Sciences Ksenia Khabarova has told the Scientific Russia portal about the purpose of the atomic clock and its working principle.

― What is the atomic clock and what is it used for?

― The atomic clock, like any other clock, is mainly used to measure time, because time, like any quantity, needs a standard. We have many different standards that we rely on: the standard of length, the standard of resistance, etc. In the very beginning, people used to measure time by Earth’s rotation around the Sun, by Moon’s rotation around the Earth, by the sunrise and sundown. Later they understood that they did not have to be tied to celestial bodies and that they could use any regular phenomenon to measure time. This led to the advent of the weight clock with an oscillating pendulum or the modern quartz wrist watch where a second is set by a quartz crystal. The atomic clock is a fundamentally new phenomenon of the past century: its name implies that it uses atoms to form a time scale.

― In what way?

― An atom has an electronic structure – a set of levels that electrons can occupy. We can change the state of an electron by giving it specific energy, for example, using a laser – a coherent source of photons. The interaction between the laser and the atoms located in a light trap created by laser beams forms the basis of the optical atomic clock that our laboratory deals with. To create this clock, one needs a long-living transition within an atom from the lower state to the upper state. Such a transition has a very small spectral width. Thus, by transferring an atom from the ground state to the excited state, we can regularly “question” it, i.e. find out whether the frequency of laser radiation fits the frequency of the clock transition in the atom. We can link the laser frequency to the atomic transition frequency to obtain a very accurate and stable optical frequency and form a time scale based on it. The higher the frequency of the radiation we use is, the more accurate and stable clock we can create.

― Could the atomic clock be useful for 3D navigation, airplanes or submarines?

― I would say that gyroscopes and accelerometers are more useful for airplanes, while optical atomic clocks are indispensable for satellite navigation. The atomic clock has one more application that has not been implemented yet, but is very interesting not only to scientists, but also to many world leaders. I mean creating a map of the Earth’s gravitational potential and an opportunity to get oriented using gravitational field lines.

― What is that ?

― Being heterogeneous, our planet is a quite curved ball in terms of gravity, scientifically it is a geoid. It means that we can make kind of a map of its gravitational potential. Our planet has seas, oceans, caves, mountains, etc., so gravity is stronger at some places and weaker at others. Creating such a map of equal gravitational potential lines is one of the most important challenges of the future, as orientation based on gravitational potential is very promising. The gravitational potential is extremely difficult to disturb, change artificially, unlike, for example, the magnetic field that we can transform using strong magnets or electric current. Thus, having a map of the gravitational potential, we can move around the territory without emitting any external signals, i.e. actually become invisible. This, of course, is very important for submarines, they would be happy to use this feature in order not to show their presence. Gravitational potential mapping is important not only for military purposes, but also for geodesy. Scientists look for cavities in the ground or places where rock density will be higher than the mean value for a given area. An atomic clock can help solve this problem too. Taking it along and moving with it around a specific territory, we can see that the clock runs slower where gravity is stronger and that it runs faster where gravity is weaker. Therefore, clock acceleration can tell us that we are approaching a large object that may be invisible to the naked eye.

― But how can we take an atomic clock along when it is so huge that it covers entire rooms?

― You are right: the atomic clock is not small enough to allow us to put it on our wrist and go on an expedition with it. However, there are quite transportable compact systems, for example, our system based on thulium atoms. One can place such a device in a car and travel with it. More likely, the problem is that we need something to compare this clock with rather than the size of this system. The clock itself is of no interest to anyone, as it has to be compared all the time, therefore, one has to be able to compare it with a reference laboratory clock on such trips to register a slowdown or acceleration of a mobile watch in relation to it. Thus, if it suddenly turns out that our transported clock has begun to change frequency vs. the reference clock, we can talk, for example, about some kind of a gravitational anomaly: for example, that here is a cave probably not known to others. However, unfortunately, today the transported atomic clock is not reliable, accurate and stable enough for such studies.

― Where was the most accurate atomic clock in the world first made? And what can you say about accuracy in Russia?

― The atomic clock has to have two important characteristics simultaneously: accuracy and stability. The most accurate and stable atomic clock existing today is based on strontium atoms. Its error and relative instability is 2 × 10^-18. Such characteristics were shown by the clock made in the USA by the Jun Ye’s group. Our domestic clock developed by the Russian Metrological Institute of Technical Physics and Radioengineering (VNIIFTRI) is also based on strontium atoms. It is built into the state standard of time and frequency to make a crucial contribution to coordinated time in Russia. Besides, our country makes a significant contribution to the international time scale – at 17%. 

This area of science also knows other strong groups from different countries of the world, for example, Japan. The team led by Hidetoshi Katori was among the first teams in the world to create a strontium atomic clock. Hidetoshi Katori found a way to remove the light shifts caused by optical grating: he discovered the so-called magic wavelength at which a clock transition does not change to cause a major breakthrough in this field of knowledge. The Japanese group has also made great progress with the cryogenic clocks, which helps suppress shifts in the frequency of a clock transition due to the thermal radiation of the environment. They are also now leading in creating transportable clocks based on strontium atoms: last year, their transportable systems showed an error being only two and a half times worse than that of the laboratory ones. I should also mention our German colleagues: the National Metrology Institute of Germany (PTB) also has its own clock based on strontium. As you can see, strontium has turned out to be a universal and generally accepted element for atomic clocks.

― Why so?

― I cannot unambiguously explain why they started using strontium for atomic clocks. One of the reasons was the good agreement of experimental data. However, this chemical element is actually quite capricious: it has a complex system of levels for cooling. When working with it, one has to use a large amount of laser, it is sensitive to black body radiation, etc.

― This is probably why the Lebedev Institute scientists decided to take a different approach and create the first-ever optical atomic clock using thulium atoms, right? Please tell us about that in more detail. Thulium was first cooled at your institute specifically, wasn’t it?

― Among other things, our laboratory searches for new chemical elements to create optical atomic clocks. You are right, thulium was first cooled in 2010 at our Lebedev Institute, right in this very laboratory. Why hadn’t anyone tried to cool thulium before us? Probably because other groups were busy with strontium, it was a very popular and promising element at that time. Thulium has a complex electronic structure and it is hard to count it. Besides, one needed lasers with a wavelength of about 400 nm, i.e. so-called blue lasers, to cool thulium, but they had not been created by that time. 

The first experiments used lasers with second harmonic generators. However, soon Japanese scientists Isamu Akasaki, Hiroshi Amano and Shuji Nakamura invented blue diodes to trigger progress: there appeared lasers that could be used to cool thulium, and we started actively developing this area. And now, when I show our results at international scientific conferences, thulium attracts much attention among researchers. Many of them take interest in this chemical element, but one needs huge investments to start working with a new element, so many scientists cannot afford such studies.

― Can atomic clocks help us solve the fundamental problems of modern physics?

― At the moment, the world has achieved the error and instability of atomic clocks at the 18th decimal place. This accuracy is so fantastic that it is even unclear where we should move to, and, which is most important, what for? One of the motivators is using atomic clocks to solve fundamental problems, for example, to search for dark matter in the Universe. We know that the Universe has a hidden mass that we cannot detect. One of the ways to come closer to its solution is to try to register changes in the frequency of atomic clocks. Frequency is the most accurate measurable quantity. Thus, for example, a clot of dark matter flies into the Earth and it somehow affects atoms, but we can determine this based on changes in clock frequency. Such studies are already on. There are many different theories to explain how interaction with dark matter affects frequency, whereas different atomic clocks can react to this mysterious mass in different ways, depending on the chemical element the clock is based on. This is one of the most interesting fundamental applications of atomic clocks. The second fundamental focus is a search for the drift of fundamental constants.

― It is difficult to imagine that fundamental constants, such as a charge of an electron, can change.

― Yes, but it is still possible. At least no one has proven yet that it is impossible. For example, the so-called fine-structure constant alpha describing the strength of an interaction between electrons and photons could have a completely different value at the beginning of time, during the Big Bang, than now. We do not know it. As for other fundamental quantities, they can remain unchanged throughout the life of mankind, but what if we look into the future? Unfortunately, we do not have an unambiguous answer either. Atomic clocks can help us in the following way. For example, we can take a thulium clock, a strontium clock, and an ytterbium clock. Next, we have to constantly compare them with each other to capture changes in their frequency correlating with changes in the fine-structure constant alpha ― but this process is different for each clock. Having three clocks of that kind, we can eliminate other causes of frequency changes, identify what is caused by a change in alpha specifically and impose a limit on its drift in this way.

The result we have today is that the alpha does not change at all. Of course, we only limit it, i.e. we determine that it remains unchanged with an experimentally established accuracy. The search for a constant drift is a fundamental search problem being of great interest to scientists. The more accurate our optical atomic clock is, the more efficiently we can register changes in fundamental constants. So far, the recent world experiments show that the fine-structure constant alpha does not change at the level of 10^-15 a year.

― Ksenia Yuryevna, Finally, let us now step back from physics. You are a young female scientist, doctor of sciences, head of a laboratory and a mother of several children. How do you manage to combine all these roles?

― Probably, each person has their own inner engine. I have clearly felt mine since my childhood: I have always been rushing to live and trying not to miss anything. It still seems to me that I will miss something important if I stop even for 15 minutes. As they say, people are divided into cubes and balls. Even when pushed, a cube will roll to the other side and continue lying. However, a ball rolls on its own after a slight touch. I think that this constant inner activity is to some extent naturally inherent in many of us. I believe there is no single way to become motivated and active. Maybe this is not only an innate inclination, but also a matter of habit. I am not used to being idle and cannot stand it – I like working. And science is just the area where you can always solve problems creatively and in a new way. I am free here. I have never had a boss to tell me what to do since the time of my postgraduate studies. They always offer me to solve a problem, and I think on how I can do that, who I have to engage, who I should ask for advice, what books on this topic I should read, etc. A scientist is a creative profession, and I would like to tell all girls that they should not be afraid to deal with science. This is a very interesting and prestigious profession that also gives a chance to see the world.

The interview was conducted under the auspices of the Ministry of Science and Higher Education of the Russian Federation and the Russian Academy of Sciences.