Thought Leaders

Building the World’s Most Accurate Clock - The Nuclear Clock

The nuClock project is a pan-European project that is bringing together the best laser and nuclear physicists, metrologists and solid-state chemists in Europe to build the world's first nuclear clock. If successful, the nuClock they create is expected to be the most accurate clock ever, with the ability to keep to time for over 20 billion years. AZoOptics spoke to the lead laser physicist of the project; Juergen Stuhler, of TOPTICA Photonics, about the physics behind the clock's design, the challenges facing the team and TOPTICA's role in the project.

JW: Please could you tell our readers about the European nuClock Project?

JS: The European nuClock project aims to produce the World’s first ever nuclear clock.

Creating a nuclear clock involves using, for the first time, optical quantum methods to manipulate an atomic nucleus in a way that allows the transitions between energy states to be used by an observer to determine the passage of time.

The project is a joint collaboration that spans many disciplines and the research team consists of nuclear physicists, clock metrologists, solid-state scientists, laser physicists and a laser manufacturing company from three European countries.

The European nuClock project aims to develop the world's first Nuclear Clock

JW: Please could you explain how a nuclear clock works?

JS: Any clock consists of three main components: An oscillator, a clockwork and a display. The oscillator has periodic motion which is converted into the movement of the hands of the clock on the display by the clockwork.

Oscillators are anything that display periodic shifts between two extremes at a defined frequency (rate). Examples of common oscillations are the rotation of the earth, the swinging of a pendulum, or the vibrating quartz crystals found in watches. The frequency of these oscillations can be used to measure the time. For example, the earth rotates every 24 hours so it has a frequency of 11.6 µHz, a 1 m long pendulum swings backwards and forwards every two seconds so it has a frequency of 0.5 Hz. Quartz crystals vibrate rapidly and have frequencies in the MHz range.

The clockwork, which consists of springs and gearwheels in conventional clocks, converts the movement of the oscillator in such a way that the hands of the clock tick every second.

The oscillations of quartz crystals and pendulums, whilst reliable, do show some error over time, which means they occasionally need resetting. With the nuclear clock we aim to produce a clock that will keep to time 10 million times more accurately than a quartz clock, as you can imagine this is no easy task.

The clockwork of conventional clock's converts the vibrations of a quartz crystal into the motion of the second hand on the display.  djgis | Shutterstock

For the nuclear clock we are using an ultraviolet (below 170 nm) laser as the oscillator. The electromagnetic field of the laser oscillates at a frequency of (1.8 × 1015 Hz).

However, as the laser frequency can be prone to drift (which would cause inaccuracies) we have had to develop a novel solution of ensuring only frequencies of 1.8 x 1015 Hz are observed. We plan to achieve this by using a Thorium (229Th) atom.

Thorium-229 has an excited energy state that is around 7.5 eV above it’s ground state, this corresponds to the 1.8 x 1015 Hz laser light that we plan to use as our oscillator. As it is a law of quantum physics that the energy states of atoms have discrete energies that do not change over time the 229Th atom is the perfect way of ensuring the laser light is always at it's required frequency.

When the thorium-229 is irradiated with laser light a transition between the ground state and 7.5 eV excited state will only occur if the laser frequency is at an exact, unknown value that is around 1.8 x 1015 Hz. These electronic transitions are observed, and the results are fed back to the laser which then corrects itself and ensures that the laser frequency never strays, ensuring that the oscillations remains completely stable, allowing highly accurate measurements over extremely long periods of time.

One of the big challenges facing the development of our clock is determining the exact energy of the excited state so we know the exact frequency of laser light to use. Whilst we know the value is close to 1.8 x 1015 Hz we have to know it's exact value to ensure the clock is accurate and functioning correctly.

The oscillations of the laser are converted into a time signal using a frequency comb, which is similar to the way atomic clocks work. This signal is then displayed as a time on an electronic display, or can be used for other measurements.

The NuClock will use an oscillating laser, kept in time by a thorium nucleus, which is converted to a time on a display using a frequency comb.

JW: How does a nuclear clock differ from an atomic clock?

JS: Atomic clocks are well-established tools that are used to measure time very precisely. In atomic clocks the frequency of microwave radiation field is adjusted so it exactly matches a transition between two hyperfine ground states of atoms. Counting the number of microwave oscillations and knowing the atomic transition frequency by prior calibration, theory or even definition (e.g. by the definition of a second) then gives the time.

National institutes all over the world use atomic clocks to measure time and distribute it via radio signal to users in their countries. Many ‘everyday’ technologies rely on accurate clocks, such as GPS, satellite TV and network synchronisation.

Our nuclear clock will not use microwave or optical electronic transitions in atoms. It will use a transition of the nucleus itself that can be created using deep-UV laser radiation. This will allow us to attain the greatest level of accuracy and precision ever achieved.  

Atomic clocks use the electronic transitions of atoms, caused by irradiation with microwaves, to measure the passage of time.

JW: What mechanisms allow nuclear clocks to be so accurate?

JS: The nuclear clock project aims to achieve two major tasks: a higher resolution and a higher accuracy.

The resolution originates from the fact that the energy difference in the nuclear clock project is even higher than in any other optical atomic clock explored so far. A higher energy difference results in more oscillations per second and therefore results in a higher precision.

The accuracy of an atomic clock is influenced by the environment. For example, electrical fields, magnetic fields or thermal radiation can change the energy difference of the contributing electronic states and lead to incorrect measurements of time.

This will not be an issue for the nuclear clock. As the nucleus is surrounded by a shell of electrons it is almost impervious to electronic fields and the nucleus has a magnetic moment that is 1,000 times less that of an atom meaning magnetic fields cause almost no disturbances. These two factors, combined with the low thermal effects of deep-UV radiation, means that nuclear transitions are less sensitive to external effects. This low susceptibility to error from external sources means the nuclear clock is expected to be the most accurate and impervious clock ever created.

As the nucleus in an atom is shielded from thermal effects and magnetic fields by their electron shells the NuClock will be far more resistant to external error. Dabarti CGI | Shutterstock

JW: What challenges are the team facing in the NuClocks development?

JS: In order to achieve good time measurements the preparation of the oscillating device must be achieved. For the nuclear clock this will involve the electromagentic trapping of a specific isotope of Thorium or implanting this isotope into a single UV-transparent crystal.

Thorium is a radioactive element with several isotopes, many of which are radioactive and decay, which will create noise in measurement signals. Preparing only one isotope (229Th) and making sure that the other isotopes do not spoil the measurement is a momentous, and unprecedented, task for nuclear physicists which will require expert skill in ion trapping, crystal growing and implanting techniques.

A second demanding issue we will have to solve is the pre-measurement of the energy of thorium’s excited state. Whilst we know the value of the excited state is around 7.5 eV we do not know it’s exact value well enough to search for the excited state using optical methods.

Assuming one has managed to correctly prepare the thorium in a suitable environment, e.g. implanted in a DUV transparent crystal or an ion trap, one still needs the correct oscillating laser to create the clock. The development of suitable laser sources is another challenge.

Lasers are required to trap thorium atoms making them an essential component in the NuClock's design. Pavel L Photo and Video | Shutterstock

JW: How do the team intend to measure the energy of the thorium excited state?

JS: It is essential for us to measure these values extremely accurately to ensure that the clock operates precisely.

Our first measurements are going to focus on energy measurements of emitted gamma radiation of different decay channels of Thorium. By determining the amount of energy given off when different thorium isotopes decay we will be able to produce an energy level map, allowing us to determine the energy value of the thorium excited state. Achieving this will require a combination of advanced and novel nuclear physics and radiation detection techniques.

Once we have a better idea of the excited states energy we will be able to make a more precise optical measurement. We will irradiate the thorium nucleus with different frequencies of radiation which lie within the possible range of nuclear energies. We will then observe if the radiation is absorbed or if the nucleus fluoresces (emits light) at the excited state’s energy. The laser frequency can be adjusted very precisely using a frequency comb such as TOPTICA’s “DFC-CORE” which is ideally suited for this task.

JW: What are the role of TOPTICA’s lasers in the project?

JS: TOPTICA's role is to develop a cw laser source with a narrow linewidth and a wavelength below 170 nm that can be used to optically excite the nuclear transition if it lies in the addressable wavelength range.

As I mentioned earlier, this is a huge challenge in itself and is essential to produce a fully operating clock. However, with our expertise in the field of laser physics we are confident that we are up to the challenge.

JW: Other than the increased accuracy what are the other benefits of nuclear clocks over conventional technologies?

Increased precision is not only a prerequisite for increased accuracy but it is also a tool in itself for scientific tasks that use frequency changes to measure other effects.

The concept of optical coherent excitation of nuclear transitions might open up new research areas and combine modern atomic physics with nuclear physics. There is potential for this coherent excitation to act as a quantum transition in systems such as quantum computers though this is something that needs to be explored further.

The individual components that have to be developed for the project will each have large impacts of their own. For example, the laser source which we aim to produce could be used as an excellent tool for high precision measurements in the semiconductor industry.

Some of the techniques developed for the nuClock project are expected to have other applications, such as in the development of quantum computers. welcomia | Shutterstock

JW: What are the expected applications of nuclear clocks?

JS: Other than the obvious one; the measurement of the time, there are many other applications the nuClock can be used in.

One of these is it's use in the study of fundamental science. By comparing readings from a nuclear clock and an atomic clock information on fundamental constants such as QCD coupling constants could be determined.

Nuclear clocks will also be very useful for navigation systems such as those found in satellites. Satellites currently use atomic or optical clocks to navigate which are less accurate and occupy more space than our nuclear clock will. Using a nuclear clock will allow the signal strength to be boosted by a significant amount.

Nuclear clocks are expected to be used in next generation satellites. Andrey Armyagov | Shutterstock

JW: Why do you think TOPTICA was chosen to be part of the project?

TOPTICA are experts in narrow linewidth tunable lasers and we are experienced at working with frequency-doubling techniques over a wide range of radiation frequencies; from the near-Infrared and visible light to the violet or ultraviolet wavelength range.

Our lasers are already used worldwide in optical atomic clocks. We have recently demonstrated that similar laser systems with wavelengths as low as 190 nm can be created using subsequent frequency-doubling steps from 760 nm to 380 nm and then to 190 nm. We have some ideas of how to extend the accessible wavelength range to below 170 nm by three times frequency-doubling of 1350-1360 nm amplified tunable diode lasers.

Our company has a long tradition in laser based metrology and high resolution spectroscopy. We have in-house experts who have previously worked for national metrology institutes, European spectroscopy laboratories and university research groups meaning we are a fantastic addition to the European NuClock Project.

JW: Where can our readers find out more about the NuClock project and TOPTICA Photonics?

JS: The nuClock homepage gives a description of the project, the involved partners and friends of the Nuclock.

The TOPTICA website provides detailed information about our unique laser systems for scientific research and industrial use as well as their major applications. Our scientific applications section also contains information on laser cooling, metrology and optical clocks.

About Dr. Jürgen Stuhler

After receiving his phd in experimental physics from the University of Konstanz/Germany in 2001, Dr. Jürgen Stuhler spent two years as researcher at the European Laboratory for Non-Linear Spectroscopy (LENS) in Florence/Italy.

Following this Jürgen spent three years as a lecturer and atom optics group leader at the University of Stuttgart, Germany. For ten years he has worked for TOPTICA Photonics AG in different functions such as (Senior) Sales Manager Scientific Lasers, Product Manager Frequency-converted Lasers and is now the Senior Director of Quantum Technologies.

He has published ~30 articles in peer-reviewed journals. In the nuclock project, he serves as the work package leader for laser systems.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

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