Optical atomic clocks are considered the future of atomic clocks. An oscillation located in the visible spectrum of light servers as their ‘pendulum’, i.e., the periodic oscillation that is at the very heart of every clock. Working at 10 000 to 100 000 times the frequency of microwave radiation used by caesium atomic clocks, they offer significant improvement of accuracy, stability, and reliability
In an optical lattice clock, strontium (or similar) atoms are confined to the interference pattern of two laser beams. In this so-called ‘optical lattice’ the oscillatory frequency of the atomic ‘pendulum’, which is the absorption frequency of the atoms, can be determined extremely accurately – allowing it to be measured to the 18th decimal place at present.
Figure 1: Schematic view of an optical lattice. Atoms are trapped in the intensity maxima of the standing wave forming the optical lattice. At the ‘magic wavelength’ both states of the clock transition experience the same energy shift by the trapping light, allowing observation of the undisturbed transition.
In the International System of Units, the oscillatory frequency of the isotope 87Sr has already been adopted as a ‘secondary representation’ of the unit of time, and its value, ν0 = 429 228 004 229 873.2 Hz, is known with an uncertainty of 0.22 Hz – limited solely by the primary caesium clocks that realize the unit of time. In fact, this means that optical clocks must be compared directly among each other in order to fully leverage their accuracy.
The working group develops and operates two optical lattice clocks based on strontium atoms. At the first of these clocks, the central goals are to continuously improve the accuracy and stability of strontium clocks as well as to implement new technology and novel techniques. In contrast to this laboratory-scale clock, which is stationary and can only be operated at PTB, the second clock is transportable and allows measurement campaigns at various locations, e.g., at other research institutes. Designing the clock we aimed for achieving sufficient improvement of compactness and robustness to allow for convenient transporting of the clock while retaining the accuracy and stability typical of a lab-sized optical atomic clock to the largest possible extent.
Each of our clocks is involved in numerous collaborative research projects and used for a broad range of research applications. For instance, comparisons of diverse as well as remote optical clocks are both essential steps towards a re-definition of the ‘second’. We contribute to such comparisons with our strontium clocks, in close cooperation with other working groups (e.g., 4.34, 4.41, 4.42, and 4.43) and the QUEST institute at PTB as well as other partners from Europe and beyond. Furthermore, optical clocks are now used to address the ‘grand questions’ of modern physics, such as a unified theory of general relativity and quantum physics, and in other applications like relativistic geodesy, where relativistic effects are used to explore gravitational fields.
As a member of various research projects, our working group works with partners and fellow researchers at PTB, Leibniz University Hannover, their joint Centre for Quantum and Space Time Research, the European Space Agency, from industry as well as other national metrology and research institutes worldwide.
Relativistic Geodesy and Gravimetry with Quantum Sensors
Within the joint Collaborative Research Cluster 1128 Relativistic Geodesy and Gravimetry with Quantum Sensors (geo-Q) of PTB, Leibniz University Hannover and other partners, we investigate novel methods of using quantum metrology for the investigation of gravitational fields. Here, atomic clocks and their recent progress offer fascinating opportunities for earth exploration by exploiting general relativity.
Fundamentals and Applications of Ultra-cold Matter
The Research Training Group 1729 consists of research groups at PTB and Leibniz University Hannover from the fields of metrology, quantum optics, atomic and molecular physics. Research within the training group is focussed on the fundamental properties and applications of ultracold gases, and some of its main goals are to involve young researchers in research at the forefront of their fields and to provide them with excellent training opportunities through courses and lectures given by renowned international guests and further a broad as well as deep understanding of the fields of research.
International Timescales with Optical Clocks
As a member of the project ITOC of the European Metrology Research Programme (EMRP), we investigate the requirements that have to be met in order to realize a future SI base unit ‘second’ with optical clocks. Therefore, we perform extensive comparisons of optical clocks and work on the development of transportable clocks to serve as so-called ‘transfer standards’.
Quantum Engineered States for Optical Clocks and Atomic Sensors
QESOCAS is another project within the EMRP, which aims at employing advanced quantum technologies to enhance the performance of optical clocks. Close collaboration with university research groups that are leading in the field of entanglement of atomic ensembles and its use for spectroscopic purposes is at its core.
Space Optical Clocks 2
In SOC2 we develop a highly compact and energy-efficient strontium lattice clock as part of a large European consortium. In doing so we are advancing the technology of optical clocks towards space readiness. SOC is also a so-called ‘candidate mission’ for the International Space Station (ISS), competing with other candidates missions for bringing the clock onto the ISS.
Optical Clocks 18
Building on our work from the ITOC project, the OC18 project of the European Metrology Programme for Innovation and Research (EMPIR) will be focussed on reducing the fractional uncertainties of optical clocks into the low 10-18 regime. In particular, our working group is going to build and operate a cryogenic strontium lattice clock, wherein the leading frequency shift in room temperature clocks, which arises from the atoms’ interaction with the thermal radiation field of their environment, is nearly eliminated.
Initial Training Network FACT
In the Initial Training Network Future Atomic Clock Technology, our partners from the research and industry sectors and we work on advancing optical clock technologies towards commercial use and future applications. Providing training and networking opportunities to early stage researchers is among the central means to achieve these goals.
Bachelor/Master/Doctoral ThesesBachelor/Master/Doctoral Theses
A list of the doctoral theses completed in our working group can be found on our publications page. If you are interested in a doctoral or master’s thesis work in our group, please submit an application.
By performing a highly accurate evaluation of the frequency shift of the clock transition due to the ambient black-body radiation (BBR) field, we have been able to reduce the uncertainty of the strontium lattice clock at PTB to few parts in 1017. (see also publications). Today, the results are used by all groups operating strontium lattice clocks to determine the BBR-induced uncertainties of their clocks. We are currently working on further reducing the major uncertainty contributions by evaluating and controlling frequency-changing effects in the clock.
Comparing our frequency standard to the primary standards at PTB and to other optical clock enables us to investigate fundamental questions, such as potential variations of constants.
Since optical clocks have surpassed primary clocks in accuracy, they need to be compared directly among each other to exploit their full accuracy. At present, remote clock comparisons can be performed either via fibre links or using a transportable clock as a transfer standard.
Beyond their immediate applications in metrology, the development of transportable optical clock is of great interest for fundamental research using clocks in space, and we plan to use our transportable clock for applications in geodesy, where frequency difference of two clocks located at different heights, i.e., the gravitational red shift, is measured. We aim for height resolution below 10 cm, which corresponds to a fractional frequency difference below 1 part in 1017, in the long run.