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If you think of passing hours, minutes and seconds when you imagine a clock, you’re not wrong, but not completely right, either. When clocks measure time very accurately, scientists can do much more than just state the time:

  • Navigation: Atomic clocks are used in satellite navigation. To locate a person or an object, the running times of the exchanged signals are analyzed. The rule of thumb: The more precisely clocks tick, the more precisely things can be successfully located.
  • Measuring the Earth’s gravitational field: Atomic clocks are highly sensitive to their environment. According to Einstein’s theory of relativity, local gravitational forces influence the passage of time and thus the rate of clocks. That is why an atomic clock ticks differently at sea level than on a mountain, for example. Today’s best clocks are already able to perceive altitude differences of just a few centimeters.
  • The search for “new physics”:  Our world is the way it is because the natural constants are what they are. If the natural constants changed, our world would change too. That leads to a fundamental question: Are the natural constants really constant? As natural constants also play a role in atomic clocks and influence the measure of time, scientists are trying to find out with the help of such clocks whether the natural constants undergo changes.

People who research atomic clocks are aware of the clocks’ fields of application: satellite navigation, geodesy, communication technology or in fundamental research.

Timely: Excited atom (glowing) in an optical clock
Timely: Excited atom (glowing) in an
optical clock

For decades, PTB has been acquiring expertise on the construction and operation of atomic clocks and is among the world’s most famous timekeepers. The fact that PTB provides the national time in Germany is just a small, though important task. The even greater task is developing the clocks of tomorrow. What are known as “optical clocks”, which use frequencies in the visible spectral range rather than in the microwave range, are the definitive step toward the next generation of clocks which work, for example, with single or an ensemble of neutral or charged atoms (ions).

With these optical clocks, the measure of time will be raised to a new level of precision from which all the practical applications which can be expected will greatly benefit – from altitude measurement in geodesy to the synchronization of networks with high-precision frequency standards. For such practical applications, however, the highly sensitive technology which tames the underlying quantum states much be transferred from a well-protected and painstakingly stabilized fundamental research laboratory to the rugged environment of practical applications, for example, to the free field in geodesy (PTB’s Sr lattice clock is already used there) or for use as a frequency standard in a server room. For this purpose, the first (and still the only) user-friendly, robust and near-commercial optical clock has been realized at PTB in the “opticlock” project in a consortium with partners from industry and academia.

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Measuring time and frequency with great accuracy is paving the way for far-reaching fields of application in areas such as communications, navigation and geodesy. PTB is investigating and developing novel optical atomic clocks using different, complementary approaches. Within the scope of the pilot project on "Optical single-ion clocks for end-users" (opticlock) funded by the BMBF, a demonstrator for a commercial optical clock is currently being developed together with partners from industry and research. To be able to exploit the potential of new optical atomic clocks to the full, the frequency must be transmitted with utmost accuracy. Such an accurate frequency dissemination is also being developed at PTB, which is among the world leaders in this area.

Stronlgy focused laser beams allow several thousands of neutral atoms to be trapped at the same time. The optical lattice's special geometry is particularly well suited for investigating atomic reference transitions, e.g. in strontium atoms. Since all trapped atoms can be interrogated at the same time, an optical clock based on this principle has a particularly high signal-to-noise ratio and allows high resolution in a short averaging period. Besides the laboratory setup, we are also operating the first portable lattice clock. Both setups have been used for geodetic studies to measure the difference in height between two clocks by exploiting the relativistic red shift from the optical path difference between the two clocks connected with each other via an optical fiber link.

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The ytterbium ion can easily be cooled with diode lasers and stored in radio-frequency traps for a long time. In contrast to other ions currently under investigation, 171Yb+ provides two reference transitions suitable for an optical clock. In addition to an electric quadrupole transition, an electric octupole transition connects the ground state and the first excited state. The two transitions differ both in their sensitivity to external electric and magnetic fields and in the optimal excitation pulse duration. Utilization of both transitions of a single trapped ion permits the realization of one of the world's most accurate optical clocks. Besides the use as a high-accuracy frequency standard, the electronic structure of the ytterbium ion enables sensitive tests of the equivalence principle and searches for so-called “new physics”.

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The quantum logic clock allows an ion which is particularly well suited as a "clock ion" to be cooled and read out by means of a "logic ion" that can efficiently and easily be manipulated and controlled. This facilitates the flexible selection of the clock ion that does not need to be directly laser-cooled or read out. The clock ion we use is Al+, which features an extremely low sensitivity to changes in external fields and thus enables the world's most accurate clocks. Applications range from fundamental physics to relativistic geodesy.

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Optical ion clocks profit from the highly accurate control of a single trapped ion, which can be positioned and studied with nm precision. This leads to a favorably low systematic uncertainty. However, the state information read out from a single quantum absorber is limited and leads to a intrinsically low stability of the clock requiring long integration times. Precision spectroscopy of Coulomb crystals offers the new possibility to realize a stable ion frequency standard. This way, relative frequency uncertainties in the 10-18 range can be obtained in a fraction of the time required by today's ion clocks. Thus, the multi-ion approach can significantly improve ion clocks and their application in tests of fundamental physics and also as sensors for the gravitational potential. 

As suitable candidates for a new optical frequency standard ensembles of Yb+ and In+ are investigated. For this purpose, a Opens external link in new windownew scalable trap technology was developed, allowing to store ion ensembles with a high level of control. This breakthrough facilitates the use of ion crystals for time and frequency metrology. 

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Within the pilot project "Optical single-ion clocks for end-users" (opticlock) funded by the BMBF, which is managed jointly by TOPTICA and PTB, the development of a demonstrator for a commercial optical clock is pursued. This project is a prime example of the application potential of quantum technologies; furthermore, it will be the first optical clock developed with the participation of industry worldwide whose frequency properties are better by one order of magnitude than the clocks and frequency references currently available.

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Applications such as optical spectroscopy with extremely high resolution and for stable interrogation lasers in optical atomic clocks require laser radiation of the highest possible stability and lowest possible linewidth. For this purpose, we stabilize the lasers' frequency onto ultrastable optical resonators of special glass or monocrystalline materials, which were developed at PTB. Developing particularly robust and portable optical resonators also allows this technology to be used in applications outside a well-controlled laboratory environment.

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