“Magnetometers and their applications including fundamental physics”
In this accessible talk (that will cover a wide range of topics), I will start by briefly reviewing optical magnetometry with atoms and color centers, and then move on to describe two new projects that are presently being setup to attempt detection of dark-matter and dark-energy constituents: the Cosmic Axion Spin Precession experiment and the Global Network of Optical Magnetometers for Exotic-physics searches. These are table-top-style experiments based on nuclear magnetic resonance and modern atomic sensors.
"Microwave Electrometry and Coherent Control of Strongly Interacting Rydberg Gases in Thermal Vapor Cells"
Rydberg atoms have unique properties like their large polarizability and their longrange interactions. Their sensitivity to AC/DC electric fields makes them very promising for sensing applications, whereas the excitation blockade can be employed as an optical non-linearity on the single photon level. Quantum based standards of length and time as well as measurements of other useful physical quantities, ex. magnetic fields, are important because quantum systems, like atoms, show clear advantages for providing stable and uniform measurements.
We demonstrate a new method for measuring microwave electric fields based on quantum interference in a Rubidium atom. Using a bright resonance prepared within an electromagnetically induced transparency window we are able to achieve a sensitivity of 30 μVcm-1Hz-1/2 with a modest setup . This method can be used for vector electrometry with a precision below 1∘  and microwave field imaging with a sub-wavelength resolution . The excitation of Rydberg atoms within Hollow-Core Photonic Crystal Fiber paves the way towards integrated sensors based on Rydberg atoms .
Furthermore we present our progress on the coherent control and investigation of Rydberg atoms in small vapor cells. We show that we are able to drive Rabi oscillations on the nanosecond timescale to a Rydberg state by using a pulsed laser excitation and are therefore faster than the coherence time limitation given by the Doppler width .
A systematic investigation reveals a clear signature for van der Waals interaction between Rydberg atoms which is the basis for quantum devices based on the Rydberg blockade. The strength of the interaction exceeds the energy scale of thermal motion (i.e. the Doppler broadening) and therefore enables strong quantum correlations above room temperature . Due to this strong interaction we observe evidence for aggregate formation of Rydberg atoms .
So in short, miniaturization and integration of Rydberg based devices and sensors is within reach.
 J. Sedlacek, et al. Nature Physics 8, 819 (2012)
2] J. Sedlacek, et al. Phys. Rev. Lett. 111, 063001 (2013)
 H.Q. Fan, et al. Opt. Lett. 39, 3030 (2014)
 G. Epple, et al, Nature comm. 5, 4132 (2014)
 B. Huber et al., Phys. Rev. Lett. 107, 243001 (2011)
 T. Baluktsian et al., Phys. Rev. Lett. 110, 123001 (2013)
 A. Urvoy et al., Phys. rev. Lett. 114, 203002 (2015)
Time and frequency are the most accurately measurable quantities today. In particular, optical clocks, that nowadays can reach a relative frequency inaccuracy as low as 10-18, will open up a new field of search for deviations in the predictions of Einstein’s general relativity, tests of modern unifying theories and the development of new sensors for gravity and navigation.
In my talk, I will introduce the concepts of optical ion clocks and the application of portable devices for relativistic geodesy. The international state-of-the-art in ion clock development and challenges in clock laser stabilization and ion control to go beyond resolutions of 10-19 will be discussed.
In order to exploit their full potential and to resolve frequencies with a fractional frequency instability of 10-18and below, optical ion clocks need to integrate over many days to weeks. For the characterisation of the clock, as well as for applications, such as relativistic geodesy, the long averaging times pose severe limits. Scaling up the number of ions for optical clock spectroscopy is a natural way to significantly reduce the integration time, but was hindered so far by the poor control of the dynamics of coupled many body systems. In our experiment, we implement linear Coulomb crystals of Yb+and In+ ions for a first evaluation for optical clock operation. For optimal control of the ion motion segmented chip-based ion traps are engineered in the clean room facilities of PTB.
"Optical clocks: towards a redefinition of the second?"
Optical clocks have reached a level of accuracy and stability that has now considerably surpassed the traditional microwave clocks realizing the SI second. A stunning illustration: the level of performance is such that knowing the altitude difference between two clocks with a resolution of 1 cm over continental distances will soon be necessary.
This field of research has led to a growing architecture of optical clocks in the world, based either on ions or on neutral atoms. In order to provide means to compare them, new methods are being developed and implemented. An optical fiber links network is presently spreading throughout Europe, with the objective to ascertain that independent technological developments lead to an agreement between the different groups.
Are we ready for a new definition of the second? In this presentation I will describe the challenges connected to the quest of a “reference” frequency, which would be free of any systematic perturbing effect and reproducible in the long run. I will present how a frequency chain works in practice and what would the advent of an “optical second” imply.