News from the lab

Quantum logic assisted state detection for molecular ions

High precision spectroscopy of molecular ions is still an outstanding goal that would enable tests of fundamental physics, such as searches for an electric dipole moment of the electron or probes for a variation of fundamental constants. One major problem limiting the achievable resolution was the lack of state detection techniques that allow efficient state readout without destroying the molecule.

Recently, we demonstrated for the first time a non-destructive state detection for a single molecular ions confined in a Paul trap. The result has now been published in Nature.

For further information see:

Journals homepage:

PTB press release:(english), (deutsch)


Figure: This is the basic concept of the experiment: MgH+ (orange) and Mg+ (green) are trapped together in a linear ion trap. The two-ion compound is cooled to the motional ground state via the atomic ion. An oscillating dipole force changes the motional state according to the rotational state of the molecular ion. This motional excitation can be detected on the atomic ion.

Isotope shift measurement

Journals homepage:

We have used photon recoil spectroscopy to measure the isotope shifts of the 2S1/22P1/2 and 2D3/22P1/2 transition for 42Ca+, 44Ca+, and 48Ca+ relative to 40Ca+ with sub-100kHz accuracy. The achieved accuracy exceeds previous measurement by an order of magnitude (see figure) and allows us to extract properties of the Ca nucleus, such as the mass shift constants, the field shift constants and the change in mean square nuclear charge radius.


Additional notes:

Two papers, recently uploaded to the arXiv use our isotope shift paper to give bounds for Higgs boson coupling

Delaunay et al.

Delauny and Soreq


Figure: The two dimensional King plot shows our measurement data in blue and the previous most accurate data in red. The boxes show our values with errorbars magnified by a factor of 30.

Hollow core fibers for UV light transmission

Journals homepage:

Transmission of UV light with high beam quality and pointing stability is desirable for many experiments in atomic, molecular and optical physics. In particular, laser cooling and coherent manipulation of trapped ions with transitions in the UV require stable, single-mode light delivery. Transmitting even around 2 mW CW light at 280 nm through silica solid-core fibers has previously been found to cause transmission degradation after just a few hours due to optical damage. We show that photonic crystal fiber of the kagomé type can be used for effectively single-mode transmission with acceptable loss and bending sensitivity. No transmission degradation was observed even after over 100 hours of operation with 15 mW CW input power.


Figure: Microscopic image of a hollow-core optical fiber

A first application for the fiber was found within our optical setup, where they replaced two periscope systems that connected two stacked platforms. We achieved typically more than 50% transmission through both fibers at approximately 5 mW input power. The distance between the end of the fibers and the ion was kept as short as possible to minimize remaining pointing fluctuations from vibrations and air currents. These pointing fluctuations lead to intensity fluctuations on the ion and can limit the laser control of its internal state. The effect of beam-pointing fluctuations was investigated by driving Rabi flops of the two hyperfine states of the ion via Raman lasers. Therefore different pulse lengths of the Raman beams were applied and the excitation probability was determined by averaging over 250 repetitions per pulse length. If the position of the laser beam fluctuates, the Rabi frequency will change between single experiments. This results in a reduction of the measured Rabi oscillation contrast. Figure 1 shows the Rabi flopping curves with and without the kagomé-fiber. One can see that the contrast between the excited and the ground state decays slower when the kagomé-fiber was used.


Figure: Raman Rabi oscillations (a) without kagomé-fiber and (b) with kagomé-fiber