News from the lab

First molecular ions produced in new chamber

We have produced the first molecular ions in our new experimental setup. For this purpose, we have loaded a Coulomb crystal of calcium ions and heated our getter pump to release hydrogen. The reaction of calcium with hydrogen to form calcium-hydride can be detected by the emergence of dark ions on the radial edge of the crystal. We can reduce the larger two-species crystal to a two ion mixed species crystal by subsequently splitting the ion chain and dumping the unwanted side.

a) Single species calcium coulomb crystal. b) after heating the getter material, hydrogen is released and reacts with the atomic ions. Calcium hydride does not fluoresce and can therefore be seen as dark ions in the outer region of the Coulomb crystal (indicated by white ellipses). c) Subsequent splitting and selection of one side of the split allows to produce a two ion crystal, containing a single calcium ion and a singe calcium hydride molecular ion.

Coherent excitation by far detuned Raman interaction

We have succeeded in driving coherent population transfer between transitions of our optical qubit in the calcium ion using a far detuned optical Raman laser system with >100THz detuning. Furthermore, we succeeded in exciting individual modes of motion of the trapped ion. The latter represents a key tool for molecular ion quantum logic spectroscopy. Also, non-classical states can be generated in this way, where the internal (electronic) state is entangled with external (motional) states.

Ground state cooling of Calcium Ions

We used Electromagnetically Induced Transparency (EIT)- cooling to achieve a mean motional excitation of 0.1 phonons. In our experiment it is a two-stage process. One set of parameters provides very rapid cooling and another, second parameter set is optimized for lowers final temperatures. The cooling performance was evaluated by probing motional sideband transitions on Calcium’s optical qubit, using a high stable laser at 729nm.

With ground state cooled Ca ions and full qubit control, we can now push for mixed species ion crystal control and quantum logic protocols.

Left: first stage EIT- cooling, right: second stage EIT- cooling. Top to Bottom: Carrier, red and blue sideband Rabi oscillations on our optical qubit.

Scheme for quantum-logic spectroscopy of oxygen ions

We have theoretically investigated a scheme for quantum logic spectroscopy of molecular oxygen ions. Similar to the scheme, developed by the NIST group for CaH, we propose to use a far detuned Raman laser system. In contrast to the NIST scheme (Chou et al. Naturevolume 545, 203–207 (2017)), we plan to use bichromatic laser interactions and resolve the rotational state dependent Zeeman structure for state discrimination. We have further investigated the direct quadrupole excitation of vibrational overtones and compared the result to the two-photon dipole excitation proposed by Carrollo et al. Atoms7, 1 (2018).

The results of our investigation can be found in Wolf et al. Prospect for precision quantum logic spectroscopy of vibrational overtone transitions in molecular oxygen ions arxiv:2002:05584 (2020)

Level scheme and angular momentum coupling in molecular oxygen ions

First calcium ions trapped in new ion trap

For the first time, we have succeeded in trapping calcium ions in the ion trap of our new experimental setup for spectroscopy of molecular ions. In the future, the calcium ions will take the role of the logic ion in our quantum logic experiments. The linear Paul trap used for our experiments is similar to the one operated by the highly charged ion experiment in our group and has several segments. This segmented trap type allows a much more flexible setting of the trapping potential for the controlled preparation of mixed species two-ion crystals.

a) image of our linear Paul trap with segmented blades. b) Two trapped calcium ions. Doppler-cooled and fluorescence detected by an EMCCD.

Fock state metrology

In a collaboration with Klemens Hammerer’s and Augusto Smerzi’s group, we have demonstrated that excited Fock states can be used for quantum enhanced sensing of amplitude and phase. In contrast to Squeezed states or Schrödinger cat states, the same quantum state can be used to achieve sub-Standard-Quantum-Limit sensitivity in botch quadratures. The results of our study are published in Wolf et al. Motional Fock states for quantum-enhanced amplitude and phase measurements with trapped ions, Nature Communications 10, 2929 (2019)

Artistic depiction of the amplitude measurement using the first excited Fock state. The surface under the pendulum depicts the product of Wigner functions of the n=1 Fock stats and the displaced n=1 Fock state. The integral over this product gives the overlap between these states. In contrast to gaussian states, the negative regions in the Wigner function can lead to vanishing state overlap, before the wavepackages are spatially separated.

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