News from the lab
We did it! After months of careful preparation, we have successfully implanted an ion of Ar13+ into a Coulomb crystal of beryllium ions. Now the work begins to deterministically prepare a two-ion crystal of one HCI and one beryllium so that we can begin our search for the clock transition at 441 nm.
We have finished commissioning of the beamline for the highly charged ions which delivers them from the EBIT to the main experimental chamber, and have observed them travelling through our cryogenic Paul trap. Now we must attempt to catch one as it travels through by switching a series of electrodes, and use a large Coulomb crystal of beryllium to dissipate the last bit of energy required before the HCI can be caught by the trap.
The first tests of the beamline have been very successful. The fast highly charged ions are extracted from the EBIT, and their energy is reduced by a factor of five on the way to the experimental chamber. Their energy spread is also greatly reduced. However, they are still much too fast to be caught by our Paul trap, so an additional deceleration step using the trap itself is needed.
An upgrade to our cryogenic supply line has resulted in a large reduction of the temperature of our ion trap region from 7 K to below 5 K. This will further reduce the background gas pressure in this region, increasing the potential storage time of the highly charged ions.
We have successfully cooled a single trapped beryllium ion to the quantum mechanical ground state of one of its three degrees of freedom using stimulated Raman transitions. Once extended to a two-ion crystal of beryllium and a highly charged ion, this technique will allow quantum logic operations to be carried out.
The EBIT is here! The electron beam ion trap that will be used to produce our highly charged ions has been safely transported to Braunschweig from the Max-Planck-Institute in Heidelberg, where it was assembled and tested.
We have observed our first stimulated Raman transitions in a cold single ion, allowing us to coherently transfer population between the two hyperfine levels of the ground state in beryllium. This is a key milestone in the project, and paves the way to cooling of the ion to the ground state of its various motional modes and quantum logic operations.
After cooling the cold stage and trap to 7 K (-266°C), we trapped our first beryllium ions. The ions are loaded into the trap via pulsed laser ablation of a beryllium wire, and single ions or large Coulomb crystals of ions can be loaded on-demand
We have assembled our new cryogenic Paul trap, designed for trapping beryllium ions together with highly charged ions. The microstructured trap is made completely from gold-coated alumina (sintered Al2O3) to avoid any damage due to differential thermal contraction that would occur between different materials during cooldown to cryogenic temperatures.
The trap was assembled with high precision under an optical microscope in PTB's cleanroom facility, and was installed into the trap chamber almost without incident…..
Publication in Applied Physics B: A tunable low-drift laser stabilized to an atomic reference
Our new ion trap just arrived! The body is manufactured of sintered alumina and will now be gold plated and post-processed to create segmented electrodes.
To store highly charged ions for a reasonable period of time without loss due to charge-exchange collisions, we require background gas pressures in the 10-14 mbar range. This can only be achieved in a cryogenic environment, however if the substantial vibrations associated with closed-cycle cryocoolers reach the ion trap they will cause line broadening and systematic perturbations to our optical clock transition via (special) relativistic time-dilation. To decouple the vibrations from the trap, a novel vibration-isolating cryogenic apparatus was recently completed at our collaborating institute, the Max-Planck-Institute for Nuclear Physics (MPIK). In March 2016, the apparatus was transported 450 km to our lab here in Braunschweig. Now that the system is in place, we will do comprehensive tests to make sure the vibration decoupling system performs as expected. Only then will we mount our ion trap and begin our initial spectroscopy on cold beryllium ions.
Figure: Massive inertial pendulum of the cryo system. This system provides a cryogenic environment in order to suppress charge exchange processes of the highly charged ions with residual gas combined with multiple vibration-suppression stages.
In many ion trapping laboratories today, the ions are produced by laser-induced photoionisation of neutral vapor of the atom of interest. Although this approach is convenient at low charge states, it is impractical for the production of highly charged ions as with each successive ionisation the remaining electrons are more tightly bound owing to the reduced screening of the nuclear charge. As such the ionisation potential rapidly increases and the photon required for further ionisation moves from the ultraviolet to the X-ray and even gamma ray regime, where convenient radiation sources are not yet available.
Another approach is to use a beam of electrons to ionise the atoms. This is the principle behind the electron beam ion trap (EBIT). Electrons emitted by a cathode are collimated by a powerful magnetic field (~1 tesla) and bombard the neutral atoms, removing electrons. The positively charged ions are then attracted to the electron beam, and thus confined and ionised further as they repeatedly cross the beam. In this way, it is possible to produce a charge state determined by the kinetic energy of the electrons, which can be over 100,000 eV (a typical UV photon used for photoionisation has an energy of only 3 eV). These energies are sufficient to strip away all of the electrons from heavy elements such as uranium, leaving behind the bare nucleus. Our requirements are somewhat more modest, we initially aim to remove thirteen of the eighteen electrons from an argon atom.
One drawback of the EBIT is the large magnetic fields required for the sufficient collimation of the electron beam. Such magnetic fields are often produced by superconducting Helmholtz coils, requiring a constant supply of liquid helium. Our design of EBIT utilises a special arrangement of strong permanent magnets to produce the necessary magnetic field, thus greatly reducing the complexity and power requirements of the experiment. This device has been designed and built as part of our collaboration with the Max-Planck-Institute for Nuclear Physics (MPIK). Our attention must now turn to the beamline that will guide our ions from the production site in the EBIT to our precision linear Paul trap in the adjacent laboratory.