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Single ions – extremely cool

Especially interesting for
  • fundamental physics
  • precision spectroscopy
  • developers of optical clocks

While looking for possible changes in the fi ne-structure constant, the aim is to measure the spectral lines (i. e. the inner structure) of atoms more accurately than ever before. One way of achieving this is quantum logic spectroscopy. For this purpose, physicists from the QUEST Institute at PTB and from Leibniz University Hanover now only need one single laser source – instead of complex laser arrangements – to bring a single magnesium ion in a quadrupole ion trap to a complete standstill and to determine the properties of another ion with its help.

Adjustment of the magnesium laser system

The new research results could help settle a scientifi c dispute – the apple of discord being the question of how to compare astronomic measurement data with laboratory references. In astronomic investigations, light is analyzed which is generated by quasars and has traversed cosmic dusts on its way down to the Earth. The elements contained in these cosmic dusts have left their characteristic "fi ngerprint", so to speak, which can be identifi ed by analyzing the light. If these "fi ngerprints" differ from those that have been generated in the laboratory for the same elements, this possibly suggests that the fi ne-structure constant has changed. To date, however, it has not been possible to measure the laboratory spectra accurately enough. Scientists from QUEST (Centre for Quantum Engineering and Space- Time Research) have made an important step in this direction.

They have developed an indirect method to examine, e. g., iron or titanium ions. They couple them with other ions of the same charge by mutual repulsion of the charged particles. Together, they form a quantum-mechanical system in which one of the partners can be manipulated and investigated and which, thus, provides information about the other partner. For this, the fi rst partner, the so-called "logic ion" (in this case, magnesium), has to be cooled down with laser light until it reaches a standstill. Then, atomic transitions can be excited in the other partner, the "spectroscopy ion" (in this case, titanium or iron) in a targeted manner. This, in turn, provokes a recoil kick which sets both ions into motion and can be detected very sensitively on the "logic ion".

The fi rst step of the laser cooling procedure has become much easier now. Instead of complicated systems with several laser sources on large optical tables, a novel and compact laser system has been developed which only needs a single source. To this end, the frequency of light emitted by a fi bre laser is multiplied with the aid of non-linear crystals up to a wavelength of 280 nm. An opto-electronic modulator generates a spectral sideband on the light which is resonant with a transition in the magnesium ion and is used for the state preparation and laser cooling of the ions. With this arrangement, a single magnesium ion in a quadrupole ion trap was successfully cooled down to the ground state of a longitudinal mode.

In a next step, this cooling scheme will be tested for an ionic crystal consisting of a magnesium ion and a calcium ion and fi nally, a frequency comb will be used as a spectroscopic laser. If this can be achieved, then elements such as titanium or iron can soon be investigated with great precision in the laboratory. This would considerably contribute to unravelling the mystery of varying fundamental constants.

Scientific publication

Hemmerling, B.; Gebert, F.; Wan, Y.; Nigg, D.; Sherstov, I.V.; Schmidt, P.O.: A single laser system for ground state cooling of 25Mg+. Applied Physics B 104 (2011) 583 – 590