Using a simple laser-cooling technique, researchers at Physikalisch-Technische Bundesanstalt (PTB) and Leibniz University of Hanover have brought a single magnesium ion to a standstill, presaging a time when such individual particles can be used as tiny sensors. Quantum logic is a new field of physics that may one day lead to the fabrication of a quantum computer. It could also aid the search for the "theory of everything" — the missing link between traditional physics and quantum physics. One of the questions churning in the field is whether fundamental constants might actually be variables. To prove this in the case of the fine-structure constant, for example, the spectral lines of atoms must be measured more accurately than ever before. Quantum logic spectroscopy provides such a method. Physicists from the QUEST (Centre for Quantum Engineering and Space-Time Research) Institute at PTB and from Leibniz University have come closer to this goal: Instead of complex laser arrangements, all they need is a single laser source to bring an individual magnesium ion to a complete standstill, then use the ion to determine the properties of another ion. The new method has been published in the journal Applied Physics B. The scientists’ results constitute another step towards the conclusion of a scientific dispute, namely the question as to who can provide better measurement results to compare astronomic measured data with laboratory references. In astronomic investigations, light generated by quasars — which traverses all kinds of elements, e.g., in cosmic dust, on its way to Earth — is analyzed. Individual materials can be identified via the spectral lines found in the quasar light. If these spectra differ from those that researchers have determined in the laboratory for the same elements, this possibly suggests that the fine-structure constant has changed. B. Hemmerling adjusting the magnesium laser system. (Photo: PTB) These reference comparisons have yielded contradictory results. In order to clarify these contradictions, different systematic aspects must be investigated. One of the fundamental aspects is the accuracy rate of the known laboratory spectra. The complex internal structure of atoms and ions, which leave a characteristic trace in the quasar light, make it difficult for traditional spectroscopic procedures to tackle them. The scientists from QUEST conceived technique to detect particles such as iron or titanium ions that are difficult to measure directly. These ions can be coupled with other ions of the same charge, by mutual repulsion of the charged particles. Together, the two partners form a quantum-mechanical system in which one can be manipulated and measured and, thus, provides information about the other partner. In this case, the first partner, the so-called "logic ion", is a magnesium ion, which is used as a "sensor" for the other ion to be investigated, such as iron, titanium or calcium. To this end, the magnesium ion first must be cooled by means of laser light. With laser cooling, so much energy is subducted from the ion that it no longer moves. Then, the researchers can excite specific atomic transitions inside the "spectroscopy ion," that is, they basically can cause electrons to move to another energy level. This, in turn, provokes a recoil kick that sets both ions into motion, which can be detected very sensitively on the "logic ion". The first step of the laser cooling procedure has become much easier now. Usually, complicated systems involving several laser sources, which fill large optical tables, are used for the purposes of cooling. Instead, the German scientists developed a novel and comparably compact laser system that only requires a single source. To this end, the frequency of light emitted by a fiber laser is multiplied with the aid of nonlinear crystals up to a wavelength of 280 nm. An optoelectronic modulator generates a sideband on the light that 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, we have succeeded in cooling a single magnesium ion in a Paul trap down to the ground state of a longitudinal mode," said Piet Schmidt, head of the QUEST Institute. "In a next step, we want to test this cooling scheme for an ionic crystal consisting of a magnesium ion and a calcium ion and then, in yet another step, a frequency comb will be used as a spectroscopic laser." If this works out, laboratory precision measurements of elements such as titanium or iron (including their isotope shifts) might come within reach. This would contribute to clarifying the question of whether the fine-structure constant possibly varies. For more information, visit: www.ptb.de