A new cooling technique (25.08.2021)

Laser cooling power transmitted through an LC circuit into another trap

Today, we report in the journal Nature (read the article here) on the first sympathetic cooling of a trapped proton using laser-cooled beryllium ions in a spatially separated trap. Energy exchange between the proton and the laser cooled beryllium ions is mediated by image currents of the ions in the trap electrodes that are transmitted through an LC circuit that connects to the two traps (see Figure below). In our first demonstration of this method, we cool the proton to a temperature about one order of magnitude lower than the cooling limit of the conventional resistive cooling method.

Fig. 1: Schematic of the implementation of the image-current cooling scheme


Cooling trapped particles is essential for any kind of precision measurement, since the remaining energy of the particle causes measurement fluctuations or systematic uncertainties. Laser-cooling has been used to prepare trapped charged particles in the motional ground state, however this technique can only be applied to a fraction of ions which have suitable closed cycle for laser cooling. Heinzen and Wineland therefore proposed a technique to extend the laser cooling from a suitable ion via image currents to a separate trap (see their publication here). The advantage is that this method can be applied to any kind of charged particle, in particular those that are difficult to cool otherwise – such as positrons, antiprotons, highly-charged ions or molecular ions.

Our motivation to implement this technique is to improve the comparison of the proton and antiproton magnetic moment, to improve our insight into the matter-antimatter balance observed in the universe. For these measurements, we require protons and antiprotons at energies of about 100 mK to resolve single spin transitions – an essential prerequisite to perform these nuclear magnetic moment measurements at all. To prepare such cold particles using resistive cooling is quite time-consuming, and our demonstration of the image-current cooling shows that this limitation can be overcome.

One experimental challenge of implementing these technique in our trap system is that the image currents are tiny. Our demonstration uses therefore a superconducting LC circuit that resonantly amplifies the image currents to make the cooling of the proton by energy transfer to the laser cooled ions and the proton possible. This new cooling method constitutes in particular an advance for antimatter physics since it can be directly applied to antiprotons. It is a stepping stone to improve the precision measurements of single trapped protons and antiprotons.


Fig. 2: Matthew Bohman (left) and Christian Smorra (right) during the installation of the trap system


Text credits: C. Smorra (JGU), Stefan Ulmer (RIKEN)

Figure and photo credits: M. Bohman (MPIK/RIKEN), F. Sämmer (JGU)


Other news outlets

JGU Press Releases: Press release (25.08.2021)

CERN Courier: Article (25.08.2021)

CERN homepage: News

RIKEN Press Release: English/Japanese

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