Physics

Focus: Cooling on the Destructive Facet

September 6, 2019• Physics 12, 98

A brand new cooling method targets unfavourable ions, that are sometimes proof against cooling strategies that work with atoms and constructive ions.

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CERN

Antimatter drop take a look at. The AEgIS experiment will measure the speed at which chilly antimatter (antihydrogen) falls in response to gravity. To chill this antimatter, researchers are engaged on intermediate steps that contain cooling negatively charged ions.Antimatter drop take a look at. The AEgIS experiment will measure the speed at which chilly antimatter (antihydrogen) falls in response to gravity. To chill this antimatter, researchers are engaged on intermediate steps that contain cooling negatively charged ions… Present extra

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CERN

Antimatter drop take a look at. The AEgIS experiment will measure the speed at which chilly antimatter (antihydrogen) falls in response to gravity. To chill this antimatter, researchers are engaged on intermediate steps that contain cooling negatively charged ions.×

Cooling atoms and ions to close absolute zero could seem routine, however surprisingly, negatively charged ions are onerous to chill down even to room temperature. Researchers have now demonstrated a way for cooling unfavourable ions by discarding the warmest ones from the group. Additional refinement of this technique might provide a option to generate chilly antimatter, which researchers want to use to check for different theories of gravity.

Utilizing an in depth toolbox of cooling methods, physicists routinely entice and chill atoms and positively charged ions to temperatures beneath 1 Ok (“subkelvin” temperatures), the place they can be utilized as precision clocks or quantum bits. In contrast, negatively charged ions, or “anions,” have been immune to those cooling strategies. Researchers have used much less environment friendly methods, comparable to electron cooling—the place anions work together with a chilly electron plasma—however the coldest temperatures have been solely about four Ok. Strategies that may attain decrease anion temperatures would profit research of chemical reactions in house, and colder anions might additionally cool antiprotons for antimatter experiments.

The problem is that “anions are fairly fragile,” says Alban Kellerbauer from the Max Planck Institute for Nuclear Physics in Germany. A unfavourable ion is an atom with an added electron, which the atom can simply let go of. In consequence, most anions can’t be put right into a secure excited state by absorbing a photon; as a substitute, photons merely knock out the additional electron. This fragility makes most anions ineligible for Doppler cooling, a extremely environment friendly cooling methodology wherein atoms are repeatedly excited by photons.

To make progress, Kellerbauer and his colleagues have turned to evaporative cooling, which has lengthy been utilized by atomic physicists. It really works by eradicating the most well liked particles from a entice, in order that the remaining fuel turns into colder. The standard method has not been used for unfavourable ions as a result of they sometimes begin off too sizzling (round 10,000 Ok) and would lose far too many ions within the course of.

The crew’s laser-assisted evaporative cooling methodology—proposed almost 30 years in the past [1]—capitalizes on the fragility of anions. The additional electron in an anion is well stripped by a laser, and as soon as this stripping happens, the now-neutral atom can escape from the electrical fields that represent the entice. To focus on sizzling ions for this liberation, the crew centered their laser a brief distance from the entice middle. That off-center location is reached primarily by essentially the most energetic ions, that are those in a position to stray far sufficient from the entice middle. The crew examined their methodology on a fuel of negatively charged oxygen ions. Beginning at an preliminary temperature of 13,000 Ok, the fuel was cooled to 3800 Ok in about 100 seconds. By the tip, between 10% and 30% of the unique inhabitants remained.

The ultimate temperature continues to be sizzling, however Kellerbauer says that the brand new method opens up a multistep pathway to subkelvin anions. The scheme would require utilizing one of many few anions that has a number of electron ranges, and lanthanum is taken into account the best choice. The ion entice utilized by Kellerbauer’s crew couldn’t accommodate lanthanum ions, however they’re at present redesigning their entice for that goal. They’re additionally exploring methods to optimize their evaporative cooling method with a second laser that may enhance the selectivity of the electron-stripping course of.

The crew’s intention is to pre-cool lanthanum ions to 100 Ok. At that time, calculations have proven that Doppler cooling may very well be used to additional cool the lanthanum ions to microkelvin temperatures [2, 3]. This subkelvin anion fuel might chill a cloud of antiprotons to close absolute zero if the 2 gases had been blended collectively. The ensuing chilly antiprotons might then be mixed with positrons to make chilly antihydrogen, which quite a lot of experiments—comparable to AEgIS—require for testing whether or not antimatter falls on the similar price in response to Earth’s gravity as regular matter.

“This examine is a vital advance for laser cooling of negatively charged species,” says Wesley Walter from Denison College in Ohio, who makes a speciality of unfavourable ion analysis. “Though the diploma of cooling achieved on this examine is modest, it’s the primary demonstration of a way that gives a step towards reaching ultracold temperatures.” Atomic physicist Michael Charlton from Swansea College within the UK additionally sees this work as “an necessary first step” within the pre-cooling of anions in the direction of the eventual aim of manufacturing chilly antihydrogen for gravity investigations.

This analysis is revealed in Bodily Evaluation Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics primarily based in Lyon, France.

References

A. Crubellier, “Idea of laser evaporative cooling of trapped unfavourable ions. I. Harmonically sure ions and RF traps,” J. Phys. B 23, 3585 (1990).C.  Walter, N.  Gibson, D.  Matyas, C. Crocker, Ok. Dungan, B. Matola, and J. Rohlén, “Candidate for laser cooling of a unfavourable ion: Observations of bound-bound transitions in La−,” Phys. Rev. Lett. 113, 063001 (2014).G. Cerchiari, A. Kellerbauer, M. Safronova, U.  Safronova, and P. Yzombard, “Ultracold anions for high-precision antihydrogen experiments,” Phys. Rev. Lett. 120, 133205 (2018).

Extra Info

Three experiments at CERN plan to check gravity with antihydrogen: AEgIS, ALPHA, and GBAR.

Topic Areas

Atomic and Molecular Physics

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