Single atoms for everyone

The Technorama ion trap

A microscope is perhaps the best-known gateway to the world of very small objects, but seeing atoms by eye requires an entirely different type of scientific instrumentation. The Technorama ion trap uses alternating and static electric fields to confine charged barium atoms; laser light is injected into the trap to cool the ions so that they are stationary. Laser light can also kick the barium ions out of their resting "ground" state into a so-called dark metastable excited state, from which the ions decay back to the ground state by emitting visible light at a wavelength of 493 nm: this wavelength is close to the peak spectral sensitivity of the human eye, making barium ions an ideal candidate for a museum exhibit. In a external page live video stream from Technorama, it is possible to watch the ions as they sit neatly in a line in their fluorescing ground state until a few of them disappear and later reappear as a consequence of laser-driven excitation.

One trap, many voices

Atoms are the building blocks of matter, but in our macroscopic lives we don't stop to ponder the atomic scale – where the reference length is the radius of an atom, about a tenth of a billionth of a metre, and objects obey the laws of quantum mechanics – as often as we may look up at the sky on a clear night. And yet astronomical distances and phenomena are no easier to grasp than those characterising atomic systems. Then again, astronomical observatories and telescopes have long been accessible to the general public, which cannot be said of the scientific instruments that make it possible to study atom-sized particles.

The new Technorama exhibit "Seeing atoms" ("Atome sehen" in German), inaugurated on 30 May 2023, wishes to bring its visitors closer to the quantum mechanical world of atoms. Thorsten-D. Künnemann, Director of Technorama, opened the event and acknowledged the unique nature of the ion trap that is now part of the Centre's attractions – not only because it was specially designed for the space, but because it is not as easily interactive as most of the other science activities at Technorama. This is precisely why designing an exhibit around the trap was crucial to provide context and allow visitors to engage more directly with the physics of trapped particles.

ETH Zurich Rector Günther Dissertori thanked Jonathan Home and his research group for pursuing the idea of making atoms visible as well as the Technorama staff for making it all happen. Home himself took the stage of Technorama's auditorium to answer the non-trivial question: what do trapped ions have to do with quantum computing? It turns out that these charged atoms are so well isolated from the surrounding environment that they are truly quantum mechanical systems that can be used to simulate different, complex systems exceedingly hard to handle by classical computing means. The inaugural speeches were rounded off by a 'science slam' featuring past and current members of Home's group: Dr Natascha Hedrich, Dr Pavel Hrmo, Dr Diana Aude Craik and Moritz Fontboté Schmidt took up the challenge of compressing fiercely technical topics – from the Doppler effect used for laser-cooling the ions to the isotope shift measurements performed on trapped particles – into a few minutes each.

The exhibit around the trap

The project that led to the present exhibit has a long history, with conversations between the Department of Physics at ETH Zurich and Technorama dating back to 2018. Once Technorama confirmed its interest, Home and members of his group sat down to figure out what it would take to build a suitable ion trap; the group secured funding from ETH and from the Swiss National Science Foundation, and they put together a team including engineer Ilia Sergachev and postdoctoral researcher Natascha Hedrich. The ion trap was delivered to Technorama in late 2021; testing of the apparatus on site, including early visitors' feedback, took place throughout 2022.

The Technorama exhibit "Seeing atoms" stimulates visitors to cast a deeper gaze at all the matter that surrounds us through interactive activities that follow the ion trap in its transparent box. For example, visitors are encouraged to measure the lifetime of a barium ion's dark excited state by counting for how many seconds an ion remains dark before becoming visible again. Thanks to a cloud chamber, visitors can also see the tracks left by electrically charged particles such as protons and muons as they fly through the Technorama building. At the end of the exhibit, visitors are encouraged to share their thoughts by leaving sticky notes on a wall.

In his speech, Dissertori said that he hopes this opportunity to see atoms sparks further interest in physics and makes visitors eager to ask more questions. Scanning the feedback wall, it seems that this is already happening: a sticky note written by a visitor reads, "Since when are there atoms?" – a beautiful question that calls for more fascinating physics.