Esta respuesta del hilo mola, la copio aunque esté en inglés
There’s a lot of people asking for more info, so I thought I’d chime in. I’m a graduate student working in a trap-physics related field, so I understand a bit of what’s going on. This photo utilizes Laser Cooling and Ion Trapping, the creators of which were given Nobel Prizes (Laser Cooling in '97 and trapping in 89) and there’s some cool shit going on.
This is a photo of a single strontium ion (Sr+). Because the particle is charged, it is (reasonably) easy to confine the particle to a small area using electric fields. Along the axis (where you see the blue / copper looking pieces), confinement is provided by applying a DC (constant) positive voltage. However, it is impossible to confine a particle in 3-D using purely static (fields that don’t change with time) fields, so a “rotating saddle” potential is formed along the direction(s) perpendicular to the axis. This is typically provided by applying a large potential (~100 Volts? I forget the typical RF voltages, but somewhere along that order of magnitude) oscillating at RF frequencies (~Mega-hertz, ~109 Hz). This is hard to picture, so here’s a decent analogy. Imagine instead of a ball, you have a positively charged ion and the RF voltages create the rotating saddle: https://www.youtube.com/watch?v=XTJznUkAmIY
This type of ion trap is called a Linear Paul Trap. See Fig 1a from the following: https://www.researchgate.net/figure/Ions-confined-in-a-trapa-A-linear-quadrupole-ion-trap-known-as-a-Paul-trap-beige_fig4_5291816
Now, how the **** do you image a single ion? Keep in mind, these particles (there can be hundreds or thousands in a trap!) are oscillating in the trap at various frequencies. If you want to do experiments with them in a very controlled manner, you need to cool (i.e. remove kinetic energy) it. In this case, Sr+ was chosen because it is capable of being laser cooled. To laser cool, you shoot a laser in at just the right frequency so when the atom is moving toward the laser, it sees the the energy of the laser blue-shifted (it’s energy shifted just below the actual energy required to absorb!) to the correct frequency. The atom then emits a photon and continues it’s oscillation. However, because of the laser de-tuning away from the required energy, the ion effectively emits away a very tiny amount of it’s motional energy. This process is very rapid ( <1s) and can get down to ~0.001 Kelvin. See https://en.wikipedia.org/wiki/Laser_cooling
Now, how do they image an individual ion? Usually the transitions for laser cooling are in the visible (or near-visible), and so many photons can be absorbed and re-emitted. Typically you see ions imaged with a CCD camera (see Fig 1 of the above link). In this case, with a long exposure you can actually image the (lone) ion in the center of the trap. If you want more evidence, there are tons of papers that have imaged individual ions. Here’s a nice photo where the group has controlled the string of ions by playing with the potentials:
And here’s a group that made a Coulomb Crystal of thousands of ions, all laser-cooled to milli-Kelvin temps: http://chapmanlabs.gatech.edu/images/Th3pCrystals.png
Lastly, to store ions for this long typically requires ultra-high vacuum (verrrrrrrrrrrrrry low pressure). For reference, room temp. air is typically ~1 atm. Ultra-high vacuum is typically around 10-10 torr, which is roughly ~10-13 atm, or 0.0000000000001 atmospheres. This is to reduce the chance of the Strontium being knocked out of the trap or neutralizing itself (and then it won’t be trapped anymore) by stealing an electron from a room temperature particle of residual gas.
EDIT: I forgot to mention: why does the particle appear so big? Those electrodes are probably on the order of ~millimeters, but the real limit here is from the camera used to image the ion. Usually, very precise CCD cameras are used for this type of thing, and even then the particle appears to be ~micrometers across. There are a LOT of photons coming off that thing, and there is still some residual motion, so the ion is emitting light at most points in it’s oscillatory motion around the trap.
TLDR: Laser cooling, long exposure photo and ion trap in a super good vacuum