Five charged calcium atoms hover in a trap, held by electric fields. Laser cooling saps their excess energy, and they line up into a tiny pyramid with a square base. But this pyramid doesn't stay put: every now and then it flips over, as if someone tosses it into the air and catches it upside down.
The rate of flips depends on how easily the ions can overcome an energy barrier. For five identical ions, the barrier is low because the ions are indistinguishable: any of them can end up at the apex. But if the top ion is replaced with a heavier isotope of calcium, the picture changes. The heavy ion can no longer swap places with the light ones as easily, and the pyramid flips much less often.
The same effect is known in chemistry: if you replace an atom in a molecule with a heavier isotope, the reaction slows down. Here it's the same, but on the scale of a few ions held in a trap. This opens the way to studying collective motions in systems where symmetry and the distinction between particles matter.
The flipping of the five-ion pyramid is like an umbrella turning inside out in a strong wind: first the dome caves in, then it snaps outward the other way.