He described the hypothetical situation of a cat in a box with vial of cyanide gas capped by a decaying radioactive atom, which would release the poison once it decayed. Under quantum theory, the atom could be in both states, that is decayed and non-decayed, meaning the cat is both dead and alive at the same time. Only if you looked into the box would you know if it was alive. If you did not look, then you could not know and would have to consider it both dead and alive. The experiment is hypothetical, as such strange quantum effects do not occur in large systems comprising countless billions of atoms.
Individual atoms are, however, a different matter.
To achieve this, the beryllium atom was cooled to close to absolute zero and isolated from all types of radiation and energy sources. The team then used lasers to force the atom's single electron into two states of spin, which also forced the atom to be in two places at the same time. The researchers then caused the situation to break down by deliberately introducing contact to the outside world via an electrical field. Then, in some cases, they were able to reverse the process. The experiments helped the scientists determine what causes a quantum state to collapse.