With very cold temperatures and magnets. The article had a link to a much better article in Science Mag [1] with more information about how neutrons are trapped

With very cold temperatures and magnets. The article had a link to a much better article in Science Mag [1] with more information about how neutrons are trapped. [1] https://science.sciencemag.org/content/360/6389/627 https://science.sciencemag.org/content/360/6389/627