Antonyms for the phrase "are in operation" would include words and phrases such as "are out of operation," "are not functioning," "are inactive," "are shut down," and "are idle." These are all terms that suggest a lack of activity, either because something is broken or because it has deliberately been turned off. Although "are in operation" generally implies that something is functioning normally, antonyms such as these can be useful for describing situations where things have gone wrong or where a particular process has been halted temporarily. Ultimately, the choice of antonym will depend on the context in which it is being used.