You are going to have to keep this tree indoors for the duration of the winter and back outside once the frost and freezes are past. THe tree has emerging leaves, which means it has virtually no ability to withstand freezing. If the buds were still intact and only swelling, you could put it back outside.
Bringing it inside to "guard it from heavy frost" is exactly the wrong thing to do. It should be exposed to one or two heavy frosts to "harden it off" for coming winter and left outside under mulch until spring. Coddling trees is a path to difficulties...