Answer: This is going to be a challenge. You will need to protect the roots from the cold temperature extremes as well as from freezing and thawing. The best thing would be to sink the pot in the ground in a sheltered location out of the wind and heap mulch over the roots but not touching the trunk.
If you can't do that, in late fall, set it in a location on your balcony that is shaded. Then insulate the pot. You can use mulch heaped over and around it (but not touching the trunk) or perhaps non-packing dry leaves such as oak leaves or possibly straw wrapped in burlap, or try bubble wrap, whatever you can improvise to insulate the roots.
Also shelter it from the wind. You can wrap the top portion of the tree gently in burlap or erect a little wind break for it. Plastic wrapped around the tree is not a good idea because it can trap moisture on sunny or warm days.
Finally, keep the soil moist until it freezes. This will ensure the tree goes into winter well hydrated and this helps it to withstand cold.
Good luck with your little tree.
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