My mother-in-law does not leave my newborn baby alone even for a second. While I know she wants to take care of the child, how do I ask her to leave me alone with my baby for sometime in the day?

If you've tried being subtle and it hasnt worked (e.g. you should take some rest; we will go have a sleep ourselves), then go out and dont tell her where you are going. If all else fails, ask your hub to have a quiet word