Combine in a small saucepan:
Bring to a boil over medium heat, stirring occasionally to dissolve the sugar, then cover and boil 5 minutes without stirring. Remove the pan from the heat and let steep for 15 minutes.
Strain into a medium bowl; discard the zest. Let cool to room temperature, then stir in:
Refrigerate until cold or quick-chill in an ice water bath. Freeze as described in Berry Granita.