1. To prepare simple syrup, combine sugar and 1/4 cup water in a microwave-safe dish; microwave at HIGH for 2 minutes. Stir until sugar dissolves; cool.
2. To prepare the cocktail, pour 3 ounces sparkling wine into a Champagne flute. Combine sloe gin, juice, and absinthe in a martini shaker with ice. Add 1 1/2 teaspoons simple syrup; reserve remaining simple syrup for another use. Shake. Strain into flute; top with bitters.