To a small saucepan, combine the sugar and water. Heat over medium-low until sugar has dissolved. Add the sprigs of tarragon and cook for an additional 1-2 minutes. Turn off the heat and allow the tarragon to steep for 10 minutes. Strain, discarding the tarragon and set aside.
To assemble a cocktail, fill a glass with ice. Pour in 1 ounce of the tarragon simple syrup (2 tablespoons), 1 ounce lime juice, 1.5 fluid ounces vodka, 4 ounces pomegranate juice and top with a splash of sparkling water. Stir and adjust alcohol to your taste. (I'm admittedly a wuss and like my drinks a bit weak.)
Garnish with a sprig of tarragon. Repeat the drink-making process until you've made all four. Serve immediately.