1. Place the gin, pomegranate juice, lemon juice, simple syrup and egg white in a cocktail shaker. Shake vigorously for 15 seconds.
2. Fill the shaker with ice and shake for 30 seconds until cold.
3. Strain the drink into a glass and top with the soda water; the egg white foam forms on top. If desired, garnish with a rosemary sprig. Serve immediately.