To make the lemon soda concentrate, combine the zest and sugar in a mortar and crush with a pestle until fragrant, moist, and tinted yellow.
Transfer the mixture to a small pitcher or lidded container and pour in the lemon juice and water. Stir until the sugar dissolves. Cover and refrigerate, agitating the mixture from time to time, until cold and infused with lemon flavor, about 2 hours.
To make the ice cubes, combine the water, sugar, and bay leaves in a small saucepan and bring to a boil, stirring to help the sugar dissolve. Boil for 1 minute, remove from the heat, and let stand for 20 to 30 minutes.
Strain the infused water through a fine-mesh sieve set over a liquid measuring cup.
Pour into ice cube molds, and freeze until solid, about 2 hours.
To serve, place 3 to 5 ice cubes in each of eight 8-ounce drinking glasses, and then add 3 tablespoons (or more, to taste) of lemon concentrate per drink.
Pour in 1/2 cup (4.3 oz / 120 g) of the sparkling water per serving and stir gently.
Serve immediately.
Do Ahead: The ice cubes can be made in advance and stored in an airtight container in the freezer for up to 3 months. The soda concentrate can be made up to 1 week ahead and stored in an airtight container in the refrigerator.Above and Beyond: At the museum, we use small half-sphere silicone molds to make flavored ice cubes, but any ice cube tray will work. Keep in mind, though, the smaller the cubes, the quicker they'll melt, and the sooner the flavors will merge in the drink. A small piece of bay leaf can be added to the cube for additional flavor and a pretty pop of color. Scented textile threads might be impossible to make at home, but my friend Daniel Patterson, chef-owner of Coi restaurant in San Francisco, came up with a great way of incorporating aromas while eating or drinking. Just dab a drop of lemon and bay essential oil (see Resources) on your wrist, and when you bring the glass to your mouth, you'll take in the fragrance.Resources: Essential oils: libertynatural.com
