This bubbly drink gets its zing from a Thai chile.
Bring 2 tablespoons water, sugar, and chile to a simmer in a small saucepan; strain. Discard chile.
In a juicer, juice pineapple, oranges, grapefruit, and lime; strain. Add yuzu juice and chile-sugar syrup.
To serve, place 2 ounces citrus-chile mixture in each Champagne flute, and top with Champagne (about 5 ounces).