Sweetener syrups are now available to use in place of sugar and you can also make your own using a powdered sweetener in the same way you'd make a sugar syrup.

Commercial brands of sugar-free sweetener syrup include Monin sweetener is best described as non-sugar sugar syrup designed for use in cocktails, coffees and milkshakes.

To make Sugar-free sweetener syrup

Stir 2 cups of powdered sweetener (e.g. Xylitol) into 1 cup of water in a pan over low heat. Allow to cool, bottle and store your sugar-free sweetener in a refrigerator.

Powdered sweeteners

XyloSweet (Xylitol)
1 spoon = 1 spoon of sugar. Dissolves like sugar and needs heat to make a 2:1 syrup. Produces a crystal clear liquid that does not foam when shaken.
Comment: Good flavour, close to that of sugar with a slightly thinner mouthfeel. Best finish of all three sweeteners I tried but not as good as sugar.
Beware: Xylitol may increase the risk of heart attack and stroke and can cause diarrhoea and liver damage. Xylitol is extremely toxic to canines (dogs) and, perhaps to a lesser degree, to cats.
Difford's Guide rating: 4/5

Green Canderel (Stevia)
1 spoon = 1 spoon of sugar. Fizzes as it dissolves in water but dissolves easily without the need to heat.
Produces a clear liqueur with thin viscosity that foams when shaken.
Comment: Not quite sweet enough. Worryingly fizzy after shaking. Unpleasant finish. 2/5

Silver Spoon Truvia (Stevia)
1/3 spoon = 1 spoon of sugar. Harder to dissolve in water than Green Canderel and seems to benefit from being warmed. Produces a clear liquid with a slight straw yellow hue that looks like sugar syrup. Foams when shaken.
Comment: Structure of cocktail made using this syrup seems to break apart very quickly with thin foam on top of a cocktail. Good taste but with a disappointing finish.
Difford's Guide rating: 3/5