This cannot be done directly with USB. USB is a shared serial bus, and only digital signals can be carried on it. If you were to use USB you would need to have a USB-interfaced device on the other end that decoded the serial commands to turn a switch on or off.
One important question for this purpose is what kind of rate of change do you need for this purpose? Another question is what precision you need in the timing. A third question is how much control you need over the pulse widths.