  We demonstrate that superconducting radio-frequency cavities can be used to
create and detect millicharged particles and are capable of extending the reach
to couplings several orders of magnitude beyond other laboratory based
constraints. Millicharged particles are Schwinger pair-produced in driven
cavities and quickly accelerated out of the cavity by the large electric
fields. The electric current generated by these particles is detected by a
receiver cavity. A light-shining-through-walls experiment may only need to
reanalyze future data to provide new constraints on millicharged particles.
