We have measured the linear response of the quasiparticle current in a Nb/AlOx/Nb junction from 75–200 GHz. Picosecond pulses of millimeter wave radiation were generated by illuminating a photoconductive switch with a mode‐locked laser. The pulses were coupled to the junction through an impedance‐mismatched, planar lithographed antenna. The broadband response of the superconductor‐insulator‐superconductor junction was measured by monitoring changes in the dc current induced by interfering two electrical pulses at the junction as a function of the time delay between them. The data are in agreement with the linear theory of photon‐assisted tunneling.