A quantum generalization of classical microwave mixer theory is developed to project the performance of nonlinear tunneling barriers at high frequencies. The results demonstrate that such devices may be considered photon detectors when their conductance varies rapidly on a scale of voltage h&slash;&ohgr;/e. Tunnel barrier mixers are thus shown to offer potential for the development of wide‐band quantum noise limited detectors in the wavelength region from 10 &mgr;m to a few millimeters.