Future of heterodyne receivers at submillimeter wavelengths

The astrophysics community is currently reflecting on the role heterodyne receivers at submillimeter wavelengths play in the next generation NASA missions. Even though they offer very high spectral resolution, they are intrinsically handicapped by quantum noise. With the proposed cold telescopes for the future NASA missions such as the SAFIR, the direct detector instruments have no longer be background noise limited, and outperform the heterodyne instruments in the sensitivity arena. In this review paper we examine these issues very carefully and describe the novel receivers being designed to make heterodyne instruments more competitive. It has shown that the heterodyne instruments still have significant roles to play in the near future.