  In their article, Wang et al. [1] report a new scheme for THz heterodyne
detection using a laser-driven LTG-GaAs photomixer [2, 3] and make the
impressive claim of achieving near quantum-limited sensitivity at room
temperature. Unfortunately, their experimental methodology is incorrect, and
furthermore the paper provides no information on the mixer conversion loss, an
important quantity that could readily have been measured and reported as a
consistency check. The paper thus offers no reliable experimental evidence that
substantiates the claimed sensitivities. To the contrary, the very high value
reported for their photomixer impedance strongly suggests that the conversion
loss is quite poor and that the actual sensitivity is far worse than claimed.
