  We propose a novel bias-free method for reconstructing the power spectrum of
the weak lensing deflection field from cosmic microwave background (CMB)
observations. The proposed method is in contrast to the standard method of CMB
lensing reconstruction where a reconstruction bias needs to be subtracted to
estimate the lensing power spectrum. This bias depends very sensitively on the
modeling of the signal and noise properties of the survey, and a misestimate
can lead to significantly inaccurate results. Our method obviates this bias and
hence the need to characterize the detailed noise properties of the CMB
experiment. We illustrate our method with simulated lensed CMB maps with
realistic noise distributions. This bias-free method can also be extended to
create much more reliable estimators for other four-point functions in
cosmology, such as those appearing in primordial non-Gaussianity estimators.
