  Although the National Institute of Standards and Technology has measured the
intrinsic quantum efficiency of Si and InGaAs APD materials to be above 98 % by
building an efficient compound detector, commercially available devices have
efficiencies ranging between 15 % and 75 %. This means bandwidth, dark current,
cost, and other factors are more important than quantum efficiency for existing
applications. This paper systematically examines the generic detection process,
lays out the considerations needed for designing detectors for non-classical
applications, and identifies the ultimate physical limits on quantum
efficiency.
