Instrumentation: Nanowatt Photodetector

The standard problem with conventional nanowatt photoreceivers is that in order to get near the shot noise, you have to use feedback resistors so gigantic that you can't maintain decent bandwidth.
This one has what I think is a completely novel photo-feedback architecture, i.e. rather than using a feedback resistor in the TIA, it uses two secondary photocurrents to cancel the input current. Putting the two secondary photodiodes in series makes the cancellation current 3 dB quieter than the shot noise, and a feedback system prevents them from fighting, as series-connected current sources normally would.

This results in a noise floor asymptotically only 10 log(1.5) ≈ 1.76 dB above the shot noise of the signal photocurrent, instead of 3 dB for straight photocurrent feedback.