3A? no no no, that should've been 3 microamp :-)

In the dual-chip system, the microcontroller doing the AD conversion + filtering might be advantageously replaced with something purpose made. The chip on this page looks really excellent:

The measurement results showed that the circuit performs as expected and it achieves a CMRR of 82 dB and a maximum SNDR of 36.1 dB with 1-V supply voltage. The total current consumption of the chip, including the output buffer, is only 3 A. Furthermore, the average current consumption of the 8th order bandpass SO-SC filter is very low, since it consumes only about 400 nA, corresponding to 50 nA/pole, which is much less than the current consumption of the SC filters in prior designs. Therefore, the circuit is very suitable for portable ECG measurement applications, like heart rate detectors.

Not sure if anything like this is available commercially though... and of course, using off-the-shelf electronics will cut the costs right down.

Read Bachelard Formation of the scientific mind and weep. If only it were so easy and blame everything on the catholic church. For a very long period of time, it looks as if entertainment value was put way above scientific rigor... that and scientific thinking is quite a recent thing. From the book, experiment held around 1700 (from vague recollection): Electricity from a battery cell passes through a liquid and the experimenter's tongue. Experimenter then "tastes" the electricity. Taste through milk? "Soft and sweet" as opposed to electricity flowing through vinegar "strong acid taste". Anyway, interesting read.

Nice one! Jitter looks a bit high at 80ps but other than that, interesting concept.

SPAD afterpulsing is probably not an issue for this project because it's looking at photon pairs, so uncorrelated random events occurring on all the SPADs won't affect the detection... but will decrease the counting/processing rate by bogging down the electronics.

For measuring concurrent events, I would've thought TTS would be much more critical, and you can't get much better than MCP-PMTs (10-20ps these days?). Just wondering if the same detection could've been done with a multi-anode MCP, although if the sensor is CMOS tech, APDs would probably be easier to incorporate onto an ASIC / SoC.