Well, current Canon sensors have a read noise of about 3-4 electrons (and it does not vary that much with pixel size), and there is not a whole lot of room for improvement.
I think 1 electron read noise would be a great improvement. 0.1, even greater.
Someday, technology may count photon hits with a digital counter, and there won't be any read noise at all. The sensors in current digital cameras aren't digital; they are the analog sensors of digital cameras. Only the ADC stage, processing, and output, are digital.
Because of the lower gain associated with small pixels,
Smaller pixels have greater gain, AOTBE, for the same ISO. If you're thinking of ADU/electrons as "gain"; that really stretches the definition of gain. Gain, AFAIU, is a black box. I don't know what voltages the ADC is looking for, so I don't know the actual gain. What I do know is that for the same camera, the gain is proportional to the ISO setting, unless the camera design uses arithmetic to achieve some ISOs.
the effect of read noise is much more prominent with small pixels. IOW, if the gain with a large pixel is 1 electron per ADU, a noise of one electron will only cause 1 ADU of noise. However, if your small pixel has a gain of 9 electrons per ADU,
That must be backward. How could a small pixel have about 9*4095 electrons at saturation, and a large pixel have about 1*4095 electrons at saturation at the same ISO?
1 electron of noise would cause a change of 9 ADUs in the noise.
OK, I now see you had a typo in the previous sentence.
Quantum efficiency could double or treble, but how would you increase the pixel density nine fold without significantly decreasing the gain. The capacitance of silicon is limited and well depth can increase only so much.
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Read noise is the issue, and it is not proportional to absolute gain. Read noise is *NOT* the amplification of existing noise in the sensor wells; it is noise *GENERATED* in the reading of the sensor. That's why it is different as enumerated in electrons, at different ISOs in Roger's experiments, and in mine.
From some of the statements you have made, you seem to think that read noise is a quantum event, like the captured electrons. When someone says"the read noise at ISO 100 is 30.1 electrons", this doesn't mean each pixel is off by some integer number of electrons, the standard deviation of which is 30.1 electrons for the entire image. It means that the read noise was measured in ADUs, and then on assumed information about the relationship between ADUs and electrons for that ISO, the ADUs are translated into units of "electrons". This figure has nothing at all to really do with sensor electrons.
P&S cameras already have nine-fold, compared to DSLRs, and they handle this small pixel thing very well, and would probably be even better with more expensive readout circuitry for a super-MP DSLR. Look at how a Canon 10D and a Sony F707 compare with the same focal length lens (45mm), from the same distance:
A 60MP DSLR could have better microlenses and readout circuitry than a P&S, I would imagine.