Once you start working with extremely sensitive sensors in very dim lighting conditions, you are basically counting the number of photons hitting each pixel. Quantum efficiency is a measure of the percentage of photons which are counted. A quick Google search turns up a paper measuring quantum efficiency of a CMOS sensor, with the sensor in question measured at 37% (meaning a 3x improvement is physically impossible):

http://www-isl.stanford.edu/~abbas/group/papers_and_pub/qe_s...

If you want to see what that translates to, look at high-end cameras. Bigger cameras tend to be more sensitive because the pixels are larger, and the support circuitry takes up a lower percentage of the surface area, leaving more of the sensor's surface area for the actual sensor. The current generation of DSLRs can go up to ISO 25,600 (or higher, actually). Searching Flickr will show you a number of pictures in extreme low light, taken without a tripod, yielding better detail than I expect my eyes would be able to discern (note: humans, compared to most animals, have excellent night vision).

http://www.flickr.com/search/?q=iso25600

If you're willing to sacrifice resolution you can get even more sensitivity, which enables you to do crazy things like shoot video of the Milky Way or a moonlit landscape. An experimental Canon video sensor shows this off:

http://petapixel.com/2013/03/04/canon-unveils-a-35mm-full-fr...

By comparison, film has quantum efficiency below 10%, at least according to Wikipedia. Photographers were quick to ditch film in the ISO 800+ range, and I rarely use film as fast as ISO 400 since digital is so much more sensitive.