Tens of thousands of photons go into making up each pixel in your standard cat photo. That's because existing cameras — even infrared night-vision ones — rely on many, many photons of light to create an image. But now physicists have photographed in almost pitch blackness, where there on average is less than one photon of light per pixel.
It's a remarkable bit of technical achievement, and it has it has pretty obvious spycraft applications. The key, as the Physics ArXiv Blog lays out, is combining two cutting-edge imaging techniques.
The first is about timing the photograph at the exact instant a photon hits the camera. This is done through heralded imaging with a pair of entangled photons. One photon acts as a trigger to announce the arrival of the other photon, which is used to make the image.
A second technique, called compressed sensing, optimises the raw image. You can see the effects of the transformed image of a USAF resolution target (right) below. Since the properties of a pixel in an image tend to follow a known statistical distribution, we can extrapolate an image from just a small number of data points using some maths. In the 300x300 pixel photograph of a USAF resolution target, an identifiable image emerges at about 7000 photons, or less than 0.2 photons per pixel.
Aside from spying in the dark, this technique could also have applications in biology. Photons can damage certain delicate samples as they pass through them. The less light, the better. We're pushing against the limits of photography here. [The Physics ArXiv Blog, ArXiv]