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Comment by gugagore | Hacker News Reader

"Honestly, the quality is probably on par with a 40 year old digital camera. On the other hand, it hopefully means that in about 40 years we will have GoPro’s with this capability strapped to our helmets showing us what’s around the corner before we even get there!"

I would have guessed there are some resolution limits. For a traditional [line of sight?] camera, the resolution is limited by the imager and https://en.wikipedia.org/wiki/Angular_resolution . What limits are at play for non-line-of-sight imaging?

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The quantization of light is going to be a serious limit. The detector elements of a camera could be made pretty sensitive, but they can't detect fewer than one photon.

In NLOS (and traditional photography) the scene to be imaged is illuminated with N photons but the detector only receives M photons worth of signal back, where M<<N. So future people are going to need:

1) helmet-mounted lasers capable of sustained very high power output, to increase the signal enough to get over the quantum detection threshold

2) to slow their roll, so a lot of imaging can occur before they round the corner

or both. There are already practical limitations on laser power output in air because the air will turn into plasma along the beam path. Similarly, for NLOS you need to not burn up and destroy the surface that you're using to bounce the light around the corner.

> The detector elements of a camera could be made pretty sensitive, but they can't detect fewer than one photon.

Or can they?

If the detector elements were part of a quantum computer (or a quantum computing "chip", whatever that will turn out to be) they would be able to analyze all the photon paths (Feynman paths) bouncing back from the subject, even those that would decohere / collapse away in a traditional detector.

IANAP, but wouldn't a quantum chip be able to perform some amount of NLOS by analyzing the paths of even a single photon?

As soon as your photon interacts with a realistic scene (thermal), I believe it loses coherence with any apparatus you might have prepared in the detector. In short, no, I don't think this is possible, unless you consider idealized environments.

> "In NLOS (and traditional photography) the scene to be imaged is illuminated with N photons but the detector only receives M photons worth of signal back, where M<<N."

How about "averaging" several attempts to compose a single shot? Would it be possible to shine lasers of multiple wavelengths to achieve better results? maybe varying angles as-well?

Averaging N images should (roughly?) make the noise go down by sqrt(N), so long as you don't introduce more sources of noise during collection. So the camera would have to be still, or the images registered to a common perspective.

The camera or subject can move, and as long as the movements are known then the reconstruction is still perfect.

High end mobile phone cameras take hundreds of shots over a second or more to reduce noise, and rely on the fact they can estimate movement of things in the frame with optical flow and a gyroscope.

This is what I meant by "registration", for the case when the subject isn't changing.

At least surfaces that are anisotropic scattering, highly absorbing surfaces, and temporal resolution of the laser and shutter will all affect the image's spatial resolution.

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