Neat. So what's it for? Improved covert communications via entangled photon pairs? Improved ability to paint a target at long distances? Improved measurements in noisy environments?

@Richard Healy. A quote from the article says one use for it This work could provide enhanced methods for low-power, single-photon lidar for high-resolution active imaging and sensing over long ranges and open up a new road for the application of long-range target recognition and earth observation.

My question is how do they know with absolute certain that it's a single photon?

I think spectral purity is important : they emit at a given well defined and stable frequency, so they look only at photons that have this exact frequency. The thinner your frequency band, the more noise you reject.

If I'm correct they use 1550 nm photons, so Infrared photons that would typically be emitted naturally by a blackbody at 1000 Kelvins or so (about 800°C but that's not a precise number anyway)
So unless you have an active volcano in the line of sight the background should be fairly quiet at those wavelength (well obviously you should not try to image the sun directly 😛).

@Robert Biloute - on diaspora-fr.org - But a single frequency is different from a single photon. Or is their implication that it's only photons of that frequency?

each photon has a frequency, so you can filter them based on that. If you send photons with a well defined frequency F, then you'll take into account only F frequency photons in the detector, everything else will be excluded as 'noise'

It's actually the same thing as radio stations : you bath in a chaos of mixed radiowaves at different frequencies, but when you tune your radio to the exact frequency of an emitter, then you'll hear that emitter and not the bazillions other ones.

aw wait, I misread your question : we are now able to count photons one by one, for example with photomulitpliers. A single photon enters the detectors, rip an electron off a material, that electron is accelerated in an electric field and will generate an avalanche of other electrons. In the end you have a huge gain, like for one photon/one electron at input, you get more than enough electrons at output to detect electronically.
Practically, you see a pulse on the screen each time a photon is detected, this is the current pulse generated in the detector. At visible wavelength in daylight, you'll have so much photons you will not see anything unless you're very very very fast.
But here thanks to thin frequency band and I guess other tricks, they manage to get a background count of some hundreds photons per seconds, which is very low and detectable individually by a fairly slow electronics.

@Robert Biloute - on diaspora-fr.org - Ah, thanks. Technology has become quite advanced in that area of science.

@Robert Biloute - on diaspora-fr.org Sweet. Thank you for putting it in terms this knucklehead can grasp. 😀