For some time, physicists have pondered the possibility that the strange phenomenon of quantum entanglement might allow them to take pictures of objects they cannot see. The idea works like this…

Create two beams of entangled photons and direct the first towards the object you want to photograph while aiming the second beam towards a camera that can record an image. The thinking is that the second beam can be used to create an image of the object thanks to the spooky effects of entanglement with the first.

Back in the 2000s, various physicists disagreed over whether this was actually possible. A couple of experiments appeared to show that it was.

But other groups later showed that quantum effects were not responsible for the resulting images and that classical light could do the trick just as well, an effect known as ghost imaging (although quantum entanglement could make the images sharper).

Now the tables have been turned once again. Anton Zeilinger at the University of Vienna in Austria and a few pals, have produced the first quantum images of an object that are entirely the result of entanglement. “The photons passing through the object are never detected, and the signal photons that are detected never interact with the object,” they say.

The technique allows them to detect an object using one wavelength and view it at another, even though the object is invisible at the wavelength used to form the image.

The trick is in the experimental setup. This consists of a single beam of green photons that is divided into two. The first of these passes through a crystal that converts each photon into an entangled pair, one of which is red and the other yellow.

The red photon goes on to pass through the target object, a cardboard cut-out shape, while the yellow is directed towards a pair of photon detectors.

The second green beam also passes through a crystal that converts each photon into a red and yellow entangled pair. The beam of red photons is combined with and interferes with the other beam of red photons that have passed through the target. This combined red beam is then dumped—these red photons are never measured.

However, the second yellow beam is combined with the first and interferes with it before hitting the photon detectors.

The important point is that there is no way of telling the photons in the final beams apart. It is impossible, therefore, to know which of the red photons interacted with the target and which didn’t. That’s true even though no measurement is made on them.

Similarly, there is no way of telling which yellow photons are entangled with the red photons that interacted with the target. It is this inability to tell the photons apart that ensures the yellow photons hold information about the interaction with the target.

Indeed, the image appears almost as if the yellow photons have actually interacted with the target. And there is a sense in which this is true since, because they are entangled, the red and yellow photons share the same existence even though they are physically separated.

The result is a clear image of the target, in this case the outline of a cat (presumably of the Schrödinger variety).

Zeilinger and co have gone to considerable trouble to exclude other explanations for the result. For example, the photon detectors they use are sensitive to yellow light but not to red or green, so the image can only be the result of the detection of yellow photons.

What’s more, they repeat the experiment with an etched silicon plate that blocks red light but is transparent to yellow light. So the target is invisible when viewed with yellow light and yet the image still appears.

That’s a fascinating result that takes quantum imaging to a level beyond the ghost imaging debate from the naughties.

And it has the potential for entirely new kinds of photographs. I’ve talked here about green, red and yellow photons to make the experiment easier to imagine but the actual colours are green, near infrared and mid infrared. So this experimental setup makes it possible to zap an object with mid-infrared photons and then create an image by detecting near infrared photons.

Indeed, a wide variety of wavelength combinations are possible. “An object can be probed with light ranging from UV through mid-infrared or possibly even the THz regime while the image is detected at a freely chosen wavelength where detectors are technologically available or exhibit superior performance,” say Zeilinger and co.

The possibilities are clearly numerous. Zeilinger and co talk about medical and environmental imaging but there are obviously other options. Suggestions please in the comments section.

Ref: arxiv.org/abs/1401.4318 : Quantum Imaging with Undetected Photons