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The strange rules of the quantum world lead to many weird phenomena. One of these is the puzzling process of quantum imaging, which allows images to form in hitherto unimagined ways.

Researchers begin by creating entangled pairs by sending a single laser  beam into a non-linear crystal, which converts single photons into entangled pairs of lower frequency photons, a process known as parametric down conversion. A continuous beam generates a series of pairs of entangled photons. 

Next, they send the entangled photons towards a pair of detectors. Each member of an entangled pair by itself fluctuates in random ways that make its time and position of arrival uncertain. 

Use one of the detectors to receive just one half of the entangled photons and the result is a blur, smeared by the process of randomness.  

But use two detectors to receive both sets of photons and the uncertainties disappear, or at least are dramatically reduced. In this case, the ‘image’ is pinsharp. The uncertainty disappears because of the quantum correlation between the entangled pairs.

Researchers have extended this technique by superimposing a pattern on the wavefront of the initial laser beam, creating shapes such as a donut. They’ve shown that a single detector alone cannot ‘see’ a such a donut image even though it appears clean and sharp when two detectors pick up both sets of the entangled pairs.

These strange pictures are called quantum images or higher order images and quantum physicists think they can use them to carry out exotic processes such as sending information secretly and performing quantum lithography.

Today, Geraldo Barbosa at Northwestern University in Evanston, Illinois, raises another interesting possibility. He asks whether it is possible for humans to see higher order images and suggests that a relatively simple experiment could settle the question. 

This experiment consists of a laser beam shaped into an image, such as the letter A. This laser then hits a non-linear crystal, generating entangled pairs of photons that retain this image shape. The set up is such that these photons are then detected, not by conventional detectors, but by human eyeballs.

The question is whether the human retina/brain combination can access the correlation that exists between the entangled pairs. If so, the human would see the letter A. If not, he or she would see only a blur. 

Of course, there are some significant experimental challenges. One is to design the experiment in a way  that ensures the subject can only receive the image through this quantum process and not through some other channel, such as talking to the experimenter. However, that should be straightforward for any psychologist to design.

Another problem, however, is that the retina can only detect photons in groups of 7 or more and these have to arrive within a specific time window. Only then can a human subject ‘see’ the result. Generating the required intensity of entangled photons is one challenge.

The key question is whether the entanglement survives this group process. If the brain can access the quantum correlations, the image will be visible. If not, the result will be a blur.

That’s a fascinating experiment not least because a positive result would be astounding. It would show that we humans can essentially ‘see’ entanglement.

Barbosa points out that new forms of imaging are not unknown in the animal world. Various animals and insects see in the infrared and ultraviolet, giving them an entirely different perspective on the world. 

There is also some evidence that birds can ‘see’ the earth’s magnetic field thanks to the quantum interaction between the field and light sensitive molecules in their retinas. 

So the possibility that new ways of seeing the world can emerge is not unprecedented. However, the idea that humans can access higher order images thanks to quantum entanglement is clearly an idea of a different ilk.

Perhaps the most exciting aspect of Barbosa’s idea is that it appears feasible now. There’s no reason why this experiment couldn’t be done in any quantum optics lab in the near future.

We’ll look forward to seeing the results.

Ref: arxiv.org/abs/1202.5434: Can humans see beyond intensity images?

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