Ghost imaging doesn’t involve anything supernatural, but it can concern itself with the spooky effects of quantum optics. Although it normally involves using computational wizardry, a new pre-print posted to arXiv – and spotted by New Scientist – explains how we can do it with the human eye.
So what the hell is ghost imaging, apart from something that indubitably sounds cool?
A 2017 paper, which goes into both quantum and classic variants, describes it as “imaging using light that has never physically interacted with the object to be imaged.” This essentially means that the visualization of something is happening without the object being directly observed. How is that possible?
According to Physics World, the process involves splitting a light beam into two separate beams. This splits high-energy photons into pairs of low-energy photons. One of those beams, or photons, are directed toward the object, and the other – the reference beam – heads straight to a light detector, one that lets all the light in.
At the same time, there is another light detector behind the object, one that lets but a single pixel of light in. We’ll call that detector the bucket, and it picks up on the photons that go through the object.
When a photon is transmitted and thus interacts with the object, its pair in the separate beam mirrors its properties. This is thanks to the fact that they are engaged in quantum entanglement, which is when two individual particles and their properties are linked to each other instantaneously despite having no physical connection.
That means that the beam of light that never itself interacts with the object is altered as it falls on the multi-pixel light detector. It doesn’t just come through as a clear image, though, which is why computers are used. They are able to use this technique to understand what the object is without getting any direct information about it.
The military, always keen to keep out of harm’s way if possible, have reportedly already developed a prototypical ghost imaging system that works at a distance of several kilometers. Unlike conventional image capture tech, taking advantage of this weird correlation of photons means that normal visual disturbances, like smoke and dust, have no effect.
It has recently been pointed out, though, that ghost imaging doesn’t have to involve any quantum alchemy at all.
Sometimes, the light bouncing off the object and into the bucket is sent as a series of alternating crisscross patterns. Lighter parts represent greater photon emission; darker parts less so. The light intensity received by the bucket are then compared and contrasted with each other using a computer.
After some careful overlapping, the shadow image is built up, again without directly photographing the object, but this time without requiring any quantum entanglement. Instead, classical physics rules the roost.
In any case, ghost imaging has always required computers to compare and contrast the received mirrored photons to build up an image. This new paper indicates we can do this ourselves.
A University of Glasgow-led team found that, as expected, showing people the reflected crisscross patterns on a projector separately doesn’t give them any idea what the object is. However, showing all of the received patterns in quick succession, say, 200 of them in 0.02 seconds, causes them to blur together enough to create an image. In this case, it was one of Albert Einstein, because of course it was.
This represents the first time ghost imaging has been achieved without using any computational technology – and a brand-new way to probe the visual systems afforded to us by millions of years of evolution.