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This new experiment will allow us to 'see' quantum entanglement with the naked eye

And it could allow us to quantum entangle two humans...

1 MAR 2016

An experiment that would allow humans to directly perceive quantum entanglement for the first time has been devised by researchers in Switzerland, and they say the same technique could be used to quantum entangle two people.

While it would be incredibly cool to be the first person ever to witness quantum entanglement with your own eyes, the experiment has been designed to answer some important and far-reaching questions, such as what does quantum entanglement actually look like, and what does it feel like to be entangled with another human being?


Quantum entanglement is a strange phenomenon where two quantum particles interact in such a way that they become deeply linked, and essentially 'share' an existence. This means that what happens to one particle will directly and instantly affect what happens to the other - even if that other particle is many light-years away.

If that sounds like too much to wrap your head around, don’t worry - even Einstein himself struggled with it. 

His problem was that for quantum entanglement to work, it had to go against his special theory of relatively, because it requires information to travel faster than the speed of light. This prompted him to refer to entanglement as "spooky action at a distance", which is basically Einstein's way of saying he thought the whole thing was absurd.

For decades, physicists denied the very existence of quantum entanglement, but now scientists are entangling particles in huge quantities in labs all over the world, and it’s the  very basis of quantum computing - a technology that's expected to change everything about how we process and store information in the future. (For example, Google's 'quantum computer' is 100 million times faster than your laptop, and it’s not even a proper quantum computer.)

Now that we can quantum entangle photons - or light particles - in the lab with relative ease, a team led by Valentina Caprara Vivoli from the University of Geneva asked the question, what will it take for humans to see this process with their own eyes?

The premise is that the human eye is basically a photon detector, so, in theory, we should be able to replace the photon detecter in an entanglement detecting experiment with a human eye, and use people to observe the process instead. 

The experiment would just need to send several pairs of entangled photons to the human photon detector, and the process would have to be repeated again and again for them to confirm statistically if entanglement has actually taken place.

Unfortunately, in reality, it’s not that simple, as MIT Technology Review explains:

"The main problem is that the eye cannot detect single photons. Instead, each light-detecting rod at the back of the eye must be stimulated by a good handful of photons to trigger a detection. The lowest number of photons that can do the trick is thought to be about seven, but in practice, people usually see photons only when they arrive in the hundreds or thousands.

Even then, the eye is not a particularly efficient photodetector. A good optics lab will have photodetectors that are well over 90 percent efficient. By contrast, at the very lowest light levels, the eye is about 8 percent efficient. That means it misses lots of photons."

That means seven photons have to be entangled for the human eye to perceive them, and that’s the bare minimum - if you want someone to reliably see it, you’re going to have to be firing off hundreds, or even thousands, of quantum entangled photons, and that’s not possible with today’s technology.

So what’s the solution? Amplify what is possible with today’s technology, so the human eye can detect it better. 

That’s what Vivoli and her team have figured out how to do in their hypothetical experiment, using a process called 'displacement operation', which causes two particles to interfere so that the phase of one is altered by the other.

As Futurism explains, physicists could use a device called a beam splitter to achieve this:

"A beam of coherent photons from a laser is aimed at a splitter, transmitted through it, but then a change of phase causes it to be reflected. If a second laser beam interferes with the first, it will change the phase of the first beam - reflecting it back instead of passing through the splitter. 

So the second beam controls whether the first is reflected or not. This 'switching beam' does not need to be as powerful as the main beam, but it does need to be coherent - that is, an actual laser - to make this successful."

So in other words, you’re using one entangled photon to change the passage of a more powerful beam to demonstrate that they’re linked, and this more powerful beam is the one human observers would be able to see

The details of the hypothetical experiment have been published on the pre-print website, and are yet to be peer-reviewed, so we’ll have to take Vivoli et al.'s word for it at this stage, but they say it "convincingly demonstrates the possibility to realise the first experiment where entanglement is observed with the eye".

Another limitation is the fact that sitting down and watching thousands - or probably tens of thousands - of beam-splitting experiments isn’t exactly glamorous, even with the possibility of being the first human to witness quantum entanglement with your own eyes. Participants are probably going to need an incentive to not succumb to microsleeps every 30 seconds. 

The answer? Quantum entangle your participants, of course.

"One way to increase this motivation would be to modify the experiment so that it entangles two humans. It's not hard to imagine a people wanting to take part in such an experiment, perhaps even eagerly," says MIT Technology Review. "That will require a modified set-up in which both detectors are human eyes, with their high triggering level and their low efficiency. Whether this will be possible with Vivoli and co's setup isn't yet clear."

If you're still a little lost about how quantum entanglement actually works, miniature versions of Derek from Veritasium are here to help:

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