Particle physics laboratory

The mystery of quantum entanglement is increasingly recreated in the laboratory – CVBJ


12/09/2021 at 08:00 CET

Nicolas bornman

“Quantum entanglement” is one of the many plot devices that appear in modern science fiction films.

Fans of Marvel superhero films, for example, will be familiar with the idea that different timelines merge and intersect, or that the fates of the characters intertwine in seemingly magical means.

But “quantum entanglement” isn’t just a science fiction buzzword. It is a very real, confusing and useful phenomenon. “Entanglement” is one aspect of the largest collection of ideas in physics known asQuantum mechanics

, which is a theory that describes the behavior of nature at the atomic, and even subatomic, level.

Understanding and taking advantage of interlacing is essential to creating many cutting-edge technologies. These include quantum computers, which can solve certain problems much faster than ordinary computers, and quantum communication devices, which would allow us to communicate with each other without the slightest chance of being overheard by a snoop. .

But what exactly is quantum entanglement?

In quantum mechanics, we say that two particles are entangled when one of the particles cannot be described perfectly without including all the information about the other: the particles are “connected” in such a way that they are not independent. one from the other.

While this sort of idea might seem logical at first glance, it’s a difficult concept to grasp, and physicists are learning even more about it.

Related topic: Quantum entanglement also works on massive objects

Play dicePlay dice

Suppose I give you and your friend Thandi a small opaque black box. Each box contains an ordinary six-sided die. They are both told to shake their boxes lightly to move the dice.

So they both go their separate ways. Thandi returns home to a South African town, Cape Town; and you return to another city, Durban. They do not communicate with each other during the journey. When you get home, each of you opens your box and looks at the number at the top of your die.

Usually there would be no correlation between the numbers you and Thandi see. She is just as likely to see any number between 1 and 6, just as you are.

More importantly, the number you see on his die would have nothing to do with the number Thandi sees on his. It’s not surprising ; in fact, this is how the world normally works.

However, if we could do this “quantum” example, the dice could behave very differently. Suppose I now tell Thandi and you to bang their boxes together, before shaking them separately and taking different paths.

In an analogy with quantum mechanics, this action of knocking the two boxes against each other would enchant the dice and tie them, or entangle them, in a mysterious way: once everyone has returned home, open their box and see how it happened. The dice: your number and Thandi’s will be perfectly correlated. If you see a ‘4’ in Durban, you know Thandi in Cape Town is guaranteed to have a ‘4’ on his die as well; if she sees a ‘6’, she will see it too.

In this analogy, the dice represent individual particles (such as atoms or particles of light called photons) and the magical act of physically bringing the boxes together is what entangles them, so the measure of a dice gives us information about each other.

Make a better messMake a better mess

As far as we know, there is no magical action to enchant a pair of dice or other objects on our human macroscopic scale (if there were any, we could experience the mechanics quantum in our daily lives and that probably wouldn’t be such a weird and confusing concept.).

For now, however, scientists have to be content with using things at the microscopic level, where it’s much easier to observe quantum effects, like charged atoms called ions or special superconducting devices called transmons.

This is the kind of work that takes place in the Structured Light Laboratory at the University of the Witwatersrand in South Africa.

However, instead of ions or transmons, researchers in the lab used particles of light, called photons, to better understand quantum mechanics and its implications.

We are interested in the use of the quantum nature of light for a variety of purposes: from designing efficient communication systems that are completely impossible to hack by a malicious third party, to creating methods to image sensitive biological samples without them. cause damage.

Studies like this often require that we start with specially created states of entangled photons. But it’s not as easy as putting two dice in separate boxes and bumping them against each other.

The processes used to create entangled photons in a real laboratory are limited by many experimental variables, such as the shape of the laser beams used in the experiments and the size of the small crystals where the entangled photons are created.

Manage the entanglementManage the entanglement

This work can give unsatisfactory results, or non-ideal states, which force researchers to selectively reject certain measurements once an experiment is performed. This is not an optimal situation: the photons are rejected and therefore energy is wasted.

A group of laboratory researchers, including myself, recently took a step toward solving this problem.

In a magazine article, we mathematically calculated what the optimal shape of the laser should be in order to best create the state of entanglement that an experimenter would like to begin their experiment with.

The method proposes to change the shape of the input laser beam at the start of an experiment, in order to maximize the process of creating entangled photons in a second phase of the experiment.

This means that there will be more photons available to run an experiment the way you want, and fewer stray photons.

Improving the efficiency of the process of creating and manipulating entanglement, using techniques such as the one proposed, will be important for maximizing the efficiency of a number of other quantum technologies, such as systems of quantum cryptography and the other technologies already mentioned.

This is especially important as the Fourth World Industrial Revolution progresses and technologies based on quantum mechanics undoubtedly become more mainstream.

Nicholas Bornman is a graduate student at the University of the Witwatersrand in South Africa. is


Top photo: Geraltd. Pixabay.