Particles have been entangled ‘on demand’ for the first time, and it’s a game changer

Quantum technology promises big things for digital security and computing power, but the very thing it relies upon – quantum entanglement – has so far proven too fickle to reliably control.

A new method for entangling particles is set to change that, providing the all-important quantum states when we want it, for as long as we need it.

Physicists from QuTech in Delft in the Netherlands are no stranger to the whimsies of quantum entanglement – the phenomenon where two particles become inextricably linked so that whatever happens to one instantly affects the other, no matter how far away it is. Famously, Einstein called entanglement ‘spooky action at a distance‘ because .

In 2015, Delft physicists nailed shut a pesky loophole that could have shown it was less ‘spooky’ than Einstein ever suggested.

In their 2015 experiment, they used transmitted photons to entangle two electrons over a distance of 1.3 kilometres.

The origins of the photons and the distances apart were used to show this whole magic act was indeed truly weird but real, experimentally confirming quantum entanglement without any loopholes for the first time.

But that same method of mixing photons with electrons has now proven useful in another way – they play a key role in a method that can generate 40 entanglements on demand in a single second.

“This is a thousand times faster than with the old method,” says physicist Peter Humphreys, putting it into perspective.

The entanglement itself is formed using their original method – two separated electrons existing in an undecided state are each hit with a photon.

The two photons are then combined into a single wave and interpreted, revealing information about the states of the two electrons. If all goes well, the electrons can be considered entangled.

This doesn’t always work out as planned, but by setting up an assembly line with some clever checks and balances, the whole process can be repeated quickly enough to promise entangled particles.

“These checks only take a fraction of the total experimental time, while allowing us to ensure that our system is ready for entanglement, without any manual action,” says the project’s leader, Ronald Hanson.

To get a better idea of how the process works, this cartoon does a great job at explaining.

So how does this work? The whole process of entanglement itself isn’t all that weird. No, really. Bear with me.

Imagine you buy a pair of shoes, but accidentally leave one of them (left or right) at the shop. When you get home, you will know immediately which one you left behind by looking in the box.

This is more or less the same as quantum entanglement. As any two particles interact, they’ll each affect the other in a predictable fashion. You can look at one to determine something about the state of the other, whether it’s momentum or spin or position.

What left Einstein flummoxed was the suggestion by fellow physicists that before you look inside the box, that shoe is both left and right at the same time. And so is the one that was left behind.

In other words, reality as we experience it doesn’t have a concrete state until it’s put into perspective by measuring it. Particles have up and down spins, are in multiple positions, and haven’t settled on a particular momentum.

So from a quantum perspective, once you see the left shoe in your box, the one that was left behind immediately clicks into being the right shoe.

Einstein thought it was ridiculous. He figured the shoes were there all along. Others disagreed, believing there was some sort of natural conspiracy between the two shoes that caused them to agree the very moment you looked.

As weird as it is, Einstein is still almost certainly wrong. And we just need to deal with that, because quantum entanglement has been proven time and time again.

Fortunately this whole undecided left shoe-right shoe issue is useful. Or rather, the mathematics that describe that maybe-state is useful, if you apply the right statistical tools to analyse it.

That’s what’s behind the business end of a quantum computer – undecided quantum states called qubits make short work of problems that would be nigh impossible if you had to crunch those same statistics the old fashioned way using normal 1s and 0s.

And Einstein’s ‘spooky’ conspiracy behind the scenes of reality? That’s also useful if you want to make sure nobody has snuck a peek at your super-secret encrypted message or transmit an entangled state in something like a quantum network.

But holding particles in a ‘left shoe/right shoe’ state is far from easy. Reality is keen to open that box for you.

Which is why this latest experiment is so exciting.

This rapid fire, trial-and-error method is combined with ways to ensure the quantum state is protected from outside interference, meaning for the first time entanglement can be created faster than it’s lost.

Picking up the pace on quantum technology is important if we’re to overcome the numerous obstacles in the way of making quantum computing a practicality. Spooky or not, we’re slowly conquering the fragile nature of reality bit by qubit.

This research was published in Nature.

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