Spooky Entanglement

Spooky season's in full swing. For some, Halloween fun comes from jump scares in horror movies. For others, it's the sweet, sweet rewards from a night of trick-or-treating. But in the world of physics, "spooky" doesn't mean frights and fun. It's instead associated with a quantum phenomenon that Einstein famously described in a letter to Max Born, a physicist paving the way in quantum mechanics, as ‘spukhafte Fernwiwirkung’, or ‘spooky action at a distance’.

This unusual phenomenon is called quantum entanglement, and it occurs when two or more quantum particles become linked. There’s no physical connection binding them together – no invisible golden string – but the behaviour of one entangled particle will influence the other, regardless of the distance between them.

When first proposed, entanglement challenged two notions of classical physics: locality, which says that an object can only be influenced by something nearby, and realism, the idea that objects have well-defined properties whether we look at them or not.

Einstein helped create quantum physics, a field of physics now over 100 years old, but he thought it was an incomplete theory. “He recognized that it was correct – he didn’t think it was completely crazy – but he thought it was just one stepping stone to a better model,” says Damian Pope, Outreach Scientist at Perimeter Institute.

Einstein’s hesitations led him to release a paper with physicists Boris Podolsky and Nathan Rosen in 1935 that questioned the completeness of quantum mechanics. Nicknamed the ‘EPR’ paper, the authors worked through a thought experiment about two entangled particles, concluding that quantum mechanics disobeyed the concepts of locality and realism, which at the time were accepted ways of understanding our physical world. Einstein and his colleagues concluded that quantum mechanics didn’t include all the elements of physical reality, and so was an incomplete theory. 

In 1964, a physicist named John Stewart Bell published a paper tackling the EPR paper’s thought experiment. His calculations – since proven correct in experiments (including work by Alain Aspect, John Clauser, and Anton Zeilinger that jointly won the Nobel Prize in Physics in 2022) – showed that there are no local hidden variables underlying the connection between entangled particles. Quantum mechanics, in other words, does in fact violate locality. Spooky indeed.

Visualizing entangled particles is no easy feat but Pope offers a simple visual to understand it. Imagine that you have two coins. Flip them at the same time and see the results: sometimes they’ll come up the same (both heads or tails), while other times, you’ll get one head and one tails and they’ll disagree. Although they’re a ‘matched pair’, one coin doesn’t affect the outcome of the other coin’s toss. This is how relations work in the familiar everyday world.

Entanglement is also connection, but instead of between coins or other familiar objects, it’s between two quantum particles. And quantum particles don’t follow the same rules as the world of classical physics. If you entangled two “quantum coins” so that one always agrees with the other, they’ll always flip up as either both heads or tails. Entanglement leads to connections that you just can't get in the everyday world.

Until they’re observed, quantum particles don’t exist in a fixed state. “Some people talk about it as a fuzzy reality,” explains Damian. “Before you look at an atom, it hasn’t decided whether it’s going to be here,” he says, pointing to the table in front of him, “or here, or here. It’s somehow in a weird limbo of not having decided. Until you look at it, and then you see it here,” he says, pointing firmly at a spot before him.

Let’s take our coin example to the quantum world. Imagine that two people are each handed a quantum entangled coin. The two people drive to opposite sides of Canada: Person A goes to Vancouver, British Columbia, and Person B to St. John’s, Newfoundland and Labrador. They both flip their coins at the same time and person A discovers that their coin is tails – instantly, faster than the speed of light, they would know that the coin in St. John’s is also tails. But here’s where things get extra tricky – Before it was flipped, the coin in Vancouver had an equal chance of being heads or tails. Only in the moment that the coin was flipped and ‘settled’ on tails, did the coin in St John’s become tails.

Swap coins for quantum particles and you have quantum entanglement: two objects whose state determines that of the other, regardless of distance. 

While there’s a lot that we understand about entanglement, says Pope, there’s still plenty more to discover. 

“We don’t really understand how it works, or really how anything works in quantum physics. But to me that’s exciting, not depressing. And even though we don’t fully understand it, we can use it as a tool,” says Pope. “I don’t fully understand how the electronics of my TV remote works, but I know how to use it and what buttons to press. To me, entanglement’s a little bit like that.”

Pope says quantum entanglement isn't really that spooky. Instead, it's more a "Swiss Army Knife in quantum physics" and Perimeter Institute physicists are using it to research quantum computers, states of matter, quantum gravity, and other emerging research topics.

So maybe it’s time to put the ‘spooky’ descriptor for quantum entanglement to rest. Let’s save the chills and thrills for Halloween night, and instead look at quantum entanglement as an exciting method to uncover the workings of the quantum world.