John Pickrell - The Best Australian Science Writing 2018

The origins of entanglement

Robyn Arianrhod

It all began in October 1927, at the Fifth Solvay Congress in Brussels. It was Louis de Broglies first congress, and he had been full of pleasure and curiosity at the prospect of meeting Albert Einstein, his teenage idol. Now 35, de Broglie happily reported: I was particularly struck by his mild and thoughtful expression, by his general kindness, by his simplicity, and by his friendliness.

Back in 1905, Einstein had helped pioneer quantum theory with his revolutionary discovery that light has the characteristics of both a wave and a particle. Niels Bohr later explained this as complementarity: depending on how you observe light, you will see either wave or particle behaviour. As for de Broglie, he had taken Einsteins idea into even stranger territory in his 1924 PhD thesis: if light waves could behave like particles, then perhaps particles of matter could also behave like waves! After all, Einstein had shown that energy and matter were interchangeable, via E = mc2.

Einstein was the first to publicly support de Broglies bold hypothesis. By 1926, Erwin Schrdinger had developed a mathematical formula to describe such matter waves, which he pictured as some kind of rippling sea of smeared-out particles. But Max Born showed that Schrdingers waves are, in effect, waves of probability. They encode the statistical likelihood that a particle will show up at a given place and time based on the behaviour of many such particles in repeated experiments. When the particle is observed, something strange appears to happen. The wave-function collapses to a single point, allowing us to see the particle at a particular position.

Borns probability wave also fitted neatly with Werner Heisenbergs recently proposed uncertainty principle. Heisenberg had concluded that in the quantum world it is not possible to obtain exact information about both the position and the momentum of a particle at the same time. He imagined the very act of measuring a quantum particles position, say by shining a light on it, gave it a jolt that changed its momentum, so the two could never be precisely measured at once.

When the worlds leading physicists gathered in Brussels in 1927, this was the strange state of quantum physics.

The official photograph of the participants shows 28 besuited, sober-looking men, and one equally serious woman, Marie Curie. But fellow physicist Paul Ehrenfests private photo of intellectual adversaries Bohr and Einstein captures the spirit of the conference: Bohr looks intensely thoughtful, hand on his chin, while Einstein is leaning back looking relaxed and dreamy.

This gentle, contemplative picture belies the depth of the famous clash between these two intellectual titans a clash that hinged on the extraordinary concept of quantum entanglement.

* * * * *

At the congress, Bohr presented his view of quantum mechanics for the first time. Dubbed the Copenhagen interpretation, in honour of Bohrs home city, it combined his own idea of particlewave complementarity with Borns probability waves and Heisenbergs uncertainty principle.

Most of the attendees readily accepted this view, but Einstein was perturbed. It was one thing for groups of particles to be ruled by chance; indeed, Einstein had explained the jittery motion of pollen in apparently still water (dubbed Brownian motion) by invoking the random group behaviour of water molecules. Individual molecules, though, would still be ruled by Isaac Newtons laws of motion; their exact movements could in principle be calculated.

By contrast, the Copenhagen theory held that sub-atomic particles were ruled by chance.

Einstein began his attack in the time-honoured tradition of reductio ad absurdum arguing that the logical extension of quantum theory would lead to an absurd outcome.

After several sleepless nights, Bohr found a flaw in Einsteins logic. Einstein did not retreat: he was sure he could convince Bohr of the absurdity of this strange new theory. Their debate flowed over into the Sixth Solvay Congress in 1930, and on until Einstein felt he finally had the pieces in place to checkmate Bohr at the seventh congress in 1933. Two weeks before that, however, Nazi persecution forced Einstein to flee to the United States. The planned checkmate would have to wait.

When it came, it was deceptively simple. In 1935 at Princeton, Einstein and two collaborators, Boris Podolsky and Nathan Rosen, published what became known as the Einstein-Podolsky-Rosen paradox, or EPR for short. Podolsky wrote up the thought experiment in a mathematical form, but let me illustrate it with jellybeans.

Suppose you have a red and a green jellybean in a box. The box seals off the jellybeans from all others: technically speaking, the pair form an isolated system, and they are entangled in the sense that the colour of one jellybean gives information about the other. You can see this by asking a friend to close her eyes and pick a jellybean at random. If she picks red, you know the remaining sweet is green.

This is key to EPR: by knowing the colour of your friends jellybean, you can know the colour of your own without disturbing it by looking at it. But in trying to bypass the supposed observer effect in this way, EPR had also inadvertently uncovered the strange idea of entanglement. The term was coined by Schrdinger after he read the EPR paper.

So now apply this technique to two electrons. Instead of a colour, each one has an intrinsic property called spin. Imagine something like the spin axis of a gyroscope. If two electrons are prepared together in the lab so that they have zero total spin, then the principle of conservation of angular momentum means that if one of the electrons has its spin axis up, the other electrons axis must be down. The electrons are entangled, just as the jellybeans were.

With jellybeans, the colour of your friends chosen sweet is fixed, whether or not she actually observes it. With electrons, by contrast, until your friend makes her observation, quantum theory simply says there is a 50 per cent chance its spin is up, and 50 per cent it is down.

The EPR attempt to strike at the heart of quantum theory now goes like this. Perhaps the spin of your friends electron was in fact determined when she picked it out. However, like a watermark that cant be detected until a special light is shone on it, the spin state is only revealed when she looks at it. Quantum spin then involves a hidden variable, yet to be described by quantum theory. Alternatively, if quantum mechanics is correct and complete, then the theory defies common sense because as soon as your friend checks the spin of her electron, your electron appears to respond instantly, because if hers is up then yours will be down.

This is because the correlation between the two spins was built into the experiment when the electrons were first entangled, just as putting the two jellybeans in a box ensures the colour of your jellybean will be opposite that of your friends. The implications are profound. Even if your friend moved to the other side of the galaxy, your electron would know that it must manifest the opposite spin in the instant she makes her observation.

Of course, instant action violated Einsteins theory of relativity: nothing can travel faster than the speed of light in a vacuum. Hence Einstein dubbed this absurd proposition spooky action at a distance.

But there was more. Spin is not the only property your friend could have chosen to observe. What EPR showed, then, is that the physical nature of your electron seems to have no identity of its own. Rather, it depends on how your friend chooses to observe her electron. As Einstein put it: Do you really believe the Moon is there only when you look at it? The EPR paper concluded: No reasonable definition of reality could be expected to permit this. Ergo, the authors believed, quantum theory had some serious problems.