Season 17, Episode 42: Quantum Entanglement with Whole Atoms

Hey StarTalkians! Episode 42 featured guest Dr. Sean Hodgman, who sat down with Neil and Chuck to discuss his research and answer some cosmic queries. He gave an overview of his research early in the show:  

Getting Entangled with Sean Hodgman- StarTalk Radio

(from 8:15)

But quantum mechanics is notoriously difficult to understand, so here’s a gentler run-through of the underlying physics and what Dr. Hodgman’s paper adds.  

Quantum Entanglement, Locality and Realism

Quantum entanglement is counter-intuitive, but you can understand it with a simple analogy.

Imagine you and a friend are stood opposite each other, each holding basketballs. Together, you throw them into the centre so they collide and bounce away. You can’t say exactly where they will bounce before you throw – it depends on the precise point of contact – but you do know they’ll bounce off in opposite directions. You can predict the whole system, in a way, but not each part individually.

Entanglement works like this except with very tiny particles and weirder variables. But unlike with the balls, there is no definite “direction” until you measure it, and then the other – instantly – takes on the corresponding value. In practice, the variables are usually things like polarity or quantum spin.

Einstein, Podolsky and Rosen criticised this, pointing out that values should be fixed before we measure them (“realism”) and that a measurement at one place shouldn’t affect one somewhere else (“locality”). They instead argued that a hidden variable (i.e. the point of contact, in our analogy) determines things definitively.

Bell’s Inequality and Proving Einstein Wrong

But they were wrong. In 1964, John Stewart Bell set a limit on what local, hidden variable theories can explain. By incorporating a “hidden variable” into his calculations, he established a limit on the amount of correlation they’d allow between entangled particles.

Later tests would show that nature does violate Bell’s inequality, exactly as predicted by quantum mechanics.

Dr. Hodgman’s Research: Violating Bell’s Inequality with Atoms

As discussed on the podcast, most of these experiments use photons. But as Dr. Hodgman’s team showed , even atoms can display these excess correlations. They used laser pulses to collide pairs of super-cooled helium atoms, recording where they landed. They showed a violation of Bell’s inequality, which had never been done with the momentum states of massive particles. The result could lead to insights into gravity at the quantum scale, or new quantum information protocols.