On Tuesday the 4th of October, 2022, the Royal Swedish Academy of Sciences awarded Alain Aspect, John F. Clauser, and Anton Zeilinger the Nobel Prize in Physics. On one hand, this was a surprise — for winning a Nobel is no small feat. On the other hand, this was no surprise at all. These three physicists are all well known both within and without physics and it has long been anticipated that they would share the prize one day. So, why did they win?
You can read the full Nobel press release here, but these are the key sections:
“Alain Aspect, John Clauser and Anton Zeilinger have each conducted groundbreaking experiments using entangled quantum states, where two particles behave like a single unit even when they are separated. Their results have cleared the way for new technology based upon quantum information.”
“John Clauser developed John Bell’s ideas, leading to a practical experiment. When he took the measurements, they supported quantum mechanics by clearly violating a Bell inequality. This means that quantum mechanics cannot be replaced by a theory that uses hidden variables.”
“Some loopholes remained after John Clauser’s experiment. Alain Aspect developed the setup, using it in a way that closed an important loophole. He was able to switch the measurement settings after an entangled pair had left its source, so the setting that existed when they were emitted could not affect the result.”
“Using refined tools and long series of experiments, Anton Zeilinger started to use entangled quantum states. Among other things, his research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance.”
That should clarify everything. Aspect, Clauser, and Zeilinger experimentally confirmed John Bell’s theorem, lay the foundations for a new generation of quantum technologies based upon entanglement, and proved that “quantum mechanics cannot be replaced by a theory that uses hidden variables.” But hold on for a second. I’m not a physicist, but that last claim doesn’t sound right — Bell’s theorem didn’t rule out hidden variables. It is a common misconception that it did, but Bell only ruled out local hidden variables (and even that is contested). Nonlocal hidden variables, such as David Bohm’s pilotwaves, remain entirely plausible. In fact, Bell was a prominent and outspoken proponent of pilot-wave theories. Sadly, most media coverage of this year’s prize has overlooked this point, and most outlets have absentmindedly parroted a whole lot of quantum nonsense. There have been some notable exceptions, such as this exceptional piece written by MIT professor David Kaiser, but on the whole entanglement has been portrayed as ‘spooky,’ ‘weird,’ and capable of the impossible.
So, what is entanglement? Kaiser explains quantum entanglement by writing that:
“Quantum theory predicts that if two or more particles are prepared in a particular way, their behavior should remain correlated, even after they have moved very far apart. Perform a measurement of some property of particle A and—wham!—particle B should display specific behavior, even if they are light-years apart. Stranger still, the particles’ properties should align even when physicists measure different properties of each particle, selecting the measurements to perform on each member of the pair at random.”
Essentially, if two particles locally interact their states become correlated so that they are indistinguishable from one another. These correlated particles can then be separated, and the correlation remains. That is fascinating, but what is really going on?
According to the standard Copenhagen Interpretation of quantum mechanics, after entanglement both of these particles are said to exist in an indefinite quantum state and remain so until the next interaction, at which point both states collapse into one definite state regardless of which particle is measured and how far apart they are. This is what Albert Einstein was talking about when he uttered the words ‘spooky action at a distance.” Entanglement itself wasn’t spooky — it is just a form of correlation — it was the nonlocal collapse that unsettled Einstein. But there are also other possibilities. For example, there could be nonlocal hidden variables, as in Bohmian Mechanics, or the measured state could have been predetermined during the original interaction, a solution promoted by superdeterminism. Admittedly, Bohmian mechanics and superdeterminism are not very popular amongst physicists — and they are both strange in their own ways — but they are listed here to show that the interpretive landscape is expansive.
So, does entanglement mean that information can travel faster than light? No. Does it mean that particles can maintain a physical connection over millions of kilometres (or miles for those still on the imperial standard)? No. Does it prove that reality is non-local (whatever that means)? No, some interpretations of quantum mechanics are non-local, but many others (such as QBism and Relational Quantum Mechanics) are decidedly local. Entanglement is just a form of quantum correlation (this is not exactly the same as ‘classical correlation,’ and involves quantum contextuality, but that is a story for another time and another blog post). Lastly, does this year’s Nobel Prize in Physics signify anything for fundamental physics as a whole? No. Aspect, Clauser, and Zeilinger have done more than their fair share for physics and the prize is well deserved, but Nobels are retrospective, and their contributions have long been incorporated into modern physics. In time, their work, and the work of countless other physicists, will be built upon and we will arrive at a consensus regarding the proper interpretation of quantum mechanics. However, for now, reality remains mysterious — and there is great beauty in the mystery.
Nevertheless, the conferral of this Nobel to Aspect, Clauser, and Zeilinger bodes well for the burgeoning field of quantum information science and the quantum technology industry more broadly. As Kaiser notes in his analysis, mainstream physics “considered entanglement to be a sideshow” and for much of the seventies, eighties, and nineties the trailblazing works of Aspect, Clauser, and Zeilinger were met with silence. Entanglement was a curious feature of quantum mechanics, but it was not considered worthy of the attention of serious physicists. Yet today entanglement sits at the centre of the rapidly developing quantum technology industry which promises to transform modes of production, war, peace, and security. Without hyperbole, Aspect, Clauser, Zeilinger, and their previously neglected works laid the foundations for the next technological revolution. It remains to be seen what this revolution will look like, but Aspect, Clauser, and Zeilinger nonetheless deserve praise for the progress they have brought about.
The Project Q team sends their belated congratulations to Alain Aspect, John F. Clauser, and Anton Zeilinger!
For more on entanglement and this years Nobel Prize in Physics, please read David Kaiser’s article here, or this excellent article by Chris Ferrie.