Why Did Quantum Entanglement Win the Nobel Prize in Physics?(script) (Patreon)

The Nobel prize in physics is typically awarded to scientists who make sense of nature; those whose discoveries render the universe more comprehensible. But the 2022 Nobel has been awarded to three physicists who revealed that the universe is even stranger than we thought.

[Intro]

This year’s physics Nobel laureates are John Clauser, Alain Aspect, and Anton Zeilinger, who together are responsible for a series of ingenious experiments that proved that the strangest prediction of quantum mechanics is actually true.

It’s the prediction that Einstein refused to accept - the idea that two quantum systems can be entangled - bound to each other such that they can influence each other instantaneously over any distance. This “spooky action at a distance” as Einstein called it, is in seeming violation of his own theory of relativity, which tells us that no causal influence can travel faster than light. Clauser and Aspect achieved the rarest of feats - they proved Einstein wrong, while Zeilinger greatly advanced our understanding and practical application of the phenomenon of quantum entanglement.

Of course, we’ve talked about quantum entanglement once or twice in the past, but today I want to tell you all about the series of brilliant experiments celebrated by this year’s Nobel. Let’s start with a simple thought experiment. You have two balls - one black, one white. You close your eyes, shuffle the balls, and place each in an identical box.

You put the first box in a rocket and send it to the moon, while you keep the second box. While that box is closed, the ball on the moon has a 50-50 chance of being black or white. Open your box and you instantly learn the color of the ball on the moon. Did you cause information to travel faster than light? Of course not - the information about the balls colors was always there with the balls - you just learned which was which.

Now imagine these are quantum balls with entangled “quantum” colors. According to quantum mechanics, we not only don’t know which is which until the box is open, but the colors of the balls are fundamentally undefined. Each ball is in a superposition state of maybe black-maybe white until a measurement is made. Opening the box causes the observed ball to have to choose a color state, which then forces the ball on the moon to choose the opposite. Now you really do seem to have an effect that travels faster than light, with the ball on the moon switching from a superposition state to a defined state as soon as you make your observation. These “quantum balls” could be any particle from subatomic to molecular scale, and the entangled property could be spin, momentum, or any other quantum property.

But why do we have to accept this wacky interpretation? Is there any real difference in the result of the experiment if the color of the balls is defined at the beginning versus when the box is open? Why even propose this entanglement and superposition stuff?

Well, because standard quantum mechanics says so. Quantum systems are described by a mathematical object called the wavefunction, which evolves according to the Schrodinger equation. The joint wavefunction of two entangled objects only contains information about the correlation between those objects, not about their specific values. They only gain specific values when observed and the wavefunction “collapses”. For our quantum balls to know their own color the whole time, there would need to be extra information not contained in their wavefunction. There are a few different interpretations of quantum mechanics that allow this hidden information, and they’re collectively known as hidden variable theories.

Einstein thought that such hidden information must exist, while others like Neils Bohr insisted that the wavefunction was the complete description of a quantum system. In almost every scientific debate, physicists side with Einstein. But not in this case. Quantum mechanics was just too successful, and Neils Bohr was aggressive in pushing his Copenhagen interpretation. It became dogma, and for a while it was career suicide to question the orthodoxy, for example by researching hidden variable theories. David Bohm got the worst of that with his pilot wave theory, which we talk about in another video.

John Stuart Bell was another hidden-variable heretic. This Irish physicist didn’t necessarily believe in them, but he wasn’t content to just accept the Copenhagen interpretation without properly testing it. And he also realized that one could reveal the existence of hidden information without actually measuring that information, or even testing a specific hidden variables theory. We’ve talked about these Bell tests previously, if you want to see exactly how this works. In short though, in 1964 Bell came up with his Bell theorem, which shows that there should be a particular statistical relationship between the measured properties of entangled particles  if the particles themselves hold the information about their internal states, and a different statistical relationship if those properties really are decided at the moment of measurement. In particular, the so-called Bell inequality would be true if there are hidden variables contained in the particles, and violated otherwise.

Finally there was a real test that could test for hidden variables. It wasn’t until 1969 that the first Bell test was performed - and that was by one of our 2022 Nobel laureates, John Clauser, along with his student Stuart Jay Freedman. Why did it take 5 years? Well, Bell tests are really hard to perform. They require the production of entangled states, which have to be manipulated and measured without breaking the exceedingly delicate correlation between the particles. But also, to do a Bell test was to question the status quo, so it was hard to get support for the significant effort required. Clauser talks about the time he proposed his idea for the experiment to Richard Feynman, who promptly kicked him out of his office. Apparently Feynman thought it was pointless because standard quantum mechanics was clearly correct.

Clauser and Freedman were not to be deterred. They’d figured out a brilliant experiment and were going to try it. It went like this. They blasted a beam of calcium atoms through the intense light of an arc lamp. That light excited electrons in calcium atoms to higher energy levels, which would then drop down again, with the lost energy carried away by photons. One of the possible electron transitions was between two states that had zero quantum spin, and which also resulted in the creation of two photons.  Spin is the quantum version of angular momentum. Because the atom’s spin hadn’t changed in this transition, in order to conserve angular momentum the pair of photons needed to have a total spin of zero, which translates to them having opposite circular polarizations.

Standard quantum mechanics says that those polarizations are undefined until measurement, when they will always turn out to be opposite to each other. Hidden variable theories, on the other hand, allow the polarization to be set at the moment the photons are created. By measuring these polarizations by passing both photons through polarizers, Clauser and Freedman could perform a Bell test. Bell’s inequality was convincingly violated in their experiment, which means quantum mechanics was working exactly as expected, implying no hidden variables. Just as Feynman had told them.

But the case wasn’t quite closed. John Bell himself pointed out that hidden variables could still be a thing, even if the Bell test said otherwise. The result of a quantum measurement depends on how you make the measurement - in the case of this experiment, the orientation of the polarizers determined which polarizations the experiment was sensitive to. Bell’s theorem assumes the choice of measurement is completely free and independent of the particle creation process. In Clauser’s experiment, the polarizers were fixed in position through the whole experiment. Their orientation was already decided when the entangled photons were produced. What if that orientation has some influence on the polarization direction of the photons at the moment of their creation? Then the photons might carry hidden information about the eventual measurement direction the whole time and sort of conspire to look like standard quantum mechanics, even with real hidden variables.

To close this loophole it would be necessary to somehow set the measurement direction after the photons were produced. That sounds incredibly difficult, because in case you didn’t know photons move pretty fast. But when you’re a brilliant experimentalist, “incredibly difficult” is your bread and butter. And so we meet our second laureate, Alain Aspect.

Aspect’s setup was very similar to Clauser's, with a beam of calcium atoms excited by light - this time a laser rather than an arc lamp. But the main difference was in the polarizers. In order to change the measurement direction of a polarizer you have to rotate it, but it’s kinda hard to do that faster than a photon can travel across your optical bench. Aspect found a way to randomize measurement direction without moving the polarizers at all. The trick was to use a type of transducer. In this case a chunk of quartz that bends the path of light in different ways depending on whether the quartz is vibrating, and that vibration can be turned on and off with an electric current. That means our entangled photons could be sent to different polarizers depending on an electrical switch - a switch that could be turned on and off quickly and randomly in the tiny interval between the creation of the photon and their arrival at the transducer.

All of this means that the photons can’t know how they’re going to be measured at the moment of their creation, which if you recall was the potential loophole in Clauser and Freedman’s experiment. The Bell inequalities were also violated in Alain Aspect’s experiment, dealing another blow to hidden variable theories.

So did Aspect close the final loophole? Not quite. There are two ways that the Bell inequalities could be violated without quantum entanglement being as spooky as Einstein feared. Well, they’re actually still spooky, but in different ways. The first I already mentioned: what if the choice of measurement is not independent of the creation of the entangled particles? Aspect’s experiment seemed to eliminate that possibility by making that choice after the particles were produced. But what if the random number generator wasn’t really random? After all, signals could have travelled to both the calcium atom and the random number generator from some common influence, causing them to conspire to violate Bell inequalities.

This idea is called superdeterminism - basically stating that the particles are not only correlated with each other, but also with the random number generator or the physicist choosing the measurement direction, so that the universe has no choice but to always hide the existence of hidden variables. We’ve discussed superdeterminism before, so you can decide for yourself whether that’s a reasonable idea. John Bell certainly thought it was implausible.

But even without superdeterminism there’s another loophole. Bell’s theorem can be used to rule out the existence of local hidden variables. It can rule out that the secret information about the entangled particle states lives in the particles themselves - that’s what local means here.

But there could still be hidden variables that exist in the global wavefunction of the entangled particles. Clauser and Aspect’s experiments ruled out local hidden variables, but that may mean they ruled out locality rather than hidden information. Any violation of locality still means that some sort of influence travels faster than light - the sort of spookiness that Einstein hated. One way or another, our Nobel laureates have revealed a universe stranger than many are comfortable with.

OK, what about our third laureate, Anton Zeilinger? Clauser and Alain’s work was all about testing the fundamentals of quantum mechanics - about getting closer to what its weirdness is really telling us about the world. But their efforts also led to very practical results. They, and others like them, advanced our ability to create and manipulate entangled quantum states. And Zeilinger put these to good use. He may be the most famous for demonstrating quantum teleportation. This is a phenomenon in which a quantum state is transferred between two particles via an interrmediate particle that’s entangled with them both. We’ll save the awesome details for another time. The important thing to know right now is that quantum teleportation and the corresponding ability to move around quantum information is critical for quantum computers. Zeilinger is responsible for a number of advances in manipulating entanglement, and has applied these to quantum cryptography and to the development of quantum computers.

This is a rare episode in which both Einstein and Feynman were wrong. Einstein because, one way or another, the quantum world is indeed quite spooky. And Feynman was right in thinking that Clauser would never disprove standard quantum mechanics - but he was wrong in thinking that Clauser shouldn’t try. Because science only moves forward when we try to push its theories to the breaking point.

We learn something whether they break or not, and perhaps find technologies that will be useful in ways no one could have predicted. In this case, our better understanding of quantum entanglement has brought us very close to the age of quantum computing and quantum cryptography. All because a few scientists were willing to challenge the status quo, and seek the hidden secrets of space time.