Saturday, September 11, 2021

The Second Quantum Revolution

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]

Quantum mechanics is more than a hundred years old. That sounds like it’s the stuff of dusty textbooks, but research on quantum mechanics is more active now than a century ago. That’s because many rather elementary facts about quantum mechanics couldn’t be experimentally settled until the 1980s. But then, by the year 2000 or so, experimental progress had totally transformed the field. Today it’s developing faster than ever. How did this happen, why does it matter, and what’s quantum teleportation? That’s what we’ll talk about today.

Albert Einstein was famously skeptical of quantum mechanics. Hi Albert. He thought quantum mechanics couldn’t possibly be a complete description of nature and he argued that something was missing from it.

You see, in quantum mechanics we can’t predict the outcome of a measurement. We can only predict the *probability for getting a particular outcome. Without quantum mechanics, I could say if I shoot my particle cannon, then the particles will land right there. With quantum mechanics I can only say they’ll land right there 50% of the time but have a small chance to be everywhere, really.

Einstein didn’t like this at all. He thought that actually the outcome of a measurement is determined, it’s just that it’s determined by “hidden variables” which we don’t have in quantum mechanics. If that was so, the outcome would look random just because we didn’t have enough information to predict it.

To make this point, in 1935 Einstein wrote a famous paper with Boris Podolsky and Nathan Rosen, now called the EPR paper. In this paper they argued that quantum mechanics is incomplete, it can’t be how nature works. They were the first to realize how essential “entangled” particles are to understand quantum mechanics. This would become super-important and lead to odd technologies such as quantum teleportation which I’ll tell you about in a moment.

Entangled particles share some property, but you only know that property for both particles together. It’s not determined for the individual particles. You may know for example that the spin of two particles must add up to zero even though you don’t know which particle has which spin. But if you measure one of the particles, quantum mechanics says that the spin of the other particle is suddenly determined. Regardless of how far away it is. This is what Einstein called “spooky action at a distance” and it’s what he, together with Podolsky and Rosen, tried to argue can’t possibly be real.

But Einstein or not, physicists didn’t pay much attention to the EPR paper. Have a look at this graph which shows the number of citations that the paper got over the years. There’s basically nothing until the mid 1960s. What happened in the 1960s? That’s when John Bell got on the case.

Bell was a particle physicist who worked at CERN. The EPR paper had got him to think about whether a theory with hidden variables can always give the same results as quantum mechanics. The answer he arrived at was “no”. Given certain assumptions, any hidden variable theory will obey an inequality, now called “Bell’s inequality” that quantum mechanics does not have to fulfil.

Great. But that was just maths. The question was now, can we make a measurement in which quantum mechanics will actually violate Bell’s inequality and prove that hidden variables are wrong? Or will the measurements always remain compatible with a hidden variable explanation, thereby ruling out quantum mechanics?

The first experiment to find out was done in 1972 by Stuart Freedman and John Clauser at the University of California at Berkeley. They found that Bell’s inequality was indeed violated and the predictions of quantum theory were confirmed. For a while this result remained somewhat controversial because it didn’t have a huge statistical significance and it left a couple of “loopholes” by which you could make hidden variables compatible with observations. For example if one detector had time to influence the other, then you wouldn’t need any “spooky action” to explain correlations in the measurement outcomes.

But in the late 1970s physicists found out how to generate and detect single photons, the quanta of light. This made things much easier and beginning in the 1980s a number of experiments, notably those by Alain Aspect and his group, closed the remaining loopholes and improved the statistical significance.

For most physicists, that settled the case: Einstein was wrong. Hidden variables can’t work. There is one loophole in Bell’s theorem, called the “free will” loophole that cannot be closed with this type of experiment. This is something I’ve been working on myself. I’ll tell you more about this some other time but today let me just tell you what came out of all this.

These experiments did much more than just confirming quantum mechanics. By pushing the experimental limits, physicists understood how super-useful entangled particles are. They’re just something entirely different from anything they had dealt with before. And you cannot only entangle two particles but actually arbitrarily many. And the more of them you entangle, the more pronounced the quantum effects become.

This has given rise to all kinds of applications, for example quantum cryptography. This is a method to safely submit messages with quantum particles. The information is safe because quantum particles have this odd behavior that if you measure just what their properties are, that changes them. Because of this, if you use quantum particles to encrypt a message you can tell if someone intercepts it. I made a video about this specifically earlier, so check this out for more.

You can also use entangled particles to make more precise measurements, for example to study materials or measure gravitational or magnetic fields. This is called quantum metrology, I also have a video about this specifically.

But the maybe oddest thing to have come out of this is quantum teleportation. Quantum teleportation allows you to send quantum information with entangled states, even if you don’t yourself know the quantum information. It roughly works like this. First you generate an entangled state and you give one half to the sender, lets call her Alice, and the other half to the receiver, Bob. Alice takes her quantum information whatever that is, it’s just another quantum state. She mixes it together with her end of the entangled state, that entangles her information with the state that is entangled with Bob, and then she makes a measurement. The important thing is that this measurement only partly tells her what state the mixed system is in. So it’s still partly a quantum thing after the measurement.

But now remember, in quantum mechanics making a measurement on one end of an entangled state will suddenly determine the state on the other end. This means Alice has pushed the quantum information from her state into her end of the entangled state and then over to Bob. But how does Bob get this information back out? For this he needs to know the outcome of Alice’s measurement. If he doesn’t have that, his end of the entangled state isn’t useful. So, Alice lets Bob now about her measurement outcome. This tells him what measurement he needs to do to recreate the quantum information that Alice wanted to send.

So, Alice put the information into her end of the entangled state, tied the two together, sent information about the tie to Bob, who can then untie it on his end. In that process, the information gets destroyed on Alice’s end, but Bob can exactly recreate it on his end. It does not break the speed of light limit because Alice has to send information about her measurement outcome, but it’s an entirely new method of information transfer.

Quantum teleportation was successfully done first in 1997 by the groups of Sandu Popescu and Anton Zeilinger. By now they do it IN SPACE… I’m not kidding. Look at the citations to the EPR paper again. They’re higher now than ever before.

Quantum technologies have a lot of potential that we’re only now beginning to explore. And this isn’t the only reason this research matters. It also matters because it’s pushing the boundaries of our knowledge. It’s an avenue to discovering fundamentally new properties of nature. Because maybe Einstein was right after all, and quantum mechanics isn’t the last word.

Today research on quantum mechanics is developing so rapidly it’s impossible to keep up. There’s quantum information, quantum optics, quantum computation, quantum cryptography, quantum simulations, quantum metrology, quantum everything. It’s even brought the big philosophical questions about the foundations of quantum mechanics back on the table.

I think a Nobel prize for the second quantum revolution is overdue. The people whose names are most associated with it are Anton Zeilinger, John Clauser, and Alain Aspect. They’ve been on the list for a Nobel Prize for quite some while and I hope that this year they’ll get lucky.

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