Experimental tests of Bell’s inequality
Experimental tests of Bell's inequality, culminating in the 2022 Nobel Prize-winning work of John Clauser, Alain Aspect, and Anton Zeilinger, confirm that quantum entanglement cannot be explained by local hidden variables. Clauser ran the first lab-scale test, but critics argued his experimental setup was too local. Aspect addressed this by using light from quasars billions of light years apart to set the polarizer directions, making any hypothetical hidden variable the size of the observable universe. Bell's inequality was still violated, which means entangled particles cannot be treated as mathematically separate systems: they share a single wave function and must be analyzed as one block.
- Clauser's key vulnerability was that he himself chose, even via a random number generator, which direction to set the polarizers, leaving open the objection that the hidden variable was local to his lab.
- Aspect's quasar experiment shifted the polarizer settings based on incoming light from celestial objects pointing in opposite directions, so the 'experimental apparatus' spanned billions of light years.
- The inability to factor the joint probability distribution of Alice's and Bob's particles into separate components is the specific mathematical signature Krishna says Feynman draws on in his 1981 talk on quantum computing.
Transcript
This chapter, from the episode video's captions · 1,161 words
19:36violation right >> there's a violation of Bell's inequality and you cannot factor this probability distribution. Okay, there's no separate components. You have to consider the entire thing in this giant no matter how far apart Alice and Bob are. Okay, John Clauser did this in the he won the 2022 Nobel Prize in physics along with Aline aspect and Anton Zylinger. >> Um these guys were instrumental at taking Bell's inequality to the absolute limit. John Clauser was the first guy to do it and these other two started considering very very like you know whenever whenever um whenever John Clauser did that there's there's people
20:17that'll come up and be like well maybe the hidden variable is like in the lab right in in the giant room that John Clauser is in because because he's making these these particles and then making it go to one end of the lab and the other end of the lab. His experiment was still kind of local, right? And then he's making choices that are that are local to him, right? He's like choosing when to when to put the polarizers in one direction or the other. >> Is the argument basically like the scope of what you're accounting for is >> is like still kind of local, right? Right. It's still kind of local. >> When is when does it not become >> Yeah. Yeah. So then Alan Aspect Alan Aspect I think one of his one of his amazing experiments he did actually I believe it was at Tenneref in the
20:58observatories there. Tenneref has these amazing observatories um optical telescopes that are across the mountain right so now what you can do the main the main problem with the with um John Clauser's experiment was that he was kind of choosing even with a random number generator he was kind of choosing himself which way to set the polarizers what choice to make for his Alice and his Bob. Um Alan aspect said okay I'm going to point telescopes in opposite directions of the universe. Okay, one is going to point at a quazar that's like billions of light years this way and one is going to point at a quazar a billion lightyears that way and depending on the light from the quazars the quaazars are
21:40going to make the choice on which direction my polarizing filter is going to be okay Bell's inequality is still violated so that means that whatever local variable has to be like the size of the universe >> the >> at that point it's like what did the hell are you what is local right and and so just to say this back to you the The idea is like who decides the position of the filter initially was not considered sufficient to say that the hidden variable is is not a thing. Yeah. Because the idea was the decider of that was still too local to the system of observation. >> And so now when we utilize these distant celestial objects >> Yeah. as the variable that decides what
22:22direction the filter is in in this matrix of options. >> The now our experimental design is billions of light years across >> versus just whatever a couple tens of yards or whatever. And so you're and then the argument that this hidden variable can exist in this billion light years across experimental apparatus no long can no longer be considered reasonable. >> Yeah. >> Um and and now we we are able to we still violate Bell >> theorem. And so we we the realists Sorry. >> Yeah. Sorry. I mean, unless you're really like doubling down and you're saying there's just a universewide local variable or something. Uh, you know, it it's getting it's getting tenuous now,
23:02right? >> Because now we're arguing over the definition of local. >> Yeah. Yeah. Yeah. Exactly. Right. Um, so the key thing here that I want you to take away, >> okay, >> is that factoring capability. Okay. >> Okay. It means that you cannot factor the physics into separate components. One for Alice and her particle and one for Bob and his particle. Okay, that's the key thing that establishes that quantum mechanics is very very weird and that's the insight that Fineman uses in his 1981 talk. >> Can you maybe phrase it in a slightly different way because I think I understand what you're saying when you say you can't factor it independently for both. Are you trying to say that there is a there's a rule set above when
23:46either of them observed or act in a system that is not independent of each other? Like I'm just trying. >> Yeah. So there's um there's like in order to really try and understand what is happening in this experiment, we have to we have to consider the entire system at once. >> We cannot consider the systems separate. The particles cannot be considered in any mathematical form to be separate from one another. They are connected by a single wave function is is is one way of putting it. Right? Like the mathematics is inherently tied. It is one block. >> Yes. >> Right. >> It's a it is a singular Lego block. You can't break it down into subsequent smaller Lego blocks. No. The whole
24:26system that you're observing at whatever scale. >> Yeah. >> Has to be considered a singular Lego block in this analogy. >> Yeah. Yeah. In order to really capture all of the quantum mechanics that is happening. Okay. there's no way to be like there's Aiden variable and so you know this guy has this thing saved that guy has that thing saved and and so >> okay >> you know what I mean okay so that's the key insight that Fman is going to use later in 1981 now this is in the 1960s and we've got a long way to go before 1981 okay cuz there's a lot more stuff that happens in parallel people are working on computation and trying to understand computation right in the early decades of the digital revolution where you've got these vacuum tubes and you're trying to understand like
25:06mathematical ical abstraction. Computation is simply that. It's just a mathematical abstraction. Okay? It's divorced from like physical substrates. Claude Shannon very famously in 1948, he shows that information is the same as entropy, but it's it's still this mathematical abstraction. Okay. Ralph Landau, he's actually at IBM research, which um we might hear a lot more about in this episode and in the next episode. More on that later. But Ralph Landau in 1961 he establishes that information is actually inherently physical. Okay? And he demonstrates this by saying that any logically irreversible operation meaning
25:47something that destroys information is going to cost you heat. Okay, here's what I mean by that. like um there
From How Quantum Computing Actually Works (Part 1)
Part I of our quantum computing deep dive traces the field from Bell and Feynman to Deutsch and Shor—and explains what quantum computers actually do differently from classical machines.