It was announced yesterday that Alain Aspect, John Clauser and Anton Zeilinger have been jointly awarded the Nobel prize in physics for their experiments to test the effects of quantum entanglement. These experiments are both extremely important and extraordinarily difficult.
The story can be said to begin with the famous Einstein-Podolsky-Rosen paper of 1935 where they seemed to suggest that the theory of quantum mechanics, by then already hailed as a massive success, had to be incomplete because there were elements of reality that were not represented in the theory. In their paper, they argued that in the standard Copenhagen interpretation of quantum mechanics that was embraced by most physicists, one could have situations where a measurement made on one particle could instantaneously influence the outcomes of a measurement on another particle however far away it was. Einstein felt that was ‘spooky’.
For a long time people thought that one could not distinguish experimentally between the so-called local reality model of nature (preferred by Einstein and spelled out in that paper) and the non-local, ‘action at a distance’ model of quantum mechanics of the Copenhagen interpretation, But John Bell showed in 1964 that such a test could be devised but the difficult experiments involved only started producing results the mid-1970s, and tended to support the Copenhagen model. So it took about 40 years to get even the first experiments done to test the EPR model. It took several more decades of careful experiments to create the current consensus that the Copenhagen interpretation, despite its unsatisfactory aspects, is the one that works best.
“It’s so weird,” Aspect said of entanglement in a telephone call with the Nobel committee. “I am accepting in my mental images something which is totally crazy.”
Yet the trio’s experiments showed it happens in real life.
“Why this happens I haven’t the foggiest,” Clauser told The Associated Press during a Zoom interview in which he got the official call from the Swedish Academy several hours after friends and media informed him of his award. “I have no understanding of how it works but entanglement appears to be very real.”
His fellow winners also said they can’t explain the how and why behind this effect. But each did ever more intricate experiments that prove it just is.
The press release announcing the award goes into some detail about what each of these experimenters did.
One key factor in this development is how quantum mechanics allows two or more particles to exist in what is called an entangled state. What happens to one of the particles in an entangled pair determines what happens to the other particle, even if they are far apart.
For a long time, the question was whether the correlation was because the particles in an entangled pair contained hidden variables, instructions that tell them which result they should give in an experiment. In the 1960s, John Stewart Bell developed the mathematical inequality that is named after him. This states that if there are hidden variables, the correlation between the results of a large number of measurements will never exceed a certain value. However, quantum mechanics predicts that a certain type of experiment will violate Bell’s inequality, thus resulting in a stronger correlation than would otherwise be possible.
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.
It is rumored that the Nobel prize committee in physics avoids giving the prize to work that has the potential to be later proven wrong and hence there is often a long period between a piece of work and its recognition. This award means that the physics establishment has decided that the results supporting non-locality are solid and unlikely to be reversed.
I often have very little knowledge of the work of Nobel prize winners even in physics and hence cannot make a judgment as to the merits of their work. But this prize seems to me to be definitely well-deserved. The experimental work that needed to be done to get the results required great ingenuity, patience, and painstaking attention to detail.
Yeah, very well-deserved. It’s about time, and it’s sort of sad that Bell died in 1990 and isn’t sharing credit with the experimentalists. I mean, nonlocality is hardly “useful” technologically speaking (not like lasers or what have you), so I guess I get why it might be harder for that sort of thing to gain traction and grab the attention of the Nobel people; but it’s pretty hard to think of any other work in fundamental physics over the last century or so that’s as important or revolutionary at a theoretical level.
The experiments certainly bore out Bell’s 1964 conclusion. But I think there’s a common misunderstanding that Bell showed that hidden variables were disfavoured. That’s simply not true. What he showed is that quantum mechanics cannot satisfy both reality and locality.
‘reality’ in this context means the outcome of a measurement of a state is completely determined, even before measurement, by a set of variables, some of which are unobservable (hidden).
What Bell meant by ‘locality’ was very particular; the measurement of the spin of one particle depends only on the local axis used to measure, and a shared set of hidden variables which embody the entanglement. So for the first particle the (pre-determined) value of the measurement is
A(a, λ)
where a is the vector defining the measurement axis, and λ is the set of hidden variables. And the outcome for the other particle is
B(b, λ)
where b defines the axis used for the second particle.
Locality here means simply that the function A cannot depend on the vector b.
Bell then showed that these assumptions (reality and locality) lead to a contradiction. So either reality (the set of hidden variables), or locality (measurement at one location doesn’t depend on settings at the other location) has to be discarded.
So Copenhagen is local in Bell’s sense, even though there is a non-local spin correlation. It should be noted that Bell himself favoured Bohmian mechanics, which has position as a hidden variable.
They definitely deserve a prize for that. Untangling multiple necklaces is hard enough…
Though not mentioned here, it seems that entangled particles maintain connections across indefinite extents of time as well as space.
Which has long led me to puzzlement about a particular question: shouldn’t entanglement apply to everything formerly part of the singularity of the Big Bang, namely everything?
Pierce @4:
In principle, yes. So, for example, long ago a neutral pion may have decayed to a pair of photons. Because the pion has spin zero, the photons are in an entangled state; if one of them later interacts in such a way that its spin is determined to be in a certain direction, the other one will interact (if it ever does) such that its spin is in the opposite direction. And of course, the pion itself may have been entangled with other particles.
So if we take two electrons and create a combination which has zero spin, it may well be that one or both of them is entangled with other particles far away. But for the purposes of the experiment, those particles don’t matter. We’re not measuring their properties.
Pierce R. Butler:
Could the Big Bang have been the Great Disentangling? (I’m just making that up. I have no clue what that could even mean.)
So extending Moore’s Law, how long till we can communicate with far-off aliens in real time? 😎
CR @1:
Better to say it’s hardly used, yet. Early days.
(https://quantum-computing.ibm.com/composer/docs/iqx/guide/entanglement)
I remember when laser was considered a solution in search of a problem.
@ ^
No you don’t.
Lasers were invented in 1960, holograms (which require lasers) in 1962. Apollo in 1969 carried a retroreflector to measure the precise distance to the Moon by interferometry.
Applications for highly collumnated light of precise frequency that could produce interference patterns were obvious immediately.
Ahem.
Let me just cut’n paste what I wrote, put it into a search engine.
https://www.google.com/search?q=laser+was+considered+a+solution+in+search+of+a+problem
Huh.
About 80,700,000 results (0.83 seconds)See for yourself.
Suppose I have two playing cards, one red and one black.
You take one of the cards, without showing it to me; then you get in a spaceship and travel halfway across the universe with your card.
At some later time, I look at the card you left behind, and I see it is black. I know at once that your card is red.
So, has the redness of your card been somehow communicated to me faster than light?
bluerizlagirl @10:
No. You know that one of the cards is red, and the other is black, before either person checks the colour. In the case of entangled particles, neither one is in a definite spin state. Before you measure the spin, you simply can’t say “this particle has either spin +1 or spin −1”. Each particle has both “plusness” and “minusness”, and measuring the spin is realizing one of those. The “spooky action at a distance” can be expressed as “if I measure my particle’s spin as +1, how does the other particle “know” that it must realize its “minusness”?”.
John Morales, #7:
I was talking about nonlocality, not entanglement. In any case, the point was not to make a statement about whether or not it ever could (in principle) be useful somehow, but to understand what might affect some of the past motivations of certain people who made certain decisions. If it is not yet useful but may be in the future, then like it or not, for better or worse, it is just the case that that doesn’t tend to have the same effect on people (the physics community, the Nobel committee, or pretty much any group of people) as something which is in fact already being used, to do some possibly very remarkable or consequential things in their actual lives.
~~~~
Rob Grigjanis, #2:
Bell himself concluded that we’re just stuck with nonlocality. If you think he was mistaken about his own result, then you should’ve at least mentioned that in your purported explanation of it. (It’s not just that he wasn’t a fan of Copenhagen or something like that.)
Well, that is a thing that you could mean by a word (although it’s definitely not what that means in any other context), and you are free to make whatever definitions you want…. But that sort of thing doesn’t need to have anything to do with Bell’s theorem.
There doesn’t seem to be any point in arguing with you about this again. So I’ll just quote this whole section from the SEP article:
cr @12:
FFS, read what I wrote. I explained exactly what Bell meant by ‘locality’ in his 1964 paper. You’ve never actually read it, have you? If he later decided to expand his definition of ‘nonlocality’ to include nonlocal correlations, that’s fine. And I did say that Copenhagen still has nonlocal correlations. You’re just manufacturing bullshit complaints.
And if Bell did conclude “we’re just stuck with nonlocality” he was mistaken. I’ll just quote a part of this SEP article (my bolding);
BTW, if you don’t like my use of ‘reality’ (philosophy of physics wonks might have used ‘realism’), despite the fact that I defined my use of the fucking term, you could simply replace it with ‘counterfactual definiteness‘.
@Rob Grigjanis, #11:
But in the case of a pair of entangled particles, you know in advance that whatever spin one of them will turn out to have when you observe it, the other one will necessarily have the opposite spin, by sole dint of the property of entanglement.
Even if you don’t look at the card you take before you fly off with it, so it has equal probabilities of being either red or black until you observe it, there are only two states into which the wave function can collapse: either the card you took is red and the one you left behind is black, or the card you took is black and the one you left behind is red. But the fact that one of the cards is red and the other is black means it is only necessary to observe one of the cards to know the states of both of them.
bluerizlagirl @15: Yes, you know that when you measure a spin, it will be either +1 or −1. But in the wacky world of quantum mechanics, that does not mean you can say “before I measure it, the spin is either +1, or −1”. That’s a crucial difference.
It’s like the difference between a classical bit and a qubit. The classical bit is always either 0 or 1. The qubit can be in a superposition;
α|0> + β|1>
where α and β are complex constants satisfying |α|² + |β|² = 1
That property, being in a sense a combination of 0 and 1, is fundamental to quantum computing.
Likewise, each particle of an entangled pair is, in a sense, a combination of both spins. But the cards are not in such a state; one is definitely red, the other definitely black.
To explore the richness of the difference between the classical and quantum cases requires a fair bit of math, and using different axes to measure the spin of each particle.
cr @12:
No, ‘that sort of thing’ is crucial to Bell’s theorem (my bolding);
Maybe you were confused because Bell, in his 1964 paper, refers to ‘additional variables’ or ‘parameters…added to quantum mechanics’ rather than ‘hidden variables’.
This might be of interest
Holms @18: The most interesting thing about the video is the apparent misunderstanding the featured physicists have about Bell’s work. From their words, you would think Bell had disproved Bohm’s hidden variable theory. In fact, Bell favoured Bohm’s theory. Two of the guys say something like “It ruled out hidden variables! Well, at least local hidden variables”. Note the parenthetic nature of the second sentence.
I say that as someone who loathes hidden variables on aesthetic grounds. But they are misrepresenting Bell’s conclusions.