Quantum entanglement is the phenomenon where the quantum state of each particle in a group cannot be described independently of the state of the others, even when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical physics and quantum physics: entanglement is a primary feature of quantum mechanics not present in classical mechanics.Note that it does not say that the state of the particle depends on the state of the distant particle. We have no proof that it does. We only know that the formalism of quantum mechanics is unable to describe the states independently. Maybe the states are independent, but we do not have the math to describe them independently.
Stanford Encyclopedia of Philosophy:
Quantum entanglement is a physical resource, like energy, associated with the peculiar nonclassical correlations that are possible between separated quantum systems. Entanglement can be measured, transformed, and purified. A pair of quantum systems in an entangled state can be used as a quantum information channel to perform computational and cryptographic tasks that are impossible for classical systems.Here, entanglement is not just a correlation in our mathematical descriptions, it is a resource like energy that can be used for communication and computation.
Is that really what it is? Quantum mechanics is a century old, and Schroedinger wrote a paper stressing the crucial importance of entanglement in 1935, but he never said it was a resource. Dozens of Nobel Prizes have been given for quantum mechanics, but none for using entanglement as a resource.
Use of energy as a resource is evident everywhere we look. Is there some compelling experiment demonstrating entanglement as a resource? A quantum computer showing quantum supremacy would do, and some people have claimed that, but the evidence is unconvincing.
There are experiments using entanglement for communication, but communication is much easier by other methods. Dr. Bee has a video on the coming quantum internet, but I do not see any practical use for it.
Sometimes people think that quantum decoherence explains entanglement, but that is not quite right. Even after Schroedinger's cat decoheres, the live cat state is still said to be entangled with the dead cat state.
I am skeptical that entanglement is anything real, like being a resource. Maybe it is just a property of the mathematical formalism, and no more.
I am guessing that most physicists will say I am wrong about this. Okay maybe so, but where is the experiment that proves me wrong? And why didn't someone get a prize for this discovery?
Maybe you will say that happened in 2022, but here is the announcement:
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. ...So they demonstrated the correlations, ruled out hidden variables, and cleared the way for new technology. In other words, they confirmed what everyone thought for a century. But where is that new technology?For a long time, the question was whether the correlation was because the particles in an entangled pair contained hidden variables, ...
“It has become increasingly clear that a new kind of quantum technology is emerging. We can see that the laureates’ work with entangled states is of great importance, even beyond the fundamental questions about the interpretation of quantum mechanics,” says Anders Irbäck, Chair of the Nobel Committee for Physics.So he is convinced that a new technology is emerging. I expect to see a prize given for that new technology, when it emerges. But I don't think we have any proof that it exists. Sean M. Carroll is probably the leading popularizer of QM, but he gets entanglement wrong, as explained in a recent paper:
A related example comes from Sean Carroll’s book From Eternity To Here: The Quest for the Ultimate Theory of Time (Dutton, 2010). This book contains a fairly lengthy section on entanglement and, nominally, the EPR thought-experiment. Carroll gives the EPR paper itself a brief mention and credit for introducing the concept of an entangled ψ function, but he does not address their motivation, how they wanted to show that quantum particles have definite values of position and momentum even before those quantities are measured. The bulk of the section is a tale of two animals for whom a joint ψ function is written. When “Miss Kitty” is observed, she is found to be either on the table or on the sofa; when “Mr. Dog” is observed, he is found to be either in the living room or in the yard.To elaborate, correlations occur in classical mechanics and in everyday life all the time. In the simplest example, imagine that two billiard balls collide and bounce apart. Knowing something about the initial conditions and measuring one ball will tell you something about the other. There is nothing mysterious about that.Even though we have no idea where Mr. Dog is going to be before we look, if we first choose to look for Miss Kitty, once that observation is complete we know exactly where Mr. Dog is going to be, even without looking for him! That’s the magic of entanglement.No, it isn’t. It’s an unremarkable possibility that could occur in everyday life. The entire buildup to this declaration is beside the point.5 None of the conceptual or mathematical apparatus of quantum theory is necessary for Carroll’s scenario, and a big sign of why is that the story considers only one observable, the location, of each character.
As the paper explains, to get the EPR paradox, you have to combine these correlations with Heisenberg uncertainty. There have to be two observables such that measuring one forces a quantum uncertainty in the other. The correlation in those uncertainties is what requires quantum rules, and cannot be modeled classically. These lame explanations like Miss Kitty and Mr. Dog miss the point of entanglement.
As the paper explains, the Einsteinian talk about elements of physical reality, as some people say EPR abbreviates, also does not get at how QM deviates from a classical theory. To get that you have to look at numerical values for correlations. The qualitative and philosophical arguments prove nothing.
The EPR paper does not look at those numerical correlations, and hence does not show that QM deviates from a classical theory. So it is not clear that Einstein understood entanglement as non-classical. Maybe he wanted a classical theory to explain quantum predictions. The proper resolution of the EPR paradox is that there is no such classical theory. That is the point of the 2022 Nobel Prize.
In a new video:
Matt O'Dowd picks apart the mystery of quantum entanglement, offering different interpretations for this baffling phenomena.Some of what he says is correct, but he says:
so this is this spooky action at a 1:20 distance and it I mean I just got 1:22 shivers right now thinking about it it 1:25 it it's weird as hell anyway we we've 1:27 now demonstrated that exactly this 1:31 you know redu absurdum that Einstein 1:33 proposed to to to 1:36 discount the idea of of that standard 1:38 quantum mechanics has proved to be real ...No, there is no spooky action at a distance, and the choice of measurement only affects the particle being measured. I thought he might get it right when he said:it it really seems like the 3:27 the choice of measurement affects both 3:30 particles
a hidden 3:03 variable interpretation would say that 3:06 this particle really knew what it was 3:08 all alongYou could say that in a classical theory, particles know what they are all along, and in a quantum theory they don't.
