Two atomic clocks have been quantum entangled for the first time | Hacker News Reader

Two atomic clocks have been quantum entangled for the first time

newscientist.com

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MIT news:

https://news.mit.edu/2020/atomic-clock-time-precise-1216

seems to be the better link, also reveals subject is a 2020 discovery, was also published in Nature:

https://www.nature.com/articles/s41586-022-05088-z [paywall]

"Abstract

Optical atomic clocks are our most precise tools to measure time and frequency1,2,3. Precision frequency comparisons between clocks in separate locations enable one to probe the space–time variation of fundamental constants4,5 and the properties of dark matter6,7, to perform geodesy8,9,10 and to evaluate systematic clock shifts. Measurements on independent systems are limited by the standard quantum limit; measurements on entangled systems can surpass the standard quantum limit to reach the ultimate precision allowed by quantum theory—the Heisenberg limit. Although local entangling operations have demonstrated this enhancement at microscopic distances11,12,13,14,15,16, comparisons between remote atomic clocks require the rapid generation of high-fidelity entanglement between systems that have no intrinsic interactions. Here we report the use of a photonic link17,18 to entangle two 88Sr+ ions separated by a macroscopic distance19 (approximately 2 m) to demonstrate an elementary quantum network of entangled optical clocks. For frequency comparisons between the ions, we find that entanglement reduces the measurement uncertainty by nearly \(\sqrt{2}\), the value predicted for the Heisenberg limit. Today’s optical clocks are typically limited by dephasing of the probe laser20; in this regime, we find that entanglement yields a factor of 2 reduction in the measurement uncertainty compared with conventional correlation spectroscopy techniques20,21,22. We demonstrate this enhancement for the measurement of a frequency shift applied to one of the clocks. This two-node network could be extended to additional nodes23, to other species of trapped particles or—through local operations—to larger entangled systems.

The New Scientist article is talking about network of two entangled optical atomic clocks, which is a step beyond entanglement within one clock.

Here's what Oxford's physics department says about it:

https://www.physics.ox.ac.uk/news/quantum-network-entangled-...

> Two atomic clocks have been connected using quantum entanglement – a property that intrinsically links them so that changes in one instantaneously affect the other.

Not this explanation again ...

ELI5 how we should be talking about it?

Here's a SO discussion:

https://physics.stackexchange.com/questions/15282/quantum-en...

"Collapsing an entangled pair occurs instantaneously but can never be used to transmit information faster than light. If you have an entangled pair of particles, A and B, making a measurement on some entangled property of A will give you a random result and B will have the complementary result. The key point is that you have no control over the state of A, and once you make a measurement you lose entanglement. You can infer the state of B anywhere in the universe by noting that it must be complementary to A.

The no-cloning theorem stops you from employing any sneaky tricks like making a bunch of copies of B and checking if they all have the same state or a mix of states, which would otherwise allow you to send information faster than light by choosing to collapse the entangled state or not."

Basically you know a that since they are complements, you know the value of the other after you measure one. No information was transmitted at all.

IANAQP. At all. Not one bit.

My layperson understanding is it's like someone giving you a penny and they keep a penny. One is heads and the other is tails.

When you eventually peak at your penny you know what the other is.

Entanglement is really "just" the quantum version of this effect, which is why it‘s also called "quantum correlation".

Take two entangled photons. Measuring both polarizations with horizontal/vertical polarizers, you find that horizontal polarization (h) of the first means vertical polarization (v) of the second. So far, everything is classical and analogous to the coin example. The source could just produce the pairs h/v and v/h each 50% of the time.

But now note that both h- and v-photons pass through a diagonal polarizer 50% of the time. So if you measure with diagonal polarizers you‘d classically expect no correlation. But in fact, if the first photon passes through a diagonal polarizer, the second will never pass the same diagonal polarizer, and always pass through a perpendicular-diagonal polarizer. This is impossible with just classical randomness.

Bell‘s inequality describes how strong the correlations between polarizations could be with any classical model, and in reality we measure them to be stronger than that.

Entanglement isn’t necessary in that scenario, though. My understanding is that peaking at your penny forces the other penny into the complement state. There’s an element of causality which is why we call it entanglement and not simply spin complements. My understanding is also that causality is instantaneous, i.e., faster than light, but still cannot be used to transmit information.

The difference being that yours being tails is actually not a fact and a probabilistic outcome? vs. it just always being tails and you not knowing?

how would we ever know the difference?

A particle is less like a coin with only two sides, and more like a little arrow which could be pointing in any direction.

Quantum mechanics allows you to measure the projection of the arrow along any one axis, and you'll always get +1 or -1. If the two people with entangled particles measure along the same axis, they will always get opposite results. But they are also free to measure along different axes.

The statistics of what results the two experimenters get when they measure their particles arrows along random axes in repeated trials are not the same as what you would get if the arrows had a predetermined direction all along.

Google "Bell inequality" for details.

Yes. That's why it's so weird.

Imagine two dice. There's no sense in which one die has a number until you throw it and one side lands upwards.

With entanglement, you have two dice but they're correlated. For example they may always land opposite sides up.

So if you see the value of one, you also know the value of the other. No matter where it is.

The hard questions are:

1. How does one dice/particle know the other has landed/been measured? This is really just a special case of the unsolved measurement problem, but over larger than usual distances.

2. Where does the entanglement information live? If you look at a single die/particle it has no physical property that shows it's entangled. Individually, entangled and unentangled particles are absolutely identical. So you have a correlation that can't be explained by the structure of each die/particle.

IANAQP but to me this suggests that you're looking at a 4D spacetime projection of an object which exists in some higher and/or more abstract space. So there's a single object in a hypothetical Quantum Space and we're seeing two views of it in our 4D spacetime. Entanglement somehow fixes the projection in a certain orientation.

I don't know how it applies specifically here, but generally until it's collapsed it will act as a wave, so you could see things occur that aren't possible for an already collapsed states.

Read about the EPR paradox and Bell's theorem. Einstein argued it was always tails and you just didn't know (local hidden variables) and Bell showed that couldn't be true.

My understanding (IANAQP) is that there is no element of causality, and so no instantaneous causality. We know that the entangled particles are in complementary states; we just don't know what states, until we peek at one of them.

There is no such effect proven, nor could we measure it if it was. Entanglement can be summarized as using physics itself to have fundamental particles contain information. They don't communicate. All it does is tell you what the state of a and b are. We could assume that they were always that way from when they were conceived, but we can't know that, because it would require measuring it, which breaks entanglement.

We can transmit that same information synthetically without entanglement, but it's not the same methods.

I imagine entanglement being the hardware implimented version, and current methods the generic C implimentation.

> We could assume that they were always that way from when they were conceived

Nope. Bell Inequality: a more complex lab experiment shows that they don't hold their state, they were not "that way" before measurement.

That's the reason behind the rants about spooky action at a distance, superdeterminism, MWI, etc.

Before someone starts yelling "You can't prove that, you can't peek the quantum states, you cheater!" I'd like to point out that it's reality showing its weird face and no human-made proofs were in use.

Isn't there some kind of physics theorem equivalent to "0.9 repeating forever equals one" about things being the same if there's no way to define them as different? If a particle can't interact with anything in the universe without having aspect X then isn't saying it doesn't have aspect X before interacting a meaningless statement? Something can't be different without making a difference.

Just FYI to you and above post, peek, not peak.

I thin kthis has the opposite problem where this explanation downplays the effect. For example, while you cannot do FTL communication using entanglement, if you have an entangled pair and send a single classical bit then you can transmit two bits of information.

They are explicitly saying this will help them keep the two atomic clocks better synchronized. Which is terribly useful for radio astronomy. But it does seem like that would imply information had been transferred. If it really does work I want to see what happens when you put one in orbit for a while. Donthe differential frames of reference decohere them?

I'm pretty sure that setting up entanglement involves information transfer.

Sure. The article implies that post-entanglement, perturbations of one particle's state will transfer to the other, which is not the case.

I don't think it does?

Atomic clocks don't exactly count atomic oscillations the way a grandfather clock counts pendulum swings. They have a more normal oscillator, and then keep that calibrated by checking it against atomic oscillations measured over some relatively short time.

Thirs is a way to let those repeated short measurements be more accurate. It doesn't mean that the oscillations of the atoms in both clocks are somehow locked together over the long term.

FTA (emphasis mine): Two atomic clocks have been connected using quantum entanglement – a property that intrinsically links them so that changes in one instantaneously affect the other.

Paywall beyond there so I don't know how they entangled the clocks, I'm just responding to the popsci description of entanglement which the article repeats.

Ah, that's the sketchy-at-best popular description of entanglement rather than anything to do with this experiment in particular. It's wrong in that it's meant for a lay audience, and to a lay audience (I think I've heard that the quantum folks define it differently?) the word "affect" implies things like causality and information transfer.

"...making a measurement on some entangled property of A will give you a random result and B will have the complementary result."

"...once you make a measurement you lose entanglement."

One entangled pair can transmit exactly one bit of information exactly once. The measurement necessarily destroys the pair.

Do I understand correctly?

No, as you can't influence the outcome of the measurement. So both parties reads a bit, which is random, they just happen to be opposite of eachother. No useful information is transmitted.

I think what the person is struggling to understand is the phrase "lose entanglement".

I also wonder what is meant here.

Are the particles no longer entangled after they are measured once, or what is being said here?

Correct. After you measure the particles once, they are no longer entangled.

It is actually quite difficult to keep the particles entangled, because the rest of the universe keeps trying to measure them. (I am not a physicist, and I have no idea how they keep the particles entangled.)

I'm struggling with the idea that reading a bit at A is random, and B is random, but that A is the inverse/opposite of B. That means B is not-random to me.

I have a feeling that terms being used are specifically defined terms of art. EG, don't mean what they mean in common english, but instead, have a different technical meaning.

B basically isn't random (that's why quantum entanglement is intersting) but you have 3 options on when to read its state ( which you can only do once):

Option 1 is that you read it before the state of A is read, in which case my undestanding is you are effectively A in this scenario.

Option 2 is that the state of A has been read, but it is unkown to you yet, because of speed of light reasons. You read B, and can infer the state of A, but because A is random, no information has been transmitted. The "sender" cannot control which state of A they read, and had you not known about it, the state of B might as well be random.

Option 3 is that you read B after recieving the state of A, in which case there is something useful in that only you can make sure that only the sender knew of the state of A, meaning you are effectively authenticating a message. But still no information has been transmitted faster than light, it's just a secure channel.

The inverse/opposite of "50% chance head, 50% chance tails" is "50% chance tails, 50% chance head". Which is the same thing.

If you are looking at A alone, A is random.

If you are looking at B alone, B is random.

If you are looking at A and B together, they are entangled, i.e. their probabilities are "50% chance A head B tails, 50% chance A tails B head".

Random means something you can't predict (other than statistically), until it actually happens. If you know the state of the entire universe except for the particle B, you can't predict A. If you know the state of the entire universe except for the particle A, you can't predict B.

You can't know that the bit is ready to be checked without separate communication

You could have pre-agreed reading time, but the bit you "send" is also random.

But hold on, this could be practical in some specific cases regardless.

Say you have two fleets of spaceships on the opposite ends of the solar system, both having plans A and B for attack. They want to surprise the enemy by being unpredictable so despite the enemy knowing about the two plans, if you decide randomly which fleet does which you'd still have an advantage. Maybe one fleet is larger so they could focus forces where they need to be if they knew the plan ahead of time.

But if you choose randomly by default you could have both fleets do plan A, which wouldn't work. But if one measures the entangled pair they both get a mutually exclusive random result and thus can make an unpredictable plan work without a pre-set decision of who does what.

A weird far fetched example to be sure, but I'd imagine cryptography nerds could find a matching case for some kind of encryption or whatever.

This is a thing! https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy

Need someone to make it a bit more accessible though…

There's something here. I think (if I understand correctly, which is kind of iffy) it's also the case that if either party reads the property early, the entanglement is used up, and the on time read is no longer correlated.

That may not be very helpful for a battle plan, but if there was concerns about enemy infiltration, the entanglement could be intentionally used early, resulting in neither ship knowing what the other is doing, although the ship that read on time wouldn't know they didn't know.

I think it would work, but for practical purposes it isn't different from picking which fleet will commit to which plan in advance, sealing it to an envelope and opening it at the agreed time.

Your use case fails to take advantage of the fact that the quantum states collapse at the time of reading.

Not exactly; nothing is "transmitted".

Is there some known property that is keeping the changes random? Or is it pure observation? Because if it is the latter then is it not correct to say that it is not known if FTL transmission can work?

QM says unmeasured quantities are uncertain, which shouldn’t seem odd on the surface.

Entanglement means measuring one thing is effectively equivalent to measuring a property of the other. It’s vaguely like randomly cutting a board in half and by measuring how long one board is you can figure out how long the other one is. The odd bit isn’t really measurement it’s all the oddity around what happens before something is measured.

qubit*

GP and all sibling comments are unfortunately incorrect.

The measurement of one _does_ instantaneously affect the other. No useful information can be transmitted, since the outcomes are random, but that does not falsify the original statement.

In fact "affect the other" has no implication of "information is transferred" - someone would need to be an expert on classical information theory and simultaneously a novice of quantum information theory to draw that conclusion.

Isn't collapse of entanglement itself useful information or is it not possible to notice it on the entangled particle?

https://www.askamathematician.com/2018/03/q-with-entangled-p...

> Tricks that use entanglement to instantly send information generally involve collapsing the state of the distant particle (by measuring your own), and assuming that the person in charge of that particle will somehow notice. Unfortunately (and this is the answer), there’s no “new-particle-smell” for particles that are still in superpositions and there’s no big flash when that superposition collapses. If you measure an electron’s spin the result is simply “spin up” or “spin down”. That’s all you get. But that doesn’t tell you what the state was before, or if the original state was a superposition of several states, or even if the particle was entangled with something else.

> If you only have access to one particle, then the two situations, 1) the other particle is inaccessible or 2) it has been quietly measured, are indistinguishable in every physical sense. Regardless of whatever else is involved, you can never notice the weirdness of entangled pairs until you directly compare them with each other.

From the complementary states of the entangled particles we know entanglement happens. But then how do we know that the collapse and "de-entaglement" happens instantaneously if we can't determine whether superposition collapsed or not?

You can't determine it at the exact moment it happens, but you can meet up later (traveling well below the lightspeed limit) and compare notes.

Thank you for the link and explanation.

Yeah, I too want to know this. If we know that the collapse has happened, then it can be used to infer that someone has tried to measure the other entangled particle. Can this be be leveraged or my understanding is wrong?

You don’t know the collapse happened. Locally you can just perform a measurement and you will concretely see one of the possible outcomes.

I had an understanding that quantum entanglement can be used to for encryption and guaranty there are no man-in-the-middle attacks by making sure the entanglement is not broken. if we can’t know the collapse happened how can we make such claim (mitm can meassure stolen briefcase put it back and no one would know)

This is incorrect. Quantum key distribution ("quantum cryptography" is a misnomer) is vulnerable to man-in-the-middle attacks. Security relies on having a pre-shared key which makes it essentially useless in practice (just use AES if you have a PSK).

AES 256 only requires millions of qubits to crack. What do we do once we think that's attainable behind close doors? It'd be better to be prepared.

https://www.fierceelectronics.com/electronics/aes-256-joins-...

This is a wildly optimistic estimate. Even if we had an error corrected quantum computer that could evaluate an AES key (complete science fiction for the forseeable future), running Grover's algorithm would take millenia assuming extremely fast gate times (1 ps).

In any case, doubling the key length would be infinitely simpler than using QKD.

What does “instantaneously affect the other” mean when there’s no absolute simultaneity? https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

“Instantaneously” kind of means that one event does not happen before or after the other, but something happening before or after should depend on the reference frame, right?

My recollection when I was once studying physics and looking into this is that some interpretations of QM do entail or allow for absolute simultaneity. Some do not, such as the prevalent Copenhagen interpretation.

This article from 2009 discusses these kinds of matters, and near the end it talks about absolute simultaneity:

"The kind of nonlocality one encounters in quantum mechanics seems to call for an absolute simultaneity, which would pose a very real and ominous threat to special relativity."

https://www.scientificamerican.com/article/was-einstein-wron...

My understanding:

2 entangled particles are 2 particles whose states are unknown, except that they are linked: e.g. 2 electrons which have opposite spins, or 2 photons which have opposite polarization. So when you measure one particle, you instantly know the state of the other even if it's light-years away (this is not FTL communication as you can't communicate this information FTL).

It's sort of like putting an "up-spin" and "down-spin" electron in a box, having someone shift around the boxes behind your back, putting the boxes far away from each other, and then opening one: before you open the box you have no idea what either box contains, after you open one you know both including the far away one.

Except it's not the same as putting an "up-spin" and "down-spin" electron in a box, because of Bell's Theorem (https://en.wikipedia.org/wiki/Bell%27s_theorem#Theorem). Electrons' spin is actually determined when you measure them: you can measure up/down, but also left/right or any other 3D direction. And if you measure both electrons at a slightly different angle, their measurements probabilistically correlate in a way which means the electrons never really had any true internal "spin" (hidden variable) until you measured one of them. The fact that measuring one electron changes the others' "spin" faster than light seemingly violates the principle of locality.

The part which confuses me in the article is that, once you measure an entangled particle, they are no longer entangled. So I figure in order for this clock to be feasible you would either need an unbelievable amount of entangled atoms, you'd need to instantaneously re-entangle, or you'd need to use a "standard" atomic clock and only break entanglement after a certain number of ticks, to re-synchronize.

I'm a dummy. Why does this make them "linked"?

If I have two legos, always put them in opposite orientations, then walk away with a random one, being careful to not change its orientation, are the two legos "linked"? When I look down at the one in my hand, I will know the other ones orientation. How is the different with entangled particles? Is "entangled" just a way to say "still related because it's undisturbed"?

So that's called "hidden variable theory". We've done tests that show it's not true. Basically, there are certain measurements you can do on entangled particles where you can prove each measurement is affecting the other measurement.

These tests you are referring to, are they a recent discovery? There is a long repeated misconception that violations of Bell's theorem has shown hidden variable theories to be false. If that's what you're referring to, then no we haven't. If, on the other hand, there's been some developments in the last 5 or 10 years, then it's very likely I've missed those.

If you are referring to violations of Bell's theorem, then what they show is that hidden variables are incompatible with locality. In other words, non-local hidden variable theories are compatible with these violations and have not been shown to be false.

The whole point is to show that something non-local is happening (or an equivalent), so of course I'm talking about local hidden variables. I don't understand your objection.

I'm being a little loose about casualty but that's because I don't want to write extra paragraphs about what "simultaneous" means.

You said "So that's called "hidden variable theory". We've done tests that show it's not true". But that claim is not true — hidden variable theory hasn’t been shown to be false, but local hidden variable theories have been.

In your latest comment, you say that "The whole point is to show that something non-local is happening". I'm not sure what thing's "whole point" you're referring to, but I suspect in the context of our conversation you're talking about the "whole point" of the implications of violations of Bell's inequality. But in that case, it's also not true that the point is that "something non-local is happening". There are QM interpretations that are entirely local, and compatible with the observed violations of Bell's inequality.

I found this paper to be a helpful breakdown of how the matter might be resolved, with some views embracing locality, and some rejecting it (some parts of the paper went over my head): https://arxiv.org/abs/1503.06413

> You said "So that's called "hidden variable theory". We've done tests that show it's not true". But that claim is not true — hidden variable theory hasn’t been shown to be false, but local hidden variable theories have been.

People usually assume locality. I was talking about local hidden variable theory.

I was using unclear language to say something true, which is different from saying something false.

> There are QM interpretations that are entirely local, and compatible with the observed violations of Bell's inequality.

They are either breaking locality or doing something equivalent. This is getting into metaphysics, not physics, isn't it? Unless a conversation is deep into the weeds of quantum physics, I feel comfortable making answers that assume a particular commonplace interpretation.

> They are either breaking locality or doing something equivalent. This is getting into metaphysics, not physics, isn't it?

We probably are into metaphysics. We're talking here about different theories that are all compatible with observations to date. Theory selection is very much a philosophical pursuit, though it's something physicists will do to.

Anyway, my main claim is that, as I understand it, the most popular or common QM interpretation (Copenhagen) embraces locality, so it's incorrect to say that "They are either breaking locality or doing something equivalent".

Here's an example of that from an article by Reinhard Werner ('What Maudlin replied to'), arguing (as I understand him) for locality. Some particles are entangled, separated, and then one particle is changed so as to impact (via entanglement) the other. However, the change on one to the other isn't real until later. "The state change only becomes effective when the results from the two labs are brought together and are jointly analyzed, which can happen centuries later."

In the other paper I linked you to, it has the realists (such as hidden variable theories) rejecting locality, with the operationalists (which, as I understand it, is the most common view) embracing it.

Sounds like the physicists maybe mixed up the concepts of correlation and causality.

Mixed up in what way?

Note that if you have two observers a light-second apart, and they only choose what measurement to do 0.01 seconds before performing it, at least one of those choices affects both results.

Using your analogy, you don’t put the legos in opposite orientation and walk away with one, it’s more like the two legos keep spinning and you walk away with one and you stop it from spinning and instantaneously the other one stops in the opposite direction.

What does “instantaneously” mean if we (afaiu) can’t even have “simultaneously” in general?

Aiui, it doesn’t at all matter which one is measured “first”, or if the two measurements are spacelike separated such that which one is first depends on frame of reference.

Whenever the two measurements are made, they will be correlated in a certain way.

You cannot communicate information from one to the other via how you choose to measure them, but you can in some cases coordinate on how your actions are correlated by choosing how to measure based on some info learned on one side.

So, if the direction along which A chooses to measure the spin depends on something A learned after A and B went far away from each-other, this can’t change the probabilities of B getting a result of 0 or 1, but it can change the probabilities of things like “A and B both get 1” or “A gets 1 and B gets 0”, etc. , even though not in a way that changes how likely it is for A to get 1, nor how likely for B to get 1.

This could conceivably be useful if uh, how useful it is for both parties to do something vs only one party doing it, changes based on the info, I guess?

All that pops into my head here, is that in this case, the two particles, when twinned, are actually the same particle... simply at different time references.

This behaviour makes zero sense to me otherwise.

That makes me think that all particles are actually the same particle, and we're only observing them at different time references. The twinning only objectifies that, so as observers, we are viewing them at time references which allow for known state.

Yes, this has a gazillion holes in it.

edit: to add to this, there is no reason to infer that because things are separated by distance, they are not the same thing. The universe does not have to play nice, to ensure that humanly derived, evolved logic thinks that things make sense from a classical perspective.

If all electrons, for example, are the same electron? Just observed at different time references and locations? Then things such as entanglement make more sense.

edit2: What I mean by 'the same particle', is that theoretically all matter in the universe was one ball of whatever prior to the big bang. Everything already is one particle, prior to diffusion and differentiation due to cooling.

Thus, every electron in the universe was at one point in time the same, single thing. Therefore, they are the same thing. If time does not exist, it means that this electron here, and over there, can be the same electron, for it can spend <unlimited time> here, and then <unlimited time> there, while at the same moment co-spanning the same time reference as we view it.

OK, I'll shut up now, because this is gibberish.

>That makes me think that all particles are actually the same particle

This sounds similar to Quantum Field Theory[1], which might interest you. I won't try to summarise it here because I don't have a good grasp on it myself, as a layperson (with a below-average understanding of physics, realistically!)

1: https://en.wikipedia.org/wiki/Quantum_field_theory

Thanks I shall poke at this.

Its... weirder than that. The issue is that the measurement of the particle changes it - and changes the entangled particle.

PBS Space Time - Quantum Entanglement and the Great Bohr-Einstein Debate https://youtu.be/tafGL02EUOA (and then watch the referenced Veritasium video)

The way they are linked is that each particle (in this case lego) actually has no orientation until you measure it, it's in a quantum superposition. But the 2 legos are guaranteed to have opposite orientations. One lego has orientation x, the other has !x

> until you measure it, it's in a quantum superposition

I assume it's been proved that it's in superposition, rather than in a static state? If so, how? Wouldn't knowing require measuring?

Or, maybe to make myself more productive, what's a good search term for this? :)

Bell's Theorem and the many experiments that followed allegedly prove that they couldn't have taken on any state before measurement, based on probability theory. If they had predetermined states before you measured them, there is a limit to how often an experiment could yield a certain result. But in these experiments, that limit is exceeded.

Search for "bell inequality".

There are reliable ways to create superposition particles: when you measure in one direction, the other direction is a superposition (https://physics.stackexchange.com/questions/43974/how-can-we...). I have no idea how to create entangled superposition particles: maybe read https://www.eurekalert.org/news-releases/473741.

You can tell the particles are in a superposition because they behave differently than those in a static state:

- Double-split experiment: https://en.wikipedia.org/wiki/Double-slit_experiment - only particles in a superposition create interference patterns

- Bell's paradox: https://www.youtube.com/watch?v=v7jctqKsUMA, https://www.youtube.com/watch?v=hiyKxhETXd8 - entangled particles in a superposition can be measured to make predictions with more confidence than if they were simply static particles with different states

I guess a good search term would be "Polarized light and quantum mechanics" because IMO polarized light is a good example which you can even simulate yourself if you have a polarizing filter (https://www.youtube.com/watch?v=zcqZHYo7ONs&t=736s, https://www.youtube.com/watch?v=ZudziPffS9E, https://chem.libretexts.org/Bookshelves/Physical_and_Theoret...)

The polarised one is really crazy as it is macroscopic and daily life.

It's also dead easy to do - use a cheap laser pointer, a tinfoil cap with a pinhole, and some polarising lenses from an optician (he gave me a couple for free).

This is also my question/confusion.

You're describing a Hidden Variable Theory, which does not work https://en.wikipedia.org/wiki/Hidden-variable_theory

Local hidden variable theories are ruled out by Bell's Theorem. Not hidden variable theories in general. I realise GP was describing a local HVT, but it's still an important distinction.

Sure it can. Bell's theorem implies you can have locality or hidden variables, but not both. de Broglie-Bohm is an example of a non-local hidden-variable theory.

From all I can tell about what physicists mean, their use of correlation is inappropriate.

It appears that their math of correlation is correct for the math of correlation, all independent of any connections with physics.

But from all I can tell, what physics they are really talking about is lack of independence. Two random independent random variables have correlation zero. But correlation zero does not mean independent.

So, if physicists keep using correlation and find correlation zero, sorry, but there can still be dependence.

So, net, in short, for what physics is considering in the real world, they want to know if two measurements, each a random variable, are independent or not, not have zero correlation or not.

Yep, it must be all of quantum physicists collectively misusing a word, and not you misunderstanding something. The hubris is unreal.

The main issue is that the explanation should not imply faster-than-light communication.

That isn't implied. A particle pair collapsing from a superposition of to one of its eigenstates upon measurement, is a change in the particle in one location that instantaneously affects the other for all intents and purposes. This still doesn't allow you to use this to transfer information from one point to another faster than light due to the way nature lets you observe and manipulate things (e.g. the no-cloning theorem crucially prevents you from by-passing the permanence of quantum collapse).

But the effect is very much real. Much more real than you might think.

You can't use it to communicate faster than light, but you absolutely can use it to coordinate faster than light. It's called quantum pseudo-telepathy. See for example the Mermin–Peres magic square game. Two players with two pairs of entangled particles can win this game with 100% probability even if they're lightyears apart, whereas players without this resource can't win 100% of the time if they're separated (e.g. by adding sufficient distance and a time limit to the game).

> This isn't implied. A particle pair collapsing from a superposition of to one of its eigenstates upon measurement, is a change in the particle in one location that instantaneously affects the other for all intents and purposes.

If you were to use this explanation to anyone that isn't already deeply familiar with how quantum entanglement works, the "instantaneous" part of it would absolutely imply either FTL communication or at the very least the idea of a hidden variable behind the particles at the point of entanglement. And that's assuming they even understand what an "eigenstate" even is.

Non local hidden variables aren’t ruled out.

It absolutely is a FTL effect, even if we can’t use it to communicate classical information.

> That isn't implied.

I disagree.

Ask 10 random people to read the phrase "links them so that changes in one affect the other", without any other disclaimers attached.

I bet some of them will think that means you can change A and see the change on B.

It's not good wording.

The wording is fine. Making people realize that a FTL effect is happening is more important than telling them that due to the no-cloning theorem we can’t directly observe it.

1. Observing the event or trying to clone entire states isn't what anyone was worried about. It's the idea that you could set even one bit to measure later (which is trivial to do with a normal particle).

2. There's no tradeoff here. "more important" doesn't matter. It's easy to make a sentence that states an FTL effect occurs and that data can't be sent.

But a type of non-classical data can be sent. See quantum pseudo telepathy.

Saying it happens "instantly" implies a preferred frame, but the process is invariant under Lorentz boosts.

It also assumes a specific way of representing the quantum state (eg a state vector over all parts) whereas other representations like Feynman paths have no corresponding notion of an instant change.

Instantly implies FTL. How is that incorrect? The effect is very much FTL.

https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy

What I'm trying to convey is that the statement "entanglement is instant" is implementation dependent. You can meet the specifications of quantum mechanics using a simulator where entanglement is instantaneous, but you can also meet those specifications using a simulator where entanglement is not instantaneous or where that concept doesn't even type check.

What we can objectively say is that entanglement doesn't care about the distinction between time-like separation and space-like separation. If two non-communicating parties are entangled and they perform an experiment, there is no observable distinction between the cases where one of them goes first or the other goes first or in whether there is time for light to travel between them in the delay between their two parts. But that is much looser than requiring the effect to be instantaneous in some unspecified frame, because only requiring that there's no observable distinction is a much looser constraint than specifying that it must be instantaneous. And the only thing that we can experimentally verify is the looser constraint.

When one collapses does it’s entangled counter part collapse to a particular state immediately?

In quantum information theory the ability of Alice to instantaneously change Bob's quantum state (and hence measurement probabilities) is referred to as quantum steering. No information can be sent, but the influencing effect can be defined and observed. Steering is distinct from both Bell-nonlocality and nonseparability. See, e.g., [1] and later papers.

[1] https://arxiv.org/abs/quant-ph/0612147

Do you mean that it was proven that observation affects the other end immediately, or is it only just a theory? If so, how it was proven, without it also being useful for sending information?

It is a theory in the sense that it is epistemic. Bob's quantum state changes depending on Alice's measurement, essentially collapsing the probability distribution to the conditional probability distribution. As science it cannot be proven, but the empirical results match the model. The paper I cited defines precise protocols to define terms like "steerable states." This is standard quantum theory, so the no-communication theorem still applies.

Is the quantum state also ontic, and in what sense? Those are open questions of interpretation.

There are any number of models that are consistent with the observed behavior. Only some of them require things to happen "instantaneously".

The accurate way to describe entanglement is probably that the entangled systems will never be observed to be in an incompatible state.

My favorite head-canon is that measurement of one particle determines the other's state at the time of entanglement. As in, the measurement changes the past, or sets the universe on a worldline compatible with the entangled pair's state.

If you have the left sock you know the other one is the right sock.

I guess my question here is when you measure one entangled bit, does it always collapse to the same state or can it be either of the two states? Based on my understanding it’s the former right?

> If you have the left sock you know the other one is the right sock.

Possibly a poor analogy; none of my socks are chiral.

It's fine and easy to understand

Sure, while also being completely wrong.

Ironically it’s correct, and it’s pedantic HNers that are wrong. It can’t be used to transmit classical information, but it’s still an instantaneous effect FTL.

https://en.wikipedia.org/wiki/Quantum_pseudo-telepathy

If enough of these or enough distance between them is achievable, would there be any experiments possible to probe the interactions between general relativity and quantum mechanics?

2m is definitely enough to measure relativistic effects in earths gravity with the most precise atomic clocks we have.

Can anyone explain what is the usual expalnation for what happens when two entangled particles are measured? That is, if one of the particles is measured, and the other is not, what happens to the second one? Is there some time where the second particle is still entangled to the first one or is it going to be entangled until measured?

> Can anyone explain what is the usual expalnation for what happens when two entangled particles are measured?

In the most extreme case of entanglement, the two systems cannot be described separately, but only by some coupled state. I.e., if you have two particles that can be spin up or spin down, you either find them both spin up, or both spin down as an example. That means, if you measure the spin of particle 1 you already know the spin of particle 2. We interpret this as the measurement affecting the other particle instantaneously even though they may be spatially separated. More generally, entanglement just means that there is an extremely strong correlation between the two systems.

> That is, if one of the particles is measured, and the other is not, what happens to the second one? Is there some time where the second particle is still entangled to the first one or is it going to be entangled until measured?

There is immediately no longer any entanglement after one of the particles is measured. If you measure the same observable again you will get the same result, and if you measure an observable incompatible to that observable on one particle you will not affect the other particle.

Could that be used to measure one-way speed of light?

https://en.m.wikipedia.org/wiki/One-way_speed_of_light

No. The standard calculations about the time dilation experienced when moving them apart don't hold without constant speed of light.

From /r/science[0]: https://news.mit.edu/2020/atomic-clock-time-precise-1216

[0]: https://old.reddit.com/r/science/comments/x8khxg/two_atomic_...

https://archive.ph/VVwbl

EDIT: ah, dang, also doesn't work

paywalled. 12ft.io doesnt help.

Full article:

http://webcache.googleusercontent.com/search?hl=en&q=cache%3...

12ft is awesome, thanks!

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