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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*