pcalau12i

The reason you are able to transfer an infinite-complex quantum state over a finite-complex classical communication channel is because the "quantum state" doesn't even exist, at least not in a single experiment. You can only ever measure it over an ensemble of systems, i.e. infinitely many systems prepared in the same way, as it is a statistical stat only. Your classical measurement results from the qubits to be "teleported" are also random, and you send that classical information over, and then that is used to construct another qubit in a particular state.

Over an ensemble of systems (repeating the experiment an infinite number of times), it's guaranteed the statistics on the qubit to be "teleported" will be transferred to now be the same as the statistics of the qubit it was "teleported" onto. Hence, you end up transferring the quantum state from one qubit to another using a classical channel. Quantum teleportation is trivial to reproduce in a classical model and it's not even inherently a "quantum" phenomena, see the Spekkens toy model for example.

It's not really that horrible of a name, because what the people who coined the term had in mind was something akin to Star Trek teleportation where you are not actually causing the object to disappear and reappear elsewhere nonlocally, but instead you are doing something "destructive" to the original object, transmitting information over a traditional communication channel, and then using that information to reassemble it with different material on the other end. Quantum teleportation is "destructive" to the original qubit in the sense that it places the qubit into a state that no longer matches what you are trying to transmit, but you gain enough information from doing this to transmit it to the other party who can use that information to (statistically) reconstruct the quantum state using their own qubits on their own end.

There are definitely more potential usages than cryptography (not really even sure I'd classify quantum direct communication as a kind of "cryptography" but that's nitpicking; it doesn't encrypt anything, it just lets you detect if someone physically disturbed the message in transit), precisely because it does it over a classical channel, that means the transfer would be much more robust to noise (and thus robust to decoherence). You need to establish an entangled Bell pair first before you can perform quantum teleportation which requires a quantum channel, but you can use quantum distillation to transfer many, many Bell pairs over a noisy network and then "distill" out a low-noise Bell pair, and then once you have achieved that you can then use quantum teleportation to transfer over a qubit via a classical communication channel.

I could also see it potentially being useful to transfer quantum information from one medium to another. Let's say you have qubits encoded in light and qubits encoded in electron spin and you want to transfer a qubit encoded on one onto the other. For example, in this paper they use quantum teleportation to transfer the information from one medium to the next.

People love to be pedantic as an "own" because they think it makes them look smart. And a lot of the times it actually works / is rewarded.

This is sadly pseudoscience, that only gets talked about because one smart guy endorsed it, but hardly anyone in academia actually takes it seriously. What you are talking about is called Orch OR, but Orch OR is filled with problems.

One issue is that Orch OR makes a lot of claims that are not obviously connected to one another. The reason this is is an issue is because, while they call the theory "falsifiable" because it makes testable predictions, even if the predictions are tested and it is found to make the correct prediction, that wouldn't actually even validate the theory because there is no way to actually logically or mathematically connect that experimental validation to all of its postulates.

Orch OR has some rather bizarre premises: (1) Humans can consciously choose to believe things that cannot be mathematically proven, therefore, human consciousness must not be computable, (2) you cannot compute the outcome of a quantum experiment ahead of time, therefore there must be an physical collapse that is fundamentally not computable, (3) since both are not computable, they must be the same thing: physical collapse = consciousness, (4) therefore we should look for evidence that the brain is a quantum computer.

Argument #1 really makes no sense. Humans believing silly things doesn't prove human decisions aren't computable. Just look at AI. It is obviously computable and hallucinates nonsense all the time. This dubious argument means that #3 doesn't follow; there is no good reason to think consciousness and "collapse" are related.

Argument #2 is problematic because physical collapse models are not compatible with special relativity or the statistical predictions of non-relativistic quantum mechanics, and so they cannot reproduce the predictions of quantum field theory in all cases, and so they aren't particularly popular among physicists, and of course there is no evidence for them. Most physicists see the "collapse" as an epistemic, not a physical, event.

Orch OR also arbitrarily insists on using the Diósi–Penrose model specifically, even though there have been multiple models of physical collapse proposed, such as GRW. There is no obvious reason to use this model specifically, it isn't connected to any of the premises in the theory. Luckily, argument #2 does present falsifiable claims, but because #2 is not logically connected to the rest of the arguments, even if we do prove that the Diósi–Penrose model is correct, it doesn't follow that #1, #3, or #4 are correct. We would just know there are physical collapses, but nothing else in the theory would follow.

The only other argument that propose something falsifiable is #4, but again, #4 is not connected to #1, #3, or #4. Even if you desperately searched around frantically for any evidence that the brain is a quantum computer, and found some, that would just be your conclusion: the brain is a quantum computer. From that, #1, #2, and #3 do not then follow. It would just be an isolated fact in and of itself, an interesting discovery but wouldn't validate the theory. I mean, we already have quantum computers, if you think collapse = consciousness, then you would have to already think quantum computers are conscious. A bizarre conclusion.

In fact, only #2 and #4 are falsifiable, but even if both #2 and #4 are validated, it doesn't get you to #1 or #3, so the theory as a whole still would remain unvalidated. It is ultimately an unfalsifiable theory but with falsifiable subcomponents. The advocates insist we should focus on the subcomponents as proof it's a scientific theory because "it's falsifiable," but the theory as a whole simply is not falsifiable.

Also, microtubules are structural. They don't play any role in information processing in the brain, just in binding cells together, but it's not just brain cells, microtubules are something found throughout your body in all kinds of cells. There is no reason to think at all they play any role in computations in the brain. The only reason you see interest in them from the Orch OR "crowd" (it's like, what, 2 people who just so happen to be very loud?) is because they're desperate for anything that vaguely looks like quantum effects in the brain, and so far microtubules are the only things that seem quantum effects may play some role, but this role is again structural. There is no reason to believe it plays any role in information processing or cognition.

I think a lot of proponents of objective collapse would pick a bone with that, haha, although it’s really just semantics. They are proposing extra dynamics that we don’t understand and can’t yet measure.

Any actual physicist would agree objective collapse has to modify the dynamics, because it's unavoidable when you introduce an objective collapse model and actually look at the mathematics. No one in the physics community would debate GRW or the Diósi–Penrose model technically makes different predictions, however, and in fact the people who have proposed these models often view this as a positive thing since it makes it testable rather than just philosophy.

How the two theories would deviate would depend upon your specific objective collapse model, because they place thresholds in different locations. For GRW, it is based on a stochastic process that increases with probability over time, rather than a sharp threshold, but you still should see statistical deviations between its predictions and quantum mechanics if you can maintain a coherent quantum state for a large amount of time. The DP model has something to do with gravity, which I do not know enough to understand it, but I think the rough idea is if you have sufficient mass/energy in a particular locality it will cause a "collapse," and so if you can conduct an experiment where that threshold of mass/energy is met, traditional quantum theory would predict the system could still be coherent whereas the DP model would reject that, and so you'd inherently end up with deviations in the predictions.

What’s the definition of interact here?

An interaction is a local event where two systems become correlated with one another as a result of the event.

"The physical process during which O measures the quantity q of the system S implies a physical interaction between O and S. In the process of this interaction, the state of O changes...A quantum description of the state of a system S exists only if some system O (considered as an observer) is actually ‘describing’ S, or, more precisely, has interacted with S...It is possible to compare different views, but the process of comparison is always a physical interaction, and all physical interactions are quantum mechanical in nature."

The term "observer" is used very broadly in RQM and can apply to even a single particle. It is whatever physical system you are choosing as the basis of a coordinate system to describe other systems in relation to.

Does it have an arbitrary cutoff like in objective collapse?

It has a cutoff but not an arbitrary cutoff. The cutoff is in relation to whatever system participates in an interaction. If you have a system in a superposition of states, and you interact with it, then from your perspective, it is cutoff, because the system now has definite, real values in relation to you. But it does not necessarily have definite, real values in relation to some other isolated system that didn't interact at all.

You can make a non-separable state as big as you want.

Only in relation to things not participating in the interaction. The moment something enters into participation, the states become separable. Two entangled particles are nonseparable up until you interact with them. Although, even for the two entangled particles, from their "perspectives" on each other, they are separable. It is only nonseparable from the perspective of yourself who has not interacted with them yet. If you interact with them, an additional observer who has not interacted with you or the three particles yet may still describe all three of you in a nonseparble entangled state, up until they interact with it themselves.

This is also the first I’ve heard anything about time-symmetric interpretations. That sounds pretty fascinating. Does it not have experimenter “free will”, or do they sidestep the no-go theorems some other way?

It violates the "free will" assumption because there is no physical possibility of setting up an experiment where the measurement settings cannot potentially influence the system if you take both the time-forwards and time-reverse evolution seriously. We tend to think because we place the measurement device after the initial preparation and that causality only flows in a single time direction, then it's possible for the initial preparation to affect the measurement device but impossible for the measurement device to affect the initial preparation. But this reasoning doesn't hold if you drop the postulate of the arrow of time, because in the time-reverse, the measurement interaction is the first interaction in the causal chain and the initial preparation is the second.

Indeed, every single Bell test, if you look at its time-reverse, is unambiguously local and easy to explain classically, because all the final measurements are brought to a single locality, so in the time-reverse, all the information needed to explain the experiment begins in a single locality and evolves towards the initial preparation. Bell tests only appear nonlocal in the time-forwards evolution, and if you discount the time-reverse as having any sort of physical reality, it then forces you to conclude it must either be nonlocal or a real state for the particles independent of observation cannot exist. But if you drop the postulate of the arrow of time, this conclusion no longer follows, although you do end up with genuine retrocausality (as opposed to superdeterminism which only gives you pseudo-retrocausality), so it's not like it gives you a classical system.

So saying we stick with objective collapse or multiple worlds, what I mean is, could you define a non-Lipschitz continuous potential well (for example) that leads to multiple solutions to a wave equation given the same boundary?

I don't know, but that is a very interesting question. If you figure it out, I would be interested in the answer.

Many of the interpretations of quantum mechanics are nondeterministic.

1. Relational quantum mechanics interprets particles as taking on discrete states at random whenever they interact with another particle, but only in relation to what they interact with and not in relation to anything else. That means particles don't have absolute properties, like, if you measure its spin to be +1/2, this is not an absolute property, but a property that exists only relative to you/your measuring device. Each interaction leads to particles taking on definite states randomly according to the statistics predicted by quantum theory, but only in relation to things participating in those interactions.

2. Time-symmetric interpretations explain violations of Bell inequalities through rejecting a fundamental arrow of time. Without it, there's no reason to evolve the state vector in a single time-direction. It thus adopts the Two-State Vector Formalism which evolves it in both directions simultaneously. When you do this, you find it places enough constructs on the particles give you absolutely deterministic values called weak values, but these weak values are not what you directly measure. What you directly measure are the "strong" values. You can interpret it such that every time two particles interact, they take on "strong" values randomly according to a rule called the Aharonov-Bergmann-Lebowitz rule. This makes time-symmetric interpretations local realist but not local deterministic, as it can explain violations of Bell inequalities through local information stored in the particles, but that local information still only statistically determines what you observe.

3. Objective collapse models are not really interpretations but new models because they can't universally reproduce the mathematics of quantum theory, but some serious physicists have explored them as possibilities and they are also fundamentally random. You assume that particles literally spread out as waves until some threshold is met then they collapse down randomly into classical particles. The reason this can't reproduce the mathematics of quantum theory is because this implies quantum effects cannot be scaled beyond whatever that threshold is, but no such threshold exists in traditional quantum mechanics, so such a theory must necessarily deviate from its predictions at that threshold. However, it is very hard to scale quantum effects to large scales, so if you place the threshold high enough, you can't practically distinguish it from traditional quantum mechanics.

Arguably, if we insist on trying to come up with the simplest way to explain non-relativistic quantum mechanics, that is to say, if we are very conservative and stick to classical explanations unless we absolutely are forced not to (rather than throwing our hands up and saying it's all magic that's impossible to understand, as most people do), then we find that it comes naturally to explain non-relativistic quantum mechanics by treating particles as excitations in a classical field. This alone can explain the interference-based paradoxes in completely classical terms, like double-slit or Elitzur-Vaidman paradox, without altering any of the postulates of the theory in any way. The extension to quantum field theory then becomes more natural and intuitive. imo

For any physical theory, you can always just ask "why x", like a child who constantly asks "why" over and over again to every answer, but you will always hit a bottom. There seems to be a popular mentality that "why x" is always a meaningful question, and from that, we can conclude that we don't know anything at all, because all our beliefs rely on a "why x" we don't know the answer to, an so they are all baseless. We can't make any truth claims about the behavior of particles, galaxies, or anything, because you can just infinitely ask "why" until we hit a bottom and then you would say "I don't know."

But, personally, I find this point of view rather bizarre, because, again, it can make it seem like we don't know anything at all and have no foundations for truth claims in the slightest, and are completely ignorant about everything. I think it makes more coherent sense to just allow for to be a bottom to the questioning. Eventually a string of "why" questions will reach a bottom, where that bottom shouldn't be answered with "I don't know" but it should be answered with "it is what it is," because, for all we know, it is indeed an accurate description of reality at a fundamental level and there is nothing beneath it.

That shouldn't be taken as a strong claim that there definitely isn't anything beneath it, as if we should just accept our current most fundamental theories are the end of the line and stop searching. It should be taken as the weaker claim that as far as we currently know it is the bottom, and so we can indeed make truth claims upon that basis. The child might ask, "why do things experience gravity?" You might say, "time dilation near matter." The child then may ask, "why does time dilate near matter?" In my opinion, the appropriate response to that is just, "as far as we know, it is what it is." That could change in the future, but, given our best scientific models at the present moment, that is the end of the line of the explanation.

That seems to be a fairly controversial point, though. Most people in my experience disagree, but I don't see how you can have a basis for truth claims at all if you claim that "why gravity" does indeed have an answer but you can't specify it, because then it would also be baseless to claim that gravity is caused by time dilation near matter, because you've not established that time actually does dilate near matter, as you would be claiming that this relies on postulates which you've not defined. It seems, again, simpler to just take the most fundamental theories as the postulates themselves, as the fundamental axioms.

There is a popular point of view that we shouldn't do this because scientific theories often change, so something you believe today can be proven wrong tomorrow. But then we end up never being allowed to believe anything at all. We always have to pretend we're clueless about nature because if we believe in any of our most fundamental theories, then our beliefs could be overturned. But personally, I don't see why this to be a problem. A person who believed Newtonian mechanics was fundamental to how nature worked back in the 1700s were shown later to be wrong, but that person's beliefs were still closer to reality than the people who rejected it and upheld outdated Aristotelian physics, or people who refused to belief in anything at all. It is fine to later be shown to be wrong, nothing to be upset about, nothing negative about that. We are better off, imo, as treating our best physical theories as indeed fundamentally how reality works, the "bottom" so to speak, until we find new theories that show otherwise, and we change our minds with the times.

That doesn't disallow speculation or research into potentially more fundamental theories. Theories of quantum gravity are such a speculation. They remain in the realm of speculation because no one has demonstrated in the real world that it's actually possible to construct a device such that quantum effects and gravitational effects are both simultaneously relevant and necessary to make predictions. The theories thus describe separate domains, and there isn't a genuine need for a new theory until we can figure out how to bridge the two domains in reality.

We don't actually know what would happen if we bridge the two domains. We may find that our theories of turning gravity quantum are all wrong and that in fact it is quantum theory that needs to be abandoned. We may also find that the domains aren't even bridgeable. We already know of certain physical limitations that make the domains unbridgeable, such as, building an interferometer sensitive enough to detect both gravitational and quantum effects simultaneously would collapse into a black hole. There may be more things like this we will discover later on that just render the two theories unbridgeable in physical reality.

Many physicists are convinced that the bridging will end up turning gravity quantum, but this is just a complete guess, there's no actual empirical evidence for it other than a complete historical coincidence that when studying the strong interaction physicists happened to accidentally stumble upon mathematics that also seemed to be able to also predict a particle that could explain gravity, giving birth to String Theory. People thought it must be correct because it wasn't intentional but discovered by accident, but this isn't a good criterion at all for suggesting it's correct, and ultimately the theory never went anywhere.

If we are to talk about theories replacing quantum mechanics and general relativity, we don't have a clue what these would look like because it's just speculation, and so it could go either way.

That's not true. If you read Schrodinger's original paper "The Present Situation in Quantum Mechanics" he's pretty clear that he was attempting to show how ridiculous it is to treat a superposition of states as if a particle is actually smeared out in multiple locations at once, because you could use that particle as the basis of a chain reaction that would eventually affect a macroscopic object, and then you would have to say the macroscopic object is smeared out in multiple places at once. The argument was a reductio ad absurdum for treating microscopic objects as if they are smeared out in multiple places at once. Its fundamental point was simply not a commentary on macroscopic objects but microscopic objects.

You don't need the wave function to do quantum mechanics, it's just a mathematical convenience, and so Schrodinger had insisted it shouldn't be interpreted as a literal physical object as if particles are actually spreading out as waves. In his book "Science and Humanism" he says that the reason he invented the wave formalism is because he didn't like Heisenberg's formalism which, even though it made all the right predictions, it didn't give intermediate states for particles, so it is as if they just hop around from interaction to interaction probabilistically, and the wave formalism was meant to "fill in the gaps" between the interactions.

However, in that book he also says that he believes this project was a failure because all the wave formalism does is move the gap between interactions to a gap between the evolution of the quantum state and observation, which made even less sense, and so he changed his mind and argued that we should abandon the notion of filling in the gaps between interactions, and the illusion of continuous transitions between states is only a macroscopically emergent feature.

I think it's boring honestly. It's a bit strange how like, the overwhelming majority of people either avoid interpreting quantum theory at all ("shut up and calculate") or use it specifically as a springboard to justify either sci-fi nonsense (multiverses) or even straight-up mystical nonsense (consciousness induced collapse). Meanwhile, every time there is a supposed "paradox" or "no-go theorem" showing you can't have a relatively simple explanation for something, someone in the literature publishes a paper showing it's false, and then only the paper showing how "weird" QM is gets media attention. I always find myself on the most extreme fringe of the fringe of thinking both that (1) we should try to interpret QM, and (2) we should be extremely conservative about our interpretation so we don't give up classical intuitions unless we absolutely have to. That seems to be considered an extremist fringe position these days.

The double-slit experiment doesn't even require quantum mechanics. It can be explained classically and intuitively.

It is helpful to think of a simpler case, the Mach-Zehnder interferometer, since it demonstrates the same effect but where where space is discretized to just two possible paths the particle can take and end up in, and so the path/position is typically described with just with a single qubit of information: |0⟩ and |1⟩.

You can explain this entirely classical if you stop thinking of photons really as independent objects but just specific values propagating in a field, what are sometimes called modes. If you go to measure a photon and your measuring device registers a |1⟩, this is often interpreted as having detected the photon, but if it measures a |0⟩, this is often interpreted as not detecting a photon, but if the photons are just modes in a field, then |0⟩ does not mean you registered nothing, it means that you indeed measured the field but the field just so happens to have a value of |0⟩ at that location.

Since fields are all-permeating, then describing two possible positions with |0⟩ and |1⟩ is misleading because there would be two modes in both possible positions, and each independently could have a value of |0⟩ or |1⟩, so it would be more accurate to describe the setup with two qubits worth of information, |00⟩, |01⟩, |10⟩, and |11⟩, which would represent a photon being on neither path, one path, the other path, or both paths (which indeed is physically possible in the real-world experiment).

When systems are described with |0⟩ or |1⟩, that is to say, 1 qubit worth of information, that doesn't mean they contain 1 bit of information. They actually contain as much as 3 as there are other bit values on orthogonal axes. You then find that the physical interaction between your measuring device and the mode perturbs one of the values on the orthogonal axis as information is propagating through the system, and this alters the outcome of the experiment.

You can interpret the double-slit experiment in the exact same way, but the math gets a bit more hairy because it deals with continuous position, but the ultimate concept is the same.

A measurement is a kind of physical interaction, and all physical interactions have to be specified by an operator, and not all operators are physically valid. Quantum theory simply doesn't allow you to construct a physically valid operator whereby one system could interact with another to record its properties in a non-perturbing fashion. Any operator you construct to record one of its properties without perturbing it must necessarily perturb its other properties. Specifically, it perturbs any other property within the same noncommuting group.

When the modes propagate from the two slits, your measurement of its position disturbs its momentum, and this random perturbation causes the momenta of the modes that were in phase with each other to longer be in phase. You can imagine two random strings which you don't know what they are but you know they're correlated with each other, so whatever is the values of the first one, whatever they are, they'd be correlated with the second. But then you randomly perturb one of them to randomly distribute its variables, and now they're no longer correlated, and so when they come together and interact, they interact with each other differently.

There's a paper on this here and also a lecture on this here. You don't have to go beyond the visualization or even mathematics of classical fields to understand the double-slit experiment.

Why interpret it as either? The double-slit experiment can be given an entirely classical explanation. Such extravagances are not necessary. As the old saying goes "extraordinary claims require extraordinary evidence." We should not be considering non-classical explanations unless they are genuinely necessary, and the only become necessary in contextual cases, which the double-slit experiment is certainly not such a case.

My impression from the literature is that superdeterminism is not the position of rejecting an asymmetrical arrow of time. In fact, it tries to build a model that can explain violations of Bell inequalities completely from the initial conditions evolved forwards in time exclusively.

Let's imagine you draw a coin from box A and it's random, and you draw coins from box B and it's random, but you find a peculiar feature where if you switch from A to B, the first coin you draw from B is always the last you drew from A, and then it goes back to being random. You repeat this many times and it always seems to hold. How is that possible if they're independent of each other?

Technically, no matter how many coins you draw, the probability of it occurring just by random chance is never zero. It might get really really low, but it's not zero. A very specific initial configuration of the coins could reproduce that.

Superdeterminism is just the idea that there are certain laws of physics that restrict the initial configurations of particles at the very beginning of the universe, the Big Bang, to guarantee their evolution would always maintain certain correlations that allow them to violate Bell inequalities. The laws don't continue to apply moment-by-moment, they just apply once when the universe "decides" its initial conditions, by restricting certain possible configurations.

It's not really an interpretation because it requires you to posit these laws and restrictions, and so it really becomes a new theory since you have to introduce new postulates, but such a theory would in principle then allow you to evolve the system forwards from its initial conditions in time to explain every experimental outcome.

As a side note, you can trivially explain violations of Bell inequalities in local realist terms without even introducing anything new to quantum theory just by abandoning the assumption of time-asymmetry. This is called the Two-State Vector Formalism and it's been well-established in the literature for decades. If A causes B and B causes C, in the time-reverse, C causes B and B causes A. if you treat both as physically real, then B would have enough constraints placed upon it by A and C taken together (by evolving the wave function from both ends to where they meet at B) to violate Bell inequalities.

That's already pretty much a feature built-in to quantum theory and allows you to interpret it in local realist terms if you'd like, but it requires you to accept that the microscopic world is genuinely indifferent to the arrow-of-time and the time-forwards and the time-reversed evolution of a system are both physically real.

However, this time-symmetric view is not superdeterminism. Superdeterminism is time-asymmetric just like most every other viewpoint (Copenhagen, MWI, pilot wave, objective collapse, etc). Causality goes in one temporal direction and not the other. The time-symmetric interpretation is its own thing and is mathematically equivalent to quantum mechanics so it is an actual interpretation and not another theory.