In the article "“Proposal for an experimental test of the many-worlds interpretation of quantum mechanics” its author R. Plaga suggested that if we trap an ion in a quantum well, we can later use it for one-time communication between multiverse branches.
The paper must be wrong. Inter-branch communication is impossible. I searched briefly for a published rebuttal, and found this guy on quora who claims that Plaga himself eventually came around to his proposal being mistaken. That’s hearsay about Plaga, but the quora response also purports to explain the exact error, and it seems sensible at a glance. (I didn’t scrutinize either the paper or the quora rebuttal; I’m more confident about the wrongness than about what exactly the error is.)
Thanks to finding this. As I understood, the rebuttal says that if you excite the ion in one branch, it becomes entangled with that branch and stops being non-entangled with anything. It is easy to see from energy conversation: if I have excited the ion I spent some energy, but if I see the ion excited in another brach, this energy would come from nowhere, which is prohibited by conversation law.
But also I think there still can be way to communicate: I think that instead of exciting ion, the senders have just to measure it (and it should be prepared in a complex state from the beginning). And the receiver will have to measure if ion remains in quantum state or is in one of eigenstates. This makes the whole setup more complex and may require more ions measurements to send just one but of data.
No matter what basis they measure in, the receiver will observe results consistent with the ion being in whatever state it was already in before the senders even did anything. This is a result of the linearity of quantum mechanics. If the overall wavefunction is a sum of two nearly-orthogonal vectors, then the evolved wavefunction is the sum of each vector evolved separately, and the terms in this sum will also be nearly-orthogonal. In equations:
If wasn't there, then would still evolve to and see the exact same outcomes. To get communication, there would have to be significant amplitudes for the universe's state to spontaneously shift from being in one world to the other. (i.e. even if world 1 is initially the only world, it still has some amplitude to end up in world 2). This is not realistic for the physics of macroscopic objects. We don't see, either theoretically or experimentally, large amplitudes for a dead cat to turn into a live one, etc, even if the initial decision to kill the cat or not was made by measuring polarization of a single photon.
EDIT: Also, it is a well known fact in QM that "one does not simply measure whether a system is an eigenstate or a superposition". If you measure a spin of up for an electron, you do not know whether it was actually spinning up, or it was spinning left and you happened to measure the "up" component of the left spin.
But if you're just concerned about energy conservation, such a complicated fix is not needed anyways: There are many systems that have multiple quantum states with identical energy, momentum, angular momentum, etc, yet are still orthogonal (i.e. perfectly distinguishable by measurement).
So the real reason it doesn't work is linearity, not energy-conservation or anything like that.
In Plaga model, the trapped ion is isolated from both branches and thus orthogonality equation is not directly applicable to it.
"one does not simply measure whether a system is an eigenstate or a superposition"
We can't do it simply, but with some efforts we can do it. We can measure was the ion in superposition or not if we repeat the experiment many times in exactly the same settings. Eigenstate will give always 1. Superposition state will give mix of 0 and 1 measurements.
The fix is needed mostly not because energy conversation, but because if we transfer any definite state, the sender destroys isolation of the ion by becoming in superposition in it (as was discussed in Quora objection to Plaga model). Here I suggest to use the mere fact of destruction of ion's quantum state as a way to transfer information.
You mean linearity equation?
If the ion is isolated, that means you take a tensor product of its state with the state of the environment. If are orthogonal, then and are still orthogonal.
Why does your "repeated measurement" method not also work to use entangled qubits to send signals faster than light? (Since measuring one qubit also collapses the state of the other.)
Or, maybe just tell me the density matrix for the ion that you expect the reciever to see if the sender sends a 0, and also the density matrix for if they send a 1?
Plaga's article has equations and was published in a scientific journal - Foundations of Physics, 1997. The journal had small impact factor at the time and its editor was Carlo Rovelli. There is no public retraction or refutation of it, except an obscure Quora post where someone said that Plaga doesn't endorse this anymore. Plaga had around 10 astrophysics articles in 90s.
He (or a person with the same name) works now on "LLM security in Germany" - not a really bad sign as H.Everett also turned to defense industry later.
1This is extremely weak signal compared to understanding the technical argument, the literature is full of nonsense that checks all the superficial boxes. Unfortunately it's not always feasible or worthwhile to understand the technical argument. This leaves the superficial clues, but you need to be aware how little they are worth.
Interesting coincident: Yesterday a new preprint appeared on the same topic
Quantum observers can communicate across multiverse branches
Maria Violaris
It is commonly thought that observers in distinct branches of an Everettian multiverse cannot communicate without violating the linearity of quantum theory. Here we show a counterexample, demonstrating that inter-branch communication is in fact possible, entirely within standard quantum theory. We do this by considering a Wigner's-friend scenario, where an observer (Wigner) can have quantum control over another observer (the friend). We present a thought experiment where the friend in superposition can receive a message written by a distinct copy of themselves in the multiverse, with the aid of Wigner. To maintain the unitarity of quantum theory, the observers must have no memory of the message that they sent. Our thought experiment challenges conventional wisdom regarding the ultimate limits of what is possible in an Everettian multiverse. It has a surprising potential application which involves using knowledge-creation paradoxes for testing Everettian quantum theory against single-world theories
https://arxiv.org/abs/2601.08102
Scott Aronson already wrote in FB that it is wrong.
If memory serves, the journal Foundations of Physics was long known as a place for people to publish wild fringe theories that would never get accepted by more mainstream physics journals.
I remember back in 2007, this was common knowledge, so it was big news that (widely respected physicist) Gerard 't Hooft was due to take over as editor-in-chief, and people in the physics department were speculating about whether he would radically change the nature of the journal. I don’t know whether that happened or not. But anyway, 1997 is before that.
Unrestricted inter-branch communication enables pooling classical compute from all the branches (which can be arbitrarily numerous). This is clearly not what you can get with quantum computing, so something is wrong or at least highly misleading here, giving this overly bailey-ish "inter-branch communication" claim.
S.Aaronson wrote that quantum computing is not distributing of computations between branches and inter-branch communication is not quantum computing, so I am not sure that comparing with quantum computing helps.
Anyway, there is a restriction in this setup: the communication can happen only between two branches.
However, one of the branches can be branched again during the experiment and return information to second branch which will later return it to first branch. Thus we can exponentially increase the number of different branches with we are communicating though this is not all branches. This gives us exponential increase of the complexity of tasks which can be solved but the price will accumulate quickly as each forking and return of information should be shorter in time.
Distributing computations between branches is a popular misconception about how quantum computing works. So Aaronson is naturally exasperated by needing to keep pointing out that it's not how it works, it works differently. These two things are not the same, in particular because only one of them is real.
If distributing computations between branches was possible, probably "quantum computing" would have a different meaning that involved distributing computations between branches. This post suggests what amounts to a method to distribute computations between branches, which would be more powerful than classical computers, but is also different and more powerful than quantum computers. Therefore it must be both wrong and not a way to test MWI, as quantum mechanics wouldn't expect an experiment that enables distributing computations between branches to work. If it works, it doesn't support MWI (in the usual sense, where it follows quantum mechanics), instead it shows that quantum mechanics is seriously wrong.
You are correct to point that distributing computation between branches is weakest point here, and in my early draft I wrote that it will not work because slicing computations 2 times between two branches is not a major gain and 1-bit channel is very thin information channel to transfer useful information about the results of computations. However, I changes my mind when I start imagine more complex constructions with multiple ions and forks.
If the branch-communication is real, advanced mind will find the ways to use it to accelerate some types of compositions.
I think the more use - if it will work - will come not from accelerating computations but from measuring some real world data in other branch. James Miller commented to the draft:
"This would have massive value. Imagine I need a dangerous operation.
It is done in one branch of the multiverse, and results are
communicated to the other before other decides if they should do it as
well. Or with computing. A branch spends 20 minutes doing some
computation and then tells the other branch if the computation was
worth doing. On a simpler level, you ask a girl on a date and report
if she said yes or no to yourself in another branch so he doesn't have
to risk rejection."
If you argue that branch communication is not real, there should be a reason: either MWI is false or in this exact setup there is some technical or theoretical flaw. As I think you are pro-MWI and there is not much technical details, there should be some theoretical problem. What it could be?
Or you argument can be interpreted socially: people are spending billions on quantum computers but did not ever tried an experiment which will cost much less then 1 million despite that they can earn hundred billions of dollars if it works (mostly via high-frequency trading). As there is no free money laying on the road, there must be obvious flaw. I am also puzzled about this.
If you argue that branch communication is not real, there should be a reason: either MWI is false or in this exact setup there is some technical or theoretical flaw.
MWI in the usual sense follows quantum mechanics, which predicts that branch communication is not real. An observation that branch communication is not real agrees with MWI, it doesn't suggest that "MWI is false".
As I think you are pro-MWI and there is not much technical details, there should be some theoretical problem. What it could be?
Like with any other metaphorical perpetuum mobile, the exact technical issue is not very interesting. To the first and second approximations, the exact issue shouldn't matter, a form of argument that demands tracking down the issue is already on the wrong track.
If we take relative states interpretation of Everett (of which MWI is simplification), there is no exact prohibition, but the interference between branches becomes infinitely minuscule very quickly. The only exception is the state of entanglement or any quantum state where a particle has quantum state and didn't yet collapsed to any of eigenvalues.
So the lack of interaction between branches doesn't apply to this experiment because we have trapped ion which still remains in its quantum state.
The problem of the experiment was explained to me in the comment below: If you excite the ion in one branch, it becomes entagled with that branch and stops being non-entagled with anything. It is easy to see from energy conversation: if I have excited the ion I spent some energy, but if I see the ion excited in another brach, this energy would come from nowhere, which is prohibited by conversation law. I think that instead of exciting ion, the senders have just to measure it. And the receiver will have to measure if ion remains in quantum state or is in one of eigenstates.
You are tracking down and patching the issue. Imagine a perpetuum mobile developer who had a specific issue pointed out to them, and who frantically redesigns the contraption around the place in its mechanism where the issue was identified.
(The perpetuum mobile is metaphorical, an analogy about methodology in reasoning around local vs. global claims, one that also carries appropriate connotations. I'm not saying that literally conservation of energy is being broken here.)
I think it ok to try to patch if costs to check are small and gains if it works are enormous. Any perpetuum mobile developer would say this.
I think that prohibiting is not that strict in the case inter-branch communication when in the case of energy conversation, so adding tricks can work.
Or you argument can be interpreted socially: people are spending billions on quantum computers but did not ever tried an experiment which will cost much less then 1 million despite that they can earn hundred billions of dollars if it works (mostly via high-frequency trading). As there is no free money laying on the road, there must be obvious flaw. I am also puzzled about this.
How can we know that no one has done this yet? If I were to do something like this, I would not publish a scientific paper or try to make it general knowledge, it would destroy my advantage in the market.
Inter-branch communication while preserving separate branches shouldn't be possible under the standard many-worlds interpretation, because it violates linearity.
"Reality is amplitudes flowing between configurations" is helpful for understanding conceptually why MWI (and quantum mechanics generally) predicts this won't work. For branch 2 to influence branch 1, the two branches' amplitudes would have to flow to the same configurations. So suppose Alice#1 (with high probability) receives a message, and is still aware that she is #1 and didn't compute the message herself. What does this mean for Alice#2?
It means that Alice#2 must have somehow arrived (with high probability) at the exact same physical configuration as Alice#1. If you believe, as is the orthodox view on LessWrong, that mental states are physical states, this implies that some physical mechanism has made Alice#2 forget, completely, down to the positions of the individual subatomic particles in the brain, that she was ever #2. The two Alices have split and then merged again.
Why can't #2 keep her memory and still change #1's outcome? Because then her branch's amplitude would be flowing to a different configuration! Specifically, a configuration that includes an Alice who remembers being #2 and witnessing the computation.
And separately, there must be some mechanism for branch 1's amplitudes to flow preferentially to the correct result of the computation to "meet up with" branch 2. This is even more unsatisfying, because it seems to require branch 1 to do the computation we hoped to avoid.
But could the amplitude flows of branch 1 depend on the state of branch 2? Not according to standard QM/MWI. This is where linearity comes in: as far as we can tell, the future state of a quantum system in superposition is always exactly equal to the sum of the future states of each individual configuration multiplied by their amplitudes: . ( just means "wait an amount of time equal to t".)
The contribution of branch 1 to this result – where the amplitudes of branch 1 flow – is , which doesn't change depending on what b is, or what turns out to be. Each branch evolves and splits into more branches, separately, except amplitudes add whenever branches meet. This is the entirety of the interactions between branches in Everettian MWI; there's no room for any other kind of influence.
Conversely...
...if we discovered that reality obeys a nonlinear quantum mechanics (all available evidence so far says this is false, but you never know!), then communication between branches becomes possible:
quotes from Polchinski (1991)
I find that forbidding EPR communication in nonlinear quantum mechanics necessarily leads to another sort of unusual communication: that between different branches of the wave function.
This same paper also has some interesting comments on the "branch selection" issue:
I have ignored previous branchings of the wave function, describing the macroscopic observer and apparatus as though they started in a definite state, as would be acceptable in the linear theory. However, the analysis of the Everett phone shows this assumption to be self-inconsistent: The evolution of the wave function will be coupled to all other possible states. Thus, while the analysis does show that branches are coupled to one another, practical communication between branches may be drowned out by the coupling to all the other branches of the wave function of the Universe.
So if you're hoping to get in touch with your Everett alter ego, I suggest keeping an eye on experiments probing the linearity of quantum physics.
Thanks for link. I think that communication is impossible when branches are completely separated, but the existence of the trapped ion prevents this complete separation. In other words, the separation has not ended yet in our case and the whole system has to be regarded as one system which evolves linearly.
"Branch separation" is itself contradictory concept as different understanding of what is it exists.
There are different interpretations of MWI, good overview here: https://iep.utm.edu/everett/
For example, if a atom decay, do branches separate everywhere in the universe with superliminal speed? Or branches are separated when we observe different outcomes? See recent discussion Is Branching Truly Global? From Cambridge Changes to Oxford Changes in the Many-Worlds Interpretation
What I mean is that for branches to affect each other, the branches must become completely not separated: every difference between the branches must be erased, including the ion and also the atoms in all the observers. This makes communication meaningless, because there's no longer anything the sender knows that the receiver doesn't also know.
I understood your idea. From early Plaga point of view the branches are in the process of separation, so they are not yet real branches and this allows short period of communication. I would be interested to see experimental test.
I suspect https://royalsocietypublishing.org/rspa/article-abstract/457/2009/1175/81027/Counterfactual-computation might be relevant.
It is similar idea but it requires quantum computer. and only provide computation results. Deutsch also suggested testing MWI via some quantum computer setup. The closes idea is quantum bomb tester - and such tester can be used for inter-branch communications if we turn off the bomb in the worlds where we want to send 0.
The main difference is that here we can send real world macro data like trade results or even movies to another branch.
I think that "at least one attempt confirm MWI" option has ambiguity. If a ion got excited once it will not prove anything. However, if we will be able to repeat the experiment 10 times - in one setting - , we can send some measurable data, like is N prime?
I understand physicists to expect probabilistic experiments to be repeated to a high degree of statistical significance... are you imagining an implementation that incurs enough cost per bit that the experimenters can't afford that?
If you can transmit information across branches, wouldn't you then be able to transmit digital copies of intelligent agents across as well, assuming you have someone on the other end willing to help set up whatever apparatus is needed to run the copy? At that point, don't you basically have a way to traverse the multiverse?
The problem that you can’t jump anywhere. You can communicate only between two branches which you carefully prepared beforehand - by preparing a ion in a trap and when splitting the branch.
For example, if I did this in the morning today and in the evening one branch has developed misaligned AI and my not (and we have enough trapped ions to send a lot of data) – this mislaigned AI can penetrate my branch. This limitations make risks smaller.
But there can be the ways to bypass this, for example, by finding some natural trapped long-term quantum states (in space? or inside crystals? - Opus said: "Nuclear spins in certain crystals can maintain coherence for remarkably long times—seconds to hours in some cases—because they're somewhat shielded from electromagnetic noise by their electronic environment. Phosphorus-31 nuclear spins in silicon are a famous example. But even these eventually decohere, and they're not truly "trapped" in isolation")
It sounds like inter-branch communication would imply that you could do that. OP does mention that as an application:
7. Superintelligence jumps from one branch to another and also gains more computational power via branch-distributed computations. The exchange of ideas between branches would accelerate science and progress in AI algorithms. This all means that inter-branch communication and the singularity would happen almost simultaneously.
Superintelligent AI will likely appear before this technology matures in the current setup, but branch communication could help AI propagate between branches and increase its dominance on Earth and in the multiverse.
In the article "“Proposal for an experimental test of the many-worlds interpretation of quantum mechanics” its author R. Plaga suggested that if we trap an ion in a quantum well, we can later use it for one-time communication between multiverse branches. In a recent experiment, it was shown that such practically long trapping – 20 minutes – is possible. We could potentially reach a world where inter-branch communication becomes as routine as television, with implications for trading, politics, visual arts, and AI.
Plaga wrote:
"The separation of worlds in the MWI is never quite complete therefore, and there should be small influences from a parallel world even after decoherence, which must be measurable in principle. This has been most clearly pointed out by Zeh [16, 17]. In Ref. [16] he discusses the possibility to observe 'probability resonances' (later further discussed by Albrecht [18]), which occur at a singular point when the amplitudes of ψ₁ and ψ₂ have exactly the same magnitude. An experiment to test the MWI against the orthodox interpretation along similar lines was proposed by Deutsch [19]. Unfortunately it is still far from practical realization, as it requires a computer which remains in a quantum mechanically coherent state during its operations and in addition possesses artificial intelligence comparable to that of humans… p. 3"
"This proposition is not realistic if the 'gateway state' is macroscopic, because the required isolation would be difficult to achieve technically (see however recent experiments with macroscopic quantum systems, e.g. Ref. [20]). Since the late 1970s it has become possible to perform precision experiments on single ions stored for long times in electromagnetic traps [21]. I will show in section 4 that these single ions are isolated from the environment to such a degree that the decoherence timescale is on the order of seconds or longer with existing technical ion-trap equipment. Moreover, it is possible to excite these atoms before they are correlated with the environment to such a degree that complete decoherence took place. In our example above, Silvia₁ switches on the microwave emitter long enough to excite an ion in a trap with a large probability. After that, Silvia₂ measures the state of the ion and finds that it is excited with some finite probability, though Silvia verified it was in the ground state before the branching took place. From that, Silvia₂ infers the existence of Silvia₁. In an obvious way, Silvia₁ and Silvia₂ can exchange information (bit strings of arbitrary length), e.g., by preparing more than one isolated ion. Single ions in traps can act as 'gateway states,' and communication between parallel worlds is possible."
Algorithm of Communication
It is surprising that we do not know whether such an experiment was ever performed, as it seems trivial and was suggested 30 years ago.
Here I will examine possible practical applications depending on split time duration.
I. Communication After 1-Second Splitting
1. MWI is definitively proved. This could be tested even in very short time periods with computers where one branch performs computations and sends the result to the other branch. The test needs to be performed many times to show that each excited state corresponds to real solutions of complex mathematical problems that were not solved on the receiver side. If the experiment fails, we can test the limits where branch communication still works. There is some similarity between this experiment and the Elitzur–Vaidman bomb tester. If the bomb is sometimes replaced in the middle of the experiment with a non-explosive one, it will work as a method of inter-branch communication.
2. High-frequency trading will gain a new way to earn money. There will be some correlation in trades—for example, if traders started selling in a parallel world, they will soon sell in ours too, even if branch communication holds for only one second. Another example involves poker-like games: one can perform a small action in one branch that immediately reveals the other side's hand and send this information to another branch where a much larger action is performed based on this information.
3. Some types of computations can achieve enormous speedup. Multiple branching can be helpful—where Alice₂ creates Alice₃ and so on, and each Alice performs a different set of computations—after which they combine answers and see which Alice solved the problem. This may help with integer factorization and thus with breaking cryptographic codes and Bitcoin. Branch-distributed computation is not the same as quantum computing—for example, we can disperse the cost of an expensive experiment between different branches, with each branch testing properties of just one molecule.
II. Longer Splits Between Branches—Days or Even Years
This section assumes larger numbers of ions, perhaps billions. This is obviously more speculative than single-ion measurement but logically follows from the possibility.
4. Experimental history becomes possible. What will happen in another branch where a different president was elected? Are they doing much better?
5. Communication with deceased relatives who are still alive in another branch.
6. Exchange of visual art and music.
7. Superintelligence jumps from one branch to another and also gains more computational power via branch-distributed computations. The exchange of ideas between branches would accelerate science and progress in AI algorithms. This all means that inter-branch communication and the singularity would happen almost simultaneously.
Superintelligent AI will likely appear before this technology matures in the current setup, but branch communication could help AI propagate between branches and increase its dominance on Earth and in the multiverse.
8. Tragic losses of communication with some branches when trapped ions are exhausted, and solving this problem by multiplying coherent ions or through other mechanisms.
Additional Considerations
It seems that communication can be bidirectional if some ions are used for sending and some for receiving. While first applications may be expensive, the technology will quickly advance into microchips able to generate many billions of coherent isolated quantum communication bits. Funding likely comes from trading and military sources.
Open Problems
Branch selection: There are infinitely many branches, but only two are communicating. How are these two selected? The answer is that they are selected when Alice performs the first quantum measurement and determines whether she will act as Alice₁ or Alice₂.
Temporal paradoxes: Can we perform the measurement before the change was made in another branch and thus obtain information about its future? Would this produce something like a time machine? Ion excitation does not violate the arrow of time, but entanglement might work—I am not sure here.
Evolutionary exploitation: Can evolution exploit this? For example, if a lion eats me in a parallel branch, do I become more anxious?
Global risks: What are the global catastrophic and geostrategic risks if branch communication becomes possible? Virus-like information propagating between branches? Time-travel-like paradoxes? Strategic instability?
I want to thank James Miller for useful comments.