Exploiting Quantum Entanglement for Spaceflight-Related Purposes

[sanman]

Okay,

so regarding use of quantum entanglement for non-local communication

If measuring the 2 members of the entangled pair gives the correlated results, then can we say that whoever measures first matters? And if the direction of measurement influences the result, then does it amount to a signal?
Can our selection of which direction to measure from then affect measurement statistics in a way that allows a signal to be passed?

Neither Alice nor Bob know the "value" of their respective qubits.  All they know is that the message that Alice transmits and the message that Bob sees on the qubits they have was pretty simple:

...---...

They still have no idea of the "value" of their respective qubits. The value is not necessary to decode the message.  The message is not the medium, to coin a phrase.

As Sanman asks, "In what crucial way does computation differ from communication?"

I'm interested in what you said above (bold emphasis mine)

user Twark_Main posted some videos on the fundamentals, one of which said the direction/orientation of measurement affects the statistics of the observations. So can a difference in these statistics amount to a signal? Isn't Bell's Inequality observing something of that nature?

In past debates, people always reply "Yes, but you need to compare notes by classical methods to know a signal was passed."

But can we cheat using extra information? We know where Alpha Centauri is, and we know where it's going to be - even if it takes light a long time to reach us from there.
If part of our Ansible is here on Earth and the other end of the Ansible is at Alpha Centauri, can we not align the detectors at both ends, so that they have identical orientations for measurement purposes?


(just referencing 4:07 from the video Twark_Main posted)



user edzieba says:

But Ed, in quantum computing you do get to choose whether the bit is 1 or 0, because you're loading your information into the entangled qubits.

You don't, because loading inputs bits into the computer and getting some answer bits out is how a classical computer works. It is not how a quantum computer works.

Greatly simplified: you can arrange your Qbits into the representation of a function you want to compute, but observing those Qbits means you get one of the possible arrangements of bits that satisfies that function, whilst also 'breaking' your Qbits. If you want to get multiple results (i.e. to build up the probability distribution you were trying to find in the first place with your function) you re-create that function multiple times and get multiple answers. If that sounds weird and awkward and a terrible way to go about computation - yes it is, for classical computation. Quantum computers are absurdly worthless for performing the functions we are used to computer performing. That are exceptional only at performing functions that can be represented in the way quantum computation works, and are valuable for the edge cases where a function can both be expressed in a way that even makes sense to attempt quantum computation on, and that function also has some property that makes it unusually difficult to compute classically.

But if we can jig this up for computation, then why can't we do so for communication as well?
It seems to me that computation is like communication, but with extra stuff added on (ie. communication is a pre-requisite for computation. if you can do computation, you can do communication)

Using your example above, if our qubits are entangled to form a function, but we already know the answers to that function, then given enough samples (ie. enough entangled setups) then shouldn't we be able to deduce which particular qubit was measured and messed up the entanglement? Because by measuring enough samples (ie. entangled qubit functions) we can tell what's consistently matching the function value it's supposed to have, versus what's looking random and no longer entangled. So isn't that a way to encode a signal?

[sanman]

Also, can we use that Monty Hall TV gameshow "Let's Make A Deal" to help us?

[sanman]

[edzieba]

user edzieba says:

But Ed, in quantum computing you do get to choose whether the bit is 1 or 0, because you're loading your information into the entangled qubits.

You don't, because loading inputs bits into the computer and getting some answer bits out is how a classical computer works. It is not how a quantum computer works.

Greatly simplified: you can arrange your Qbits into the representation of a function you want to compute, but observing those Qbits means you get one of the possible arrangements of bits that satisfies that function, whilst also 'breaking' your Qbits. If you want to get multiple results (i.e. to build up the probability distribution you were trying to find in the first place with your function) you re-create that function multiple times and get multiple answers. If that sounds weird and awkward and a terrible way to go about computation - yes it is, for classical computation. Quantum computers are absurdly worthless for performing the functions we are used to computer performing. That are exceptional only at performing functions that can be represented in the way quantum computation works, and are valuable for the edge cases where a function can both be expressed in a way that even makes sense to attempt quantum computation on, and that function also has some property that makes it unusually difficult to compute classically.

But if we can jig this up for computation, then why can't we do so for communication as well?
It seems to me that computation is like communication, but with extra stuff added on (ie. communication is a pre-requisite for computation. if you can do computation, you can do communication)

Using your example above, if our qubits are entangled to form a function, but we already know the answers to that function, then given enough samples (ie. enough entangled setups) then shouldn't we be able to deduce which particular qubit was measured and messed up the entanglement? Because by measuring enough samples (ie. entangled qubit functions) we can tell what's consistently matching the function value it's supposed to have, versus what's looking random and no longer entangled. So isn't that a way to encode a signal?

Because you're still not transmitting any information that way. You're both able to conclude that 1+1=2 (or whatever your function is) but you don't get to choose the inputs. Even if you can deduce the inputs from the outputs, you then just have knowledge of what those unchooseable inputs were at both ends: in exactly the same position as just straight-up measuring the entangled pairs directly.


New Physics would be the appropriate place for this thread, as transmitting information superluminally via entanglement is not consistent with either Relativistic physics or QM.

[InterestedEngineer]

Here's a spaceflight scenario that uses quantum communication to get everyone to the correct rendezvous.

Suppose we make a spaceship that can go to Alpha Centauri, a triple start system in 10 years per light year, or Proxima Centauri in 43 years.  (Proxima, Centauri A, Centauri B).

Supposes when they get to Proxima (the first star), they have a choice of which start to go to next, either Centauri A or Centauri B.  They can flip a coin, doesn't matter which.

Now, before they left Earth, they entangled some quantum bits and one bit is on Earth.

Now, Earth needs to know which wait to aim its interstellar capable tight beam radio or laser.  So they know to aim it a Proxima at the start.

Now, when spaceship at Proxima rolls the dice, they do so by measuring the entangled quantum bit.  If it points up, they go to Centauri A. If it points down, they go to Centauri B.

Earth instantly knows which way to aim the antenna, without a round trip message from Proxima saying "We are going to X".  They just look at their version of the bit, and point the antenna using inverted logic that the spaceship is using.

Now, the same thing applies if that spaceship decides to head to one of two other different star systems, and Earth wants to send a spaceship to rendezvous.

Now this is kind of silly, because they could have planned the route in advance, using a coin instead of a quantum bit.

But it's not silly if there's a war involved, because making random choices at the last second helps confuse the enemy and prevents spies at the origin of the spacecraft from leaking the predetermined choice.  The enemy is now forced to spread its defenses between Centauri A and Centauri B in the above example, and can't use spies to concentrate its forces.

What I described is no different than quantum encryption.  The spy/snooper can't see the random number that the communicating parties can instantly share.

What it doesn't get you is any useful information. If our spaceship arrives at Proxima and spies a nice planet in the Goldilocks zone of Centauri A (or a concentration of forces), it can't tell Earth about it instantly, its going to take 4.3 years to tell Earth.

[sanman]

Thanks for great replies, gentlemen

Because you're still not transmitting any information that way. You're both able to conclude that 1+1=2 (or whatever your function is) but you don't get to choose the inputs. Even if you can deduce the inputs from the outputs, you then just have knowledge of what those unchooseable inputs were at both ends: in exactly the same position as just straight-up measuring the entangled pairs directly.


New Physics would be the appropriate place for this thread, as transmitting information superluminally via entanglement is not consistent with either Relativistic physics or QM.

Okay, fair enough, I now see your point. But so when IBM claims their quantum chips will have built-in "quantum error correction" within a few years, is that consistent with known physics?

And what about the idea of "weak measurement", which is about trying to measure a quantum state in a very weak way, to minimize disruption of the entanglement? Is that an actual thing? Has it actually been done?

Here's a spaceflight scenario that uses quantum communication to get everyone to the correct rendezvous.

Suppose we make a spaceship that can go to Alpha Centauri, a triple start system in 10 years per light year, or Proxima Centauri in 43 years.  (Proxima, Centauri A, Centauri B).

Supposes when they get to Proxima (the first star), they have a choice of which start to go to next, either Centauri A or Centauri B.  They can flip a coin, doesn't matter which.

Now, before they left Earth, they entangled some quantum bits and one bit is on Earth.

Now, Earth needs to know which wait to aim its interstellar capable tight beam radio or laser.  So they know to aim it a Proxima at the start.

Now, when spaceship at Proxima rolls the dice, they do so by measuring the entangled quantum bit.  If it points up, they go to Centauri A. If it points down, they go to Centauri B.

Earth instantly knows which way to aim the antenna, without a round trip message from Proxima saying "We are going to X".  They just look at their version of the bit, and point the antenna using inverted logic that the spaceship is using.

Now, the same thing applies if that spaceship decides to head to one of two other different star systems, and Earth wants to send a spaceship to rendezvous.

Now this is kind of silly, because they could have planned the route in advance, using a coin instead of a quantum bit.

But it's not silly if there's a war involved, because making random choices at the last second helps confuse the enemy and prevents spies at the origin of the spacecraft from leaking the predetermined choice.  The enemy is now forced to spread its defenses between Centauri A and Centauri B in the above example, and can't use spies to concentrate its forces.

What I described is no different than quantum encryption.  The spy/snooper can't see the random number that the communicating parties can instantly share.

What it doesn't get you is any useful information. If our spaceship arrives at Proxima and spies a nice planet in the Goldilocks zone of Centauri A (or a concentration of forces), it can't tell Earth about it instantly, its going to take 4.3 years to tell Earth.

Great story. So the moral of your story is that when we can accept or actually want randomness, then quantum entanglement can keep us in the know. As a variant on your story, if we wanted to distribute space mines across some region of space in a random pattern, we could use quantum entanglement to decide the distribution, and still secretly know where all the mines were.

So quantum entanglement can be used to let the qubit(s) communicate to us and dictate our choices/information, even if we can't use entanglement to communicate to each other.

If the weak measurement thing was possible, then how would that change things?

[InterestedEngineer]

Great story. So the moral of your story is that when we can accept or actually want randomness, then quantum entanglement can keep us in the know. As a variant on your story, if we wanted to distribute space mines across some region of space in a random pattern, we could use quantum entanglement to decide the distribution, and still secretly know where all the mines were.

I like it!  Quantum mine deployment, or Ansible Mine Deployment.  The "secretly know" could be warships light years away and adjust course accordingly, which is far cheaper in energy (the farther away, the cheaper it is to change course, triangles being what they are).

Would make a great plot device for a military sci-fi.  I'm pretty sure it is plausible.

One might posit that QE allows one party to create a random leg of a triangle which starts at their location and another party at a distance to follow the hypotenuse (shortest path) of that triangle.

Here's the TL;DR version of QE:

tired:  God doesn't play dice
wired:  It doesn't matter where you play entangled dice

[InterestedEngineer]

Here's a paper on the rendezvous problem with quantum entanglement involved:

https://iopscience.iop.org/article/10.1088/1367-2630/acb22d/meta

Rendezvous is an old problem of assuring that two or more parties, initially separated, not knowing the position of each other, and not allowed to communicate, are striving to meet without pre-agreement on the meeting point. This problem has been extensively studied in classical computer science and has vivid importance to modern and future applications. Quantum non-locality, like Bell inequality violation, has shown that in many cases quantum entanglement allows for improved coordination of two, or more, separated parties compared to classical sources. The non-signaling correlations in many cases even strengthened such phenomena. In this work, we analyze, how Bell non-locality can be used by asymmetric location-aware agents trying to rendezvous on a finite network with a limited number of steps. We provide the optimal solution to this problem for both agents using quantum resources, and agents with only 'classical' computing power. Our results show that for cubic graphs and cycles it is possible to gain an advantage by allowing the agents to use the assistance of entangled quantum states.

[sanman]

I like it!  Quantum mine deployment, or Ansible Mine Deployment.  The "secretly know" could be warships light years away and adjust course accordingly, which is far cheaper in energy (the farther away, the cheaper it is to change course, triangles being what they are).

Would make a great plot device for a military sci-fi.  I'm pretty sure it is plausible.

Hah, I wonder if anyone's filed for a patent on that yet?

One might posit that QE allows one party to create a random leg of a triangle which starts at their location and another party at a distance to follow the hypotenuse (shortest path) of that triangle.

So quantum entanglement correlation represents a symmetry that can be instantaneously projected across unlimited distance, while being premised off a random state.

So if we had different spaceships arbitrarily far apart, their maneuvers could be coordinated off some random source values that become instantly known to all.

Randomness can travel Faster-Than-Light
(is that why space can travel Faster-Than-Light? because space is full of randomness - ie. random quantum fluctuations?)

Randomness cannot be controlled (by definition)

Randomness originates in indeterminacy / indeterminacy gives rise to randomness

Here's the TL;DR version of QE:

tired:  God doesn't play dice
wired:  It doesn't matter where you play entangled dice

But what advantages come from entangling more and more particles / qubits?

[sanman]

Here's a paper on Quantum Radar, which uses entanglement to save energy:

https://www.nature.com/articles/s41567-023-02113-4

Article
Published: 29 June 2023
Quantum Advantage in Microwave Quantum Radar
R. Assouly, R. Dassonneville, T. Peronnin, A. Bienfait & B. Huard
Nature Physics volume 19, pages1418–1422 (2023)Cite this article

Abstract
A central goal of any quantum technology consists in demonstrating an advantage in their performance compared to the best possible classical implementation. A quantum radar improves the detection of a target placed in a noisy environment by exploiting quantum correlations between two modes, probe and idler. The predicted quantum enhancement is not only less sensitive to loss than most quantum metrological applications, but it is also supposed to improve with additional noise. Here we demonstrate a superconducting circuit implementing a microwave quantum radar that can provide more than 20% better performance than any possible classical radar. The scheme involves joint measurement of entangled probe and idler microwave photon states after the probe has been reflected from the target and mixed with thermal noise. By storing the idler state in a resonator, we mitigate the detrimental impact of idler loss on the quantum advantage. Measuring the quantum advantage over a wide range of parameters, we find that the purity of the initial probe-idler entangled state is the main limiting factor and needs to be considered in any practical application.

https://phys.org/news/2023-07-quantum-radar-outperforms-classical.html

[edzieba]

Great story. So the moral of your story is that when we can accept or actually want randomness, then quantum entanglement can keep us in the know. As a variant on your story, if we wanted to distribute space mines across some region of space in a random pattern, we could use quantum entanglement to decide the distribution, and still secretly know where all the mines were.

So quantum entanglement can be used to let the qubit(s) communicate to us and dictate our choices/information, even if we can't use entanglement to communicate to each other.

You're not actually communicating anything there, either. What you are instead doing is effectively distributing a one-time-pad in a manner that can be proven to not have been intercepted before you read it. That's the basis of Quantum Crypt ography: not being 'more secure' than other crypt ographic methods (because the actual data is encrypted classically), but instead providing a guarantee that your key has not been intercepted in transit.

If the weak measurement thing was possible, then how would that change things?

'Weak measurement' does not really work, because Quanta are Quanta: you've either measured it or you haven't.

[sanman]

You're not actually communicating anything there, either. What you are instead doing is effectively distributing a one-time-pad in a manner that can be proven to not have been intercepted before you read it. That's the basis of Quantum Crypt ography: not being 'more secure' than other crypt ographic methods (because the actual data is encrypted classically), but instead providing a guarantee that your key has not been intercepted in transit.

Well, if some information being communicated is not of your own choosing, then isn't it still communication?
The qubit(s) being evaluated at one end then make their information known to the qubits evaluated at the other end. It's an information exchange between them, and it's not information of our choosing. The values being communicated are determined by the collapse of their wave function, rather than being determined by us.

[Twark_Main]

You're not actually communicating anything there, either. What you are instead doing is effectively distributing a one-time-pad in a manner that can be proven to not have been intercepted before you read it. That's the basis of Quantum Crypt ography: not being 'more secure' than other crypt ographic methods (because the actual data is encrypted classically), but instead providing a guarantee that your key has not been intercepted in transit.

Well, if some information being communicated is not of your own choosing, then isn't it still communication?

No, it's not. To both parties it's indistinguishable from a perfect random number generator.

Worse yet, you can't "ring" the other device. Neither party even knows when or if the other party has actually tried to communicate any information or not. In fact it's meaningless to even ask the question, because the question assumes some system of absolute (or even merely self-consistent) simultaneity, which violates SR.

[sanman]

No, it's not. To both parties it's indistinguishable from a perfect random number generator.

Worse yet, you can't "ring" the other device. Neither party even knows when or if the other party has actually tried to communicate any information or not. In fact it's meaningless to even ask the question, because the question assumes some system of absolute (or even merely self-consistent) simultaneity, which violates SR.

So you feel there's no "spooky action at a distance" - it's all as if someone first deposited similar/related information into 2 envelopes, and then sent them to separate destinations to be opened/discovered later.

Can you explain how the "bomb experiment" works without "spooky" weirdness?

[Twark_Main]

No, it's not. To both parties it's indistinguishable from a perfect random number generator.

Worse yet, you can't "ring" the other device. Neither party even knows when or if the other party has actually tried to communicate any information or not. In fact it's meaningless to even ask the question, because the question assumes some system of absolute (or even merely self-consistent) simultaneity, which violates SR.

So you feel there's no "spooky action at a distance"

No, that's not what I'm saying. That's what Einstein was (wrongly) saying.

There is spooky action at a distance. However it can't be used to build a faster-than-light communication device.

Both statements are simply the conventionally accepted beliefs in physics, of course. I'm not proposing anything extraordinary or "woo" here.

- it's all as if someone first deposited similar/related information into 2 envelopes, and then sent them to separate destinations to be opened/discovered later.

No again. This would be called a local hidden variable theory, and these have been experimentally ruled out by the loophole-free Bell tests.

Can you explain how the "bomb experiment" works without "spooky" weirdness?

Nope, and I doubt anybody else could either! "Spooky action at a distance" really is how the universe works, despite Einstein's initial objections.

[1]

In past debates, people always reply "Yes, but you need to compare notes by classical methods to know a signal was passed."

This is the root of the misunderstanding by so many; resulting in a multitude of FTL threads over the years.

What you need to understand is that entanglement is NOT an inherent property of a particle, as one would consider charge or mass to be. It's a statistical correlation between two or more particles that manifests in the quantum regime, but not in the classical regime. Because of that, you need to compare measurements of two or more particles to determine if they can be said to exist in an entangled state. If those comparisons show a correlation that can be explained by quantum mechanics, but not by classical mechanics, only then can BOTH particles be determined to definitively be (or have been) in an entangled state.

Asking if a single particle is in an entangled state is akin to asking if a single particle is "faster". Unless you have something else to compare it to, the question itself is meaningless. This is why the classical channel is needed. It's not there just to say 'okay, we measured something!'. I need to know the speed of particle 2 to determine if particle 1 is indeed "faster". One particle cannot fundamentally tell you anything at all about a second if you assume no prior knowledge. You can't say that the second particle has been measured, or remains in an entangled state, or even that the second particle even still exists, as it could have been destroyed by an external antiparticle between "then" and "now". It's the classical channel that's needed to tell us all of that.

One side note. Although the exact nature of expansion/collapse of a wavefunction is an open question in physics, it's actually not really relevant this discussion. Various physicists have various opinions on how to interpret it, but pretty much nobody believes any interpretation will allow for FTL communication. I would personally love for there to be some way to achieve FTL-anything, but entanglement is simply not going to be the mechanism to take us there.

[InterestedEngineer]

Worse yet, you can't "ring" the other device. Neither party even knows when or if the other party has actually tried to communicate any information or not. In fact it's meaningless to even ask the question, because the question assumes some system of absolute (or even merely self-consistent) simultaneity, which violates SR.

You don't think you can coordinate atomic clocks on relativistic spaceships?

I think you can. If you synchronize at the beginning, and dead-reckon your acceleration, you can keep them in sync.

You can even post incremental corrections by broadcasting your current timer value, the red or blue shift in your transmitted signal tells the other party part of the correction value.

It still means you have to have a pre-determined time to look at your entangled bits.  But you can get pretty close to simultaneous, close enough for anything useful like rendezvous.

[InterestedEngineer]

In past debates, people always reply "Yes, but you need to compare notes by classical methods to know a signal was passed."

Asking if a single particle is in an entangled state is akin to asking if a single particle is "faster".

If you know they were entangled to begin with, one can assume with some probability that they continue to be entangled.  One can qualify an entanglement process to prove that things (say electrons) stay entangled under certain conditions, and then use that process every time.

One is not measuring a relative variable like "faster", either.  For example electron spin is a binary 1 or 0 (-1/2 or +1/2).  If one of two entangled electrons is +1/2, the other is going to be -1/2.

It's random which is +/-, so you aren't communicating anything.   It's just a replacement for STL transmission of a one-time pad.   Unlike a one-time pad, it can't be intercepted.

Literally, distributed dice.

[1]

Yes, that's what I was referring to when I mentioned prior knowledge. But this doesn't help us, because the converse is also true. Which is to say, you cannot know, by only measuring that single particle, when the particle instead loses its entangled state.

One is not measuring a relative variable like "faster", either.  For example electron spin is a binary 1 or 0 (-1/2 or +1/2).  If one of two entangled electrons is +1/2, the other is going to be -1/2

Actually, yes, that's a relative measurement. The reason is that you're not measuring the magnitude of spin, but rather the orientation, and that's actually an arbitrary choice. It's usually phrased/chosen as spin up and spin down, but you could just as easilly choose spin left and spin right as your basis. What Bells experiment actually did was deliberatley alter the basis of choice and track how the results correlated. If your choice of spin up/down is, say, 20 degrees off from my choice of spin up/down, interesting things happen. But you need to know exactly what I measured for it to have any significance.

[sanman]

Yes, that's what I was referring to when I mentioned prior knowledge. But this doesn't help us, because the converse is also true. Which is to say, you cannot know, by only measuring that single particle, when the particle instead loses its entangled state.

One is not measuring a relative variable like "faster", either.  For example electron spin is a binary 1 or 0 (-1/2 or +1/2).  If one of two entangled electrons is +1/2, the other is going to be -1/2

Actually, yes, that's a relative measurement. The reason is that you're not measuring the magnitude of spin, but rather the orientation, and that's actually an arbitrary choice. It's usually phrased/chosen as spin up and spin down, but you could just as easilly choose spin left and spin right as your basis. What Bells experiment actually did was deliberatley alter the basis of choice and track how the results correlated. If your choice of spin up/down is, say, 20 degrees off from my choice of spin up/down, interesting things happen. But you need to know exactly what I measured for it to have any significance.

But while, velocity is relative, acceleration is absolute (acceleration being derivative of velocity wrt time)
Since time doesn't seem to be relative in quantum physics, can we use number of quanta instead?
What happens as we extend our entanglement to more and more members? Does it matter?

Quantum stuff is based on statistics and probability.
From statistics we know that as we flip more coins, we get a smoother Bell Curve.
From probability we know that integral of probability density is cumulative probability across intervals (or conversely, derivative of cumulative probability is probability density). Can we somehow take more derivatives from there?

Or alternatively, since quanta are discrete, we can say summation instead of integral. Does size of quanta matter?
Because technically, any object can be in superposition - even asteroids.
Even a planet or a star can be said to have a DeBroglie wavelength, even if it's really tiny.

Our tiny subatomic particles have multiple quantum numbers. Yet we only talk of spin.
Can these different quantum numbers cross-correlate with each other, to offer us more information? Or are they completely separate and distinct?
I'd imagine that if a particle in superposition suffers wavefunction collapse wrt a property like spin, then it could be more likely to also suffer wavefunction collapse in other quantum number properties.
Could that be usefully exploited?

Orbital angular momentum is also a quantum property having multiple possible values, typically more than spin.
Does that increased number of possible values then offer us any advantage? (eg. more Gaussian, more like Bell Curve)
What does it take to measure Orbital Angular Momentum?