How fast is quantum entanglement? Scientists investigate it at attosecond scale(phys.org)
phys.org
How fast is quantum entanglement? Scientists investigate it at attosecond scale
https://phys.org/news/2024-10-fast-quantum-entanglement-scientists-attosecond.html
55 comments
Reality doesn't necessarily agree with its model though, so it makes sense to verify?
We model reality incorrectly all the time. A lot of math was wrong, too. So, it’s always good to test the models. In security, we also tested in normal, abnormal, and hostile environments.
Einstein’s relativity theory is a model that is quite fundamental, are there wrong parts of it as well? Asking not as challenge, more out of interest.
Yes, of course, 100 year old theories start to crack, because of all new data discovered recently, but we are far from a new theory to take it place.
Pilot wate theory is slowly move in, for example. In 20-50 years it may predict reality better in some cases than existing theories.
Pilot wate theory is slowly move in, for example. In 20-50 years it may predict reality better in some cases than existing theories.
What does instant mean in a relativistic universe? Does it imply there is a simultaneous now for both entangled things?
It would imply faster than light communication between particles, but as someone else said, models are only models. Faster than light communication is impossible for us to achieve, no reason to assume there's not a way that's inaccessible to us.
Sight correction, at least in my understanding (physics major but only undergrad) this explicitly isn't communication.
It's not possible to use entanglement to transmit information faster than light, even though the entanglement itself could be simultaneous.
It's not possible to use entanglement to transmit information faster than light, even though the entanglement itself could be simultaneous.
It's not the speed I am interested in; it's that relativity says there is no such thing as a simultaneous now (for things in different reference frames anyway).
Well you could imagine seeing a process that causes entanglement and just trying to halt this process somehow. Before a certain time you would simply see no entanglement at all and after some time you wouldn't see any effect on the entanglement. In between something might happen, but what exactly is a bit hard to tell.
None of this is beyond current models though, so I imagine you could predict the results quite well if the calculations were feasible. And if that can be done then that's probably exactly what they did.
None of this is beyond current models though, so I imagine you could predict the results quite well if the calculations were feasible. And if that can be done then that's probably exactly what they did.
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The simulations are investigating how long quantum entanglements take to form, not how long it takes for non-local action / pilot wave / wave function collapse (if you subscribe to these views of QM) to propagate.
(There is a theory in which non-local variables propagate at superluminal speeds, but not instantaneously, but this is a whole other matter).
(There is a theory in which non-local variables propagate at superluminal speeds, but not instantaneously, but this is a whole other matter).
Might I ask, what is this theory of superluminal non-local variables? I would love to read more about it.
It looks like they measured the time of the process where the entanglement occurred, not a time of entanglement itself. I guess they can use this to put an upper bound on the time for entanglement, which is a valid thing to do for an instantaneous process.
This seems to come up in every discussion of quantum entanglement.
It doesn't travel faster than the speed of light. Information can't travel faster than c.
No need to worry about Trisolarans.
It doesn't travel faster than the speed of light. Information can't travel faster than c.
No need to worry about Trisolarans.
I thought information could only travel as fast as light because that was fastest medium we're currently aware of that can carry information. But that doesn't necessarily preclude other, faster mediums from existing, no?
Not quite. There are two speeds in relativity: the maximum speed and non-maximum speeds (i.e. everything else). It just so happens that light travels at the maximum speed.
Supra-luminal signals will almost necessarily imply that it's possible to violate causality by arranging a series of supra-luminal communications. Not every supra-luminal communication violates causality, but it's hard to think of a way to build a consistent theory that would both preclude causality violations and permit at least some supra-luminal signaling.
Supra-luminal signals will almost necessarily imply that it's possible to violate causality by arranging a series of supra-luminal communications. Not every supra-luminal communication violates causality, but it's hard to think of a way to build a consistent theory that would both preclude causality violations and permit at least some supra-luminal signaling.
Supersonic speed doesn't violate laws of nature or violates casuality. Superluminal speed in matter just produces Cherenkov's radiation.
At superluminal speed, we will be able to hit photons in any order, including reverse order, or emit photons and catch them later. Why this is a problem for casuality?
At superluminal speed, we will be able to hit photons in any order, including reverse order, or emit photons and catch them later. Why this is a problem for casuality?
I know next to nothing about physics, but isn't the "It just so happens" part of what you said potentially misleading? Isn't it that light is massless, so that directly implies that it would travel at the maximum speed?
It's a distinction with no difference. Only massless particles can move at the "maximum speed", in our Universe these are photons and gluons. And maybe gravitons, if they exist.
Similarly, massless particles can't move below the "maximum speed", as they'll have no energy or momentum and won't be able to interact with anything.
Similarly, massless particles can't move below the "maximum speed", as they'll have no energy or momentum and won't be able to interact with anything.
Why would the absence of mass force something to travel at the maximum speed? I could understand it being able to travel at that speed; why would it always do so though?
Theoretically, a massless particle can move slower than light. But then it'll have zero energy and momentum, so it won't be able to interact with anything.
Thanks. The equations give the answer in the end, I found this explanation the best [1].
The only way to get a zero mass is if the energy is zero, or the velocity is c.
[1] https://profoundphysics.com/why-do-photons-have-no-mass-simp...
The only way to get a zero mass is if the energy is zero, or the velocity is c.
[1] https://profoundphysics.com/why-do-photons-have-no-mass-simp...
No. It's an absolute, inviolable, universal limit on any and all interactions in the universe. It was recently confirmed that even gravitational waves are limited to the speed of light. This (and the Fermi Paradox) are the strongest evidence that faster than light travel or communication of any kind is impossible.
We just call it the "speed of light" because it's the speed that massless particles (like photons) travel at in a vacuum.
We just call it the "speed of light" because it's the speed that massless particles (like photons) travel at in a vacuum.
Ok, but why?
Per relativity, any medium that could conceptually facilitate transfer of information or of causality (which are basically the same thing) faster than the speed that light happens to travel at would also be able to reverse direction and arrive at their source in space earlier than the time that they left. That effect is completely unrelated to light itself.
"If time travel is possible, then there is no such thing as time."
— Kurt Gödel, Einstein's great friend at Princeton's Institute For Advanced Study.
Einstein once remarked that he went to his office "just to have the privilege of walking home with Kurt Gödel."
https://imgur.com/a/avUwwHk
https://archive.ph/6doQh#selection-707.279-711.1
https://www.amazon.com/When-Einstein-Walked-G%C3%B6del-Excur...
Einstein once remarked that he went to his office "just to have the privilege of walking home with Kurt Gödel."
https://imgur.com/a/avUwwHk
https://archive.ph/6doQh#selection-707.279-711.1
https://www.amazon.com/When-Einstein-Walked-G%C3%B6del-Excur...
IIRC It’s not that light is a medium. It is that light goes that particular speed max (photons can go slower in fact) because that is the speed of causality.
I recall there was research some years ago that predicted when entangled particles are measured, their wave function collapses (and entangled particles' properties take on definite values between each other) at a speed which the lower bound was supposed to be about 10,000x light speed (c).
https://www.science.org/content/article/quantum-physics-gets...
But this seems to measure the speed at which particles can become entangled in the first place, which I always assumed would've been on the scale of Planck time, but 232 attoseconds is pretty long compared to 10^-43 seconds.
But this seems to measure the speed at which particles can become entangled in the first place, which I always assumed would've been on the scale of Planck time, but 232 attoseconds is pretty long compared to 10^-43 seconds.
this somehow reminds me of the lovely mermin-peres-magic-square game, beautifully described by mr. greg-egan here : https://www.gregegan.net/SCIENCE/MPMS/MPMS.html
Complete noob here.
Why are there no practical applications of entanglement?
Why are there no practical applications of entanglement?
Quantum computer is a (sort of) practical application of entanglement. At least it's useful for breaking cryptography.
As explained elsewhere, entanglement cannot be used for faster than light communication because even though change in one particle affects the other particle, without additional information observer sees only a random change.
As explained elsewhere, entanglement cannot be used for faster than light communication because even though change in one particle affects the other particle, without additional information observer sees only a random change.
Entanglement is like splitting a coin in half, so you have a tails and a head half. Then you put one in your left pocket and give the other to a friend.
Then, just by looking in your left pocket, you can learn that your friend has the heads half—instantly!
In other words, it’s not as spooky as it is made out to be.
Then, just by looking in your left pocket, you can learn that your friend has the heads half—instantly!
In other words, it’s not as spooky as it is made out to be.
That's not correct. With entanglement you get correlations that you cannot get if the states of a pair of particles are determined at creation time.
There's a neat game called the CHSH game that illustrates this. Here's a description [1].
Here's a puzzle equivalent to CHSH but that might be easier for programmers to visualize [1].
[1] https://news.ycombinator.com/item?id=41393075
[2] https://news.ycombinator.com/item?id=35905284
There's a neat game called the CHSH game that illustrates this. Here's a description [1].
Here's a puzzle equivalent to CHSH but that might be easier for programmers to visualize [1].
[1] https://news.ycombinator.com/item?id=41393075
[2] https://news.ycombinator.com/item?id=35905284
First, I assume I’m missing some critical detail and am wrong somewhere.
Both the ERP, and the explanation of the CHSH with the difference being cos^2(theta) an isn’t that just Malus’s law? So in the case of the ERP experiment, if you fired single polarized particles at a polarizing filter at one angle or the other you still get cos^s(theta) as the difference without requiring entanglement, no?
That implies, in the case of entangled particles there is more than one dimension of “whatever” that causes the polarizing filter to “choose” whether to extinguish the particle on non-equal angles - like azimuth/elevation instead of just theta? It just seems to me that rather than disproving a “hidden variable”, it requires one?
Like I said, I assume I’m missing something and am wrong.
Both the ERP, and the explanation of the CHSH with the difference being cos^2(theta) an isn’t that just Malus’s law? So in the case of the ERP experiment, if you fired single polarized particles at a polarizing filter at one angle or the other you still get cos^s(theta) as the difference without requiring entanglement, no?
That implies, in the case of entangled particles there is more than one dimension of “whatever” that causes the polarizing filter to “choose” whether to extinguish the particle on non-equal angles - like azimuth/elevation instead of just theta? It just seems to me that rather than disproving a “hidden variable”, it requires one?
Like I said, I assume I’m missing something and am wrong.
Yes, it is just Malus's law. The key is what angle is relevant.
Suppose you did CHSH but instead of pairs of entangled photons with was pairs of non-entangled photons that were polarized in the same direction. They players do the same thing as with entangled photons: use the bit from the referee to pick their measurement angle. A measures at 0 or 45 degrees, where 0 is the axis the photons were polarized on. B measures at -22.5 or 22.5.
Let's say the 0 degrees the players are using is the direction of the original polarization axes.
When the referee gives a 0 to A then A is measuring on the same axis the photon was polarized on, so will get a 1. When the referee gives a 1 to A then A measures at 45 and Malus gives a 50/50 chance of 1.
Player B is always measuring 22.5 from 0, so Malus says B gets a 1 85% of the time.
That gives us this:
If the player's setup isn't aligned with the initial axis the result will differ. For example let's say their set up is 10 degrees off from the setup described above. Then their angles are 10 and 55 for A and -12.5 and 32.5 for B. If I did the numbers right they will win around 62% of the time.
Without entanglement when each player measures a photon the θ for Malus's law is the angle between the axis they measure and the axis the photon was originally polarized with.
With entanglement the θ is the angle between the two axis that the players used.
Suppose you did CHSH but instead of pairs of entangled photons with was pairs of non-entangled photons that were polarized in the same direction. They players do the same thing as with entangled photons: use the bit from the referee to pick their measurement angle. A measures at 0 or 45 degrees, where 0 is the axis the photons were polarized on. B measures at -22.5 or 22.5.
Let's say the 0 degrees the players are using is the direction of the original polarization axes.
When the referee gives a 0 to A then A is measuring on the same axis the photon was polarized on, so will get a 1. When the referee gives a 1 to A then A measures at 45 and Malus gives a 50/50 chance of 1.
Player B is always measuring 22.5 from 0, so Malus says B gets a 1 85% of the time.
That gives us this:
Ref A Ref B A's 1/0 chances B's 1/0 chances
0 0 100/0 85/15
0 1 100/0 85/15
1 0 50/50 85/15
1 1 50/50 85/15
If the player's setup isn't aligned with the initial axis the result will differ. For example let's say their set up is 10 degrees off from the setup described above. Then their angles are 10 and 55 for A and -12.5 and 32.5 for B. If I did the numbers right they will win around 62% of the time.
Without entanglement when each player measures a photon the θ for Malus's law is the angle between the axis they measure and the axis the photon was originally polarized with.
With entanglement the θ is the angle between the two axis that the players used.
Appreciate the reply! Now I have math and thinking to do :)
You might find this interesting [1]. It's a report from a couple of probably 3rd or 4th year undergraduate physics students who did a CHSH experiment with entangled polarized photons for one of their physics lab classes. They measured the correlations with entangled pairs and with non-entangled pairs.
[1] https://columbia.edu/~ask2262/CourseProjects/KudinoorEntangl...
[1] https://columbia.edu/~ask2262/CourseProjects/KudinoorEntangl...
What if there are hidden properties that affect each particle of a pair after they are split, in predetermined ways? that would certainly create correlations that seem to be created on the fly, but they would simply be the result of the inner workings of particles that don't communicate.
For example, if we have two balls, each containing a spin mechanism inside them which makes them spin in relation, let's say, to the magnetic north, and we throw one of them to the other...and later we discover that their spin is somehow correlated.
That is not action at a distance, that is the result of the inner mechanism of each ball.
Why not a similar thing happens with particles?
For example, if we have two balls, each containing a spin mechanism inside them which makes them spin in relation, let's say, to the magnetic north, and we throw one of them to the other...and later we discover that their spin is somehow correlated.
That is not action at a distance, that is the result of the inner mechanism of each ball.
Why not a similar thing happens with particles?
Those kind of mechanisms can't create the kind of correlations that are observed in real particles, so we know that something else is going on.
You can play around with this by trying to design the pair of devices that were described in my second link (https://news.ycombinator.com/item?id=35905284).
To recap, you want to design a pair of devices that each have 3 buttons labeled A, B, and C, a red LED, a green LED, and a counter. The counter starts at 1000. When you press any one of the buttons one of the LEDs flashes and the counter decrements. When the counter reaches 0 the device stops responding.
You should also specify a way that if the devices are brought together the pair of them can be reset.
You can specify any kind of non-quantum hardware you want in the devices. As much computing as you need, as much RAM and ROM and disk as you want, and physical sensors. Include clocks if you need to. You can include true random number generators. It doesn't have to be limited to current technology--it just has to be limited to known physics and not use quantum entanglement.
What are need to achieve with that hardware and whatever algorithms you specify is:
1. Suppose someone has used one of the devices, and recorded the results of a very large number of interactions.
Suppose that a statistician is given a list of 5-tuples (P, F, n, R, t) of those interactions with one of the devices, where P is which button was pressed, F is which LED flashed, n is the value on the counter when the button was pressed, and R is how many times the device has been reset (i.e., R = 0 the first 1000 times the device is used, then when it and the other device are reset R = 1 for the next 1000 uses and so on), and finally t is the time at which the button was pressed.
It should not be possible using any known statistical test on that list of 5-tuples for the statistician to distinguish the device from a device whose algorithm is simply:
A couple things to note.
1. The above has to hold even if the users take the devices very far apart from each other before they start pressing buttons.
In particular the users might choose to take the devices so far apart before they start pressing buttons that each has finished their run of 1000 before any possible communications from their device could reach the other.
2. The users might wait a long time before starting a run of 1000, and they might wait a long time between presses within a run.
3. The users are determining when to press independently so you can't count on them alternating. You can't even count on them overlapping: one might do all 1000 presses before user the other starts.
4. The users might use a true random number generator to determine which buttons to press.
You can play around with this by trying to design the pair of devices that were described in my second link (https://news.ycombinator.com/item?id=35905284).
To recap, you want to design a pair of devices that each have 3 buttons labeled A, B, and C, a red LED, a green LED, and a counter. The counter starts at 1000. When you press any one of the buttons one of the LEDs flashes and the counter decrements. When the counter reaches 0 the device stops responding.
You should also specify a way that if the devices are brought together the pair of them can be reset.
You can specify any kind of non-quantum hardware you want in the devices. As much computing as you need, as much RAM and ROM and disk as you want, and physical sensors. Include clocks if you need to. You can include true random number generators. It doesn't have to be limited to current technology--it just has to be limited to known physics and not use quantum entanglement.
What are need to achieve with that hardware and whatever algorithms you specify is:
1. Suppose someone has used one of the devices, and recorded the results of a very large number of interactions.
Suppose that a statistician is given a list of 5-tuples (P, F, n, R, t) of those interactions with one of the devices, where P is which button was pressed, F is which LED flashed, n is the value on the counter when the button was pressed, and R is how many times the device has been reset (i.e., R = 0 the first 1000 times the device is used, then when it and the other device are reset R = 1 for the next 1000 uses and so on), and finally t is the time at which the button was pressed.
It should not be possible using any known statistical test on that list of 5-tuples for the statistician to distinguish the device from a device whose algorithm is simply:
if any_button_pressed():
r = uniform_true_random_from_0_to_1()
if r < 0.5:
flash(GREEN)
else:
flash(RED)
A couple things to note.
1. The above has to hold even if the users take the devices very far apart from each other before they start pressing buttons.
In particular the users might choose to take the devices so far apart before they start pressing buttons that each has finished their run of 1000 before any possible communications from their device could reach the other.
2. The users might wait a long time before starting a run of 1000, and they might wait a long time between presses within a run.
3. The users are determining when to press independently so you can't count on them alternating. You can't even count on them overlapping: one might do all 1000 presses before user the other starts.
4. The users might use a true random number generator to determine which buttons to press.
So helpful. Ok, I’ve made the classical game here:
https://claude.site/artifacts/41c76c01-bfcf-47d7-81fb-cc704a...
I’ll work on the quantum next
I’ll work on the quantum next
I mean the person you were responding to said "like". I think splitting a coin is a reasonable but not perfect analogy.
The splitting a coin analogy doesn't capture anything about entanglement though. It is just capturing a property of all distinguishable persistent objects: if you have two distinguishable persistent objects and you know one is at location A and one is at location B, then if you find out which one is at A you know what the one at B must be.
To be a reasonable analogy it has to capture something this is different between entangled and not entangled particles. That's the thing that is sometimes described as "spooky" and completely missing from the split coin analogy.
To be a reasonable analogy it has to capture something this is different between entangled and not entangled particles. That's the thing that is sometimes described as "spooky" and completely missing from the split coin analogy.
Its capturing that the two particles are correlated instead of transfering information. I feel like that is the core part to understand.
But correlations like that happen all the time. You put the tape that was supposed to "Back Door Sluts 9" into the VCR and see that it is "Lord of the Rings" then you immediately know that when you gave your kids "Lord of the Rings" to take to a friends house you got the tapes swapped and the kids are now walking around with the hottest porno ever made. You know this because the two tapes are correlated because you rented them together and have no other rental tapes. No information had to be transferred for you to know the kids have BDS9.
Or your daughter Lisa calls from school and tells you there is meat in her lunch. You immediately know that you must have mixed up the bag lunches that morning and Bart had the vegan meal. You make the lunches together, one vegan and one not, so they are correlated without information transfer.
What makes entangled particles different from other pairs of correlated things is that they correlate in ways the seem impossible without communication.
It's like if someone answered the question of why frogs sometimes rain from the sky by saying that because frogs cannot fly and are heavier than air, just like hail or rain or snowflakes, they feel a downward force from gravity which pulls them to the ground.
It's right...but no one has trouble understanding the gravitational aspect of frogs falling from the sky. The thing people want to understand is how a bunch of freaking frogs got into the sky in the first place.
Or your daughter Lisa calls from school and tells you there is meat in her lunch. You immediately know that you must have mixed up the bag lunches that morning and Bart had the vegan meal. You make the lunches together, one vegan and one not, so they are correlated without information transfer.
What makes entangled particles different from other pairs of correlated things is that they correlate in ways the seem impossible without communication.
It's like if someone answered the question of why frogs sometimes rain from the sky by saying that because frogs cannot fly and are heavier than air, just like hail or rain or snowflakes, they feel a downward force from gravity which pulls them to the ground.
It's right...but no one has trouble understanding the gravitational aspect of frogs falling from the sky. The thing people want to understand is how a bunch of freaking frogs got into the sky in the first place.
Classical analogies only go so far. What's spooky about entanglement is that it works even if you change the measurement basis (as long as the two parts are using the same basis), which you cannot do with a coin.
In this case the coin is neither heads nor tails until you check it and then it becomes one or the other and it's counterpart becomes the opposite, no matter how far apart they are. So yeah it's kind of spooky.
My understanding is that entanglement has nothing to do with the particles and everything to do with what is known about the particles in advance by dint of the experimental setup.
For example, that two electrons are created in a closed system with a net spin of zero, so as long as you do not allow their waveforms to collapse (EG, so long as the system remains closed) then no matter where they go they must maintain that net spin of zero when perceived in relation to one another.
But if this is the case then the "entanglement" precedes the creation of the particles: it is nothing more than an accounting that they are required to obey in concert by the symmetries of physics.
For example, that two electrons are created in a closed system with a net spin of zero, so as long as you do not allow their waveforms to collapse (EG, so long as the system remains closed) then no matter where they go they must maintain that net spin of zero when perceived in relation to one another.
But if this is the case then the "entanglement" precedes the creation of the particles: it is nothing more than an accounting that they are required to obey in concert by the symmetries of physics.
> In other words, it’s not as spooky as it is made out to be.
That's not really the spooky part of entanglement. The rabbit hole goes much deeper, like here[1] or here[2].
[1]: https://arxiv.org/abs/1203.4834
[2]: https://arxiv.org/abs/1209.4191
That's not really the spooky part of entanglement. The rabbit hole goes much deeper, like here[1] or here[2].
[1]: https://arxiv.org/abs/1203.4834
[2]: https://arxiv.org/abs/1209.4191
Gotta love the confidence to casually think every physicist for the past 100 years is an idiot.
It’s more like looking in your pocket and finding that sometimes it’s actually become a head due to beta decay, and spookily your friend now instantly has tails.
It's more like splitting the timeline in half: in one branch it's heads, in the other it's tails, and you don't know which timeline you're in until you look at the coin. The Bell's inequality says the coin has no state until it's been observed.
Surely anything else would imply a mechanism with additional complexity and indirectly void the entire concept. Delayed entanglement effect doesn't quite have the same cleanness for lack of better word