Optical loss along the fibre. Optical loss is basically environmental vacuum noise coupling into the system and causing decoherence, destroying your quantum correlations. Air is usually much lower loss depending on the wavelength
One of the biggest challenges in distributing entanglement is loss.
It's easy to send a classical signal over 30 km of fibre. You just send a lot of photons and a significant portion of them will get through. Big pulse goes in, smaller pulse comes out. All you care about is that there was a pulse.
When you're trying to distribute a pair of entangled photons, it really is important that you're sending one photon to point A and one photon to point B. One photon goes in, and maybe it comes out. Maybe. If one half of a pair gets where it's going, then you need to be lucky for the other half to get where it's going too, otherwise you have to throw the one that got through out. Typical quantum entanglement experiments have multiple massive sources of loss. Need to use a 50/50 beam-splitter somewhere? There goes half your photons. Any given mirror incurs some loss. Detectors may only be able to detect a tiny portion of the photons that hit them. The list goes on and on.
The experiment here was done using typical telecom fibre. Although telcom fibre is "good" at transmitting certain wavelengths of light, we're still talking about shining light through 30 km of glass here. That's a lot of loss. Think about looking through a window that's 30 km thick. A free-space link with a satellite is going to have significantly less loss. First off, the air you're breathing right now is a lot less optically dense than glass. Go up 30,000 km, and it's even less dense. By the time you get to the satellite, it's vacuum. In total, a free-space satellite link incurs a lot less loss than a fibre link this long.
The distance of this experiment is indeed far shorter than the freespace one, but they're dealing with much higher loss. The motivation for this experiment was practical quantum networks using existing and installed fibre. These are both impressive experiments and you're right that "distance" should be qualified a bit here, but free-space and telecomm fibre experiments really are apples and oranges. Free-space experiments have their own challenges, and free-space links have their own applications. If you want to build a QKD network that's for commercial use, a fibre based one is probably a lot more practical.
This is entangling two atoms (two atom-photon pairs are created and then the two photons are entangled together). The Chinese record is for photons only.
> While photons have been entangled over great distances before, this study marks a new distance record for entangling two atoms, which could function as “quantum memory” nodes, over fiber optics.
- If so are we then able to retrieve (examine) the information at the other end without it being corrupted?
- What implications does this have for Einstein's theories? Do we place them into the same category we did for Newtonian Physics where they're ok for some applications but ultimately an approximation?
It's definitely not FTL information transfer. If it were, you'd know. It would probably be on the front page of every newspaper as the biggest scientific discovery of the last century.
It's not even a transfer since you can't control the state of the measured atom. It just collapses into one state which happens to also affect the other atom. So you could never choose to transfer actual data
My understanding is that it's more like putting two copies of a letter into an envelope, mailing one copy halfway around the world, and then opening the other copy. You now know what they both contain because you know they contain the same thing.
Is this at least somewhat directionally correct? I know that quantum physics principles are very difficult to define without going into the actual math.
That was Einstein's view, but it was disproved in the seventies by experimental measurements of Bell's inequality. I'm just a computer guy, so someone else can chime in with a more detailed explanation. It's also easy to find more info on Google.
Your example is akin to the Hidden variable theory (in the example the hidden variable will be the fact the two letters are copies). Bell's inequality tests have ruled out the hidden variable theory. Veritasium has a good video explaining Bell's inequality - https://www.youtube.com/watch?v=ZuvK-od647c
My interpretation is that is not better for communication than putting the same random number in two envelopes, sending them somewhere and then opening both at the same time.
Okay, so how do we know that A wasn't like that all along?
edit; Been trying to read up on this, i think i get it; Its a random outcome whether B has been looked at or not, but when you check them later, they are opposite pairs randomly distributed.
Yes exactly. Nothing "changes" at the other end (although, there has been arguments regarding the Wigner's Friend thought experiments about this... see Guerin 2021)
Just be careful when reading up on this about a subtlety here - as long as both A and B measure the same property, say "spin along Y axis", a measurement they will always both measure the opposite of, the experiment will seem just like the "unopened envelopes" popular description of entanglement which is not really magical. It's annoying that this particular description keeps being made..
The interesting things happen when you start mixing up the measurements so A and B measure different, but correlated, properties. In the context of Bell's tests, they will not measure in the exact same basis (like vertical polarization), one of them will rotate the basis slightly, say 30 or 60 degrees. And the resulting correlations between A and B in this scenario can't be explained by hidden variables ("unopened envelopes describing the result of all possible measurements") but is predicted correctly by quantum mechanics.
Quantum entanglement cannot be used to transmit information but can be used to check if the message was interrupted, so it's still important for quantum cryptography.
(I'm just a programmer but had a course quite some time ago on quantum computing thought by a physics professor.)
The message being interrupted is itself information, so I'm not sure you can say that information can't be transmitted. If the entangled counterparty can notice message interruptions then binary messaging and information passage is possible.
The entangled photons can't detect the interruption alone, you need the message too. If you are interested how I suggest watching Veritasium's video about Bell's theorem as a start. The cryptographic method is called Ekert protocol and it uses Bell's theorem to detect if the quantum spin was measured in an incorrect "direction" (called base). Just write down the expected outcome in 60 degrees tilted base and you will see how it changes compared to being measured in the correct direction. Add enough photons and possible directions and you can detect eavesdropping with a very high certainty.
I dislike all writings on quantum entanglement as they seem more interested in clickbaiting than actually communicating actual knowledge.
Two contradictory talking points seem to be pervasive:
- Altering one atom affects the other instantaniously
- There's no FTL communication going on, it's just a correlation, even though we have told you before that quantum correlations are not like classic correlations, and even though we have told you that the atoms don't have any settled state until you measure them
Basically, never take seriously anything written about quantum physics in pop-science magazines.
Say i entangle two atoms and send one of the atoms to mars, which is 8 lightseconds away. I tell you before you leave that if the poles of the atom where to flip, the british are coming. If i were to flip the atom in one second, you were to measure it the next second you were still able to exchange information 4X faster than the speed of light.
Or am i missing something and no one actually understands any of this?
You would have to know when to measure. If you on mars measure too soon, youre the one forcing the other atom to flip. You also wouldnt know that you had done that. (I think, IANAQP)
This is not how entanglement works, and while people do understand how entanglement works, it's a little subtle.
When you measure your atom on earth, you will get results with certain probabilities. When I measure my atom on the moon, I will get results with certain probabilities. My probabilities don't depend on how you performed your measurement - I can't tell what measurement you performed from just the results of my measurement.
However, our results will be correlated in unusual ways depending on our measurements. Maybe we both get a spin up result, or both get a spin down result.
These correlations go beyond the amount of correlation it's possible for classical objects to have, which is the essence of the Bell experiment, and specifically the CHSH inequality.
So entanglement can't be used to tell someone that the British are coming faster than normal classical communication. But it could be use to coordinate our responses (maybe both attack or both retreat, or one of each, with certain probabilities) in ways that can't be reproduced classically without using communication.
If I understand it correctly, two entangled qbits can can have their spin measured in the same order of planes (XY, YZ, XZ) to get the same result? So you can send a binary bit {0:no british, 1:british} or a qbit{XY:british probability, some other strategy dependent on XY spin and order of spin planes?}
It's only possible to achieve correlations that are stronger than are classically possible without communication. Communication itself still cannot be achieved, by any sequence of measurements.
Taken from your wiki link with a twist; "spooky correlation at a distance" does seem to be some amount of information. The paul revere joke was entirely out of scope, but the universe does seem to have some way to pull the strings behind the curtain here.
I don't have the background to really understand this but it sounds like: in order for information (we correlated our spins) to be passed you would first have to know what the other side is doing which requires side band communication which itself would be subject to the speed of light. But I might be missing something in my understanding.
There is something to that - there's a concept called "Quantum Nonlocality", which says that an accurate description of an entangled quantum system must be nonlocal - to predict what will happen, we need to look at the whole system at once.
But nonlocality is different from the capacity for communication. To make accurate predictions about the measurements, one would need information from the earth and from mars. However, the results of the measurements cannot be used to transmit information.
These two concepts - what is necessary for prediction and what the results can accomplish - often line up in the classical world, but in the quantum world they are quite distinct. So your intuition has merit, but it reveals quantum nonlocality, not quantum superluminal communication.
> Two contradictory talking points seem to be pervasive:
They are not really contradictory.
> - Altering one atom affects the other instantaniously
It kind-of does, if by "affecting the other" you mean changing the physical state of the pair from "the A/B system is in state S0" to "A is in state SA and B is in state SB".
> - There's no FTL communication going on,
There isn't unless you want to define as "communication" the change in the physical state described above.
> Basically, never take seriously anything written about quantum physics in pop-science magazines.
When i read the title, as a non-physicist, non-scientist, it sounds like the precursor to...teleportation! Over 20 miles distance. What is wrong with me for thinking that :)
not teleportation. i see it as the precuror to literal instant messaging. writing a message with two entangled receivers could mean sending messages faster than the speed of light.... just me dreaming though
A fellow non-physicist, non-scientist here. If I understand correctly the problem is that if you attempt to change the state of one particle, two particles are no longer entangled. In other words you can observe one particle and be sure that the other particle has the same properties but the moment you decide to change one particle's state you can't say anything about the other particle.
You have two sealed envelopes, one carrying a red card and the other a black. You shuffle the envelopes randomly and give one to your friend who goes to Mars. Once there, your friend opens their envelope and immediately knows the color of the card in your envelope.
You can know the other by observing one, the entanglement is then broken. You cannot change the state remotely. We can't say that the entangled particles were X and Y to begin with because that requires measuring them, which breaks entanglement. Quantum is heavily misreported on, often times by scientific bodies for the purposes of funding.
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[ 0.23 ms ] story [ 123 ms ] threadI'm no expert, but the following link suggests otherwise?
https://www.scientificamerican.com/article/china-shatters-ld...
edit: I understand this experiment was over fiber, but I still have issues with the quote above.
It's easy to send a classical signal over 30 km of fibre. You just send a lot of photons and a significant portion of them will get through. Big pulse goes in, smaller pulse comes out. All you care about is that there was a pulse.
When you're trying to distribute a pair of entangled photons, it really is important that you're sending one photon to point A and one photon to point B. One photon goes in, and maybe it comes out. Maybe. If one half of a pair gets where it's going, then you need to be lucky for the other half to get where it's going too, otherwise you have to throw the one that got through out. Typical quantum entanglement experiments have multiple massive sources of loss. Need to use a 50/50 beam-splitter somewhere? There goes half your photons. Any given mirror incurs some loss. Detectors may only be able to detect a tiny portion of the photons that hit them. The list goes on and on.
The experiment here was done using typical telecom fibre. Although telcom fibre is "good" at transmitting certain wavelengths of light, we're still talking about shining light through 30 km of glass here. That's a lot of loss. Think about looking through a window that's 30 km thick. A free-space link with a satellite is going to have significantly less loss. First off, the air you're breathing right now is a lot less optically dense than glass. Go up 30,000 km, and it's even less dense. By the time you get to the satellite, it's vacuum. In total, a free-space satellite link incurs a lot less loss than a fibre link this long.
The distance of this experiment is indeed far shorter than the freespace one, but they're dealing with much higher loss. The motivation for this experiment was practical quantum networks using existing and installed fibre. These are both impressive experiments and you're right that "distance" should be qualified a bit here, but free-space and telecomm fibre experiments really are apples and oranges. Free-space experiments have their own challenges, and free-space links have their own applications. If you want to build a QKD network that's for commercial use, a fibre based one is probably a lot more practical.
> While photons have been entangled over great distances before, this study marks a new distance record for entangling two atoms, which could function as “quantum memory” nodes, over fiber optics.
- Is this really FTL transmission of information?
- If so are we then able to retrieve (examine) the information at the other end without it being corrupted?
- What implications does this have for Einstein's theories? Do we place them into the same category we did for Newtonian Physics where they're ok for some applications but ultimately an approximation?
Is this at least somewhat directionally correct? I know that quantum physics principles are very difficult to define without going into the actual math.
Of course many scientists don’t like violating locality or statistical independence but there’s no philosophical reason to a priori discard them.
If yes, that's binary communication. If no, then how do we know B changed at all?
edit; Been trying to read up on this, i think i get it; Its a random outcome whether B has been looked at or not, but when you check them later, they are opposite pairs randomly distributed.
Just be careful when reading up on this about a subtlety here - as long as both A and B measure the same property, say "spin along Y axis", a measurement they will always both measure the opposite of, the experiment will seem just like the "unopened envelopes" popular description of entanglement which is not really magical. It's annoying that this particular description keeps being made..
The interesting things happen when you start mixing up the measurements so A and B measure different, but correlated, properties. In the context of Bell's tests, they will not measure in the exact same basis (like vertical polarization), one of them will rotate the basis slightly, say 30 or 60 degrees. And the resulting correlations between A and B in this scenario can't be explained by hidden variables ("unopened envelopes describing the result of all possible measurements") but is predicted correctly by quantum mechanics.
Two contradictory talking points seem to be pervasive:
- Altering one atom affects the other instantaniously
- There's no FTL communication going on, it's just a correlation, even though we have told you before that quantum correlations are not like classic correlations, and even though we have told you that the atoms don't have any settled state until you measure them
Basically, never take seriously anything written about quantum physics in pop-science magazines.
Say i entangle two atoms and send one of the atoms to mars, which is 8 lightseconds away. I tell you before you leave that if the poles of the atom where to flip, the british are coming. If i were to flip the atom in one second, you were to measure it the next second you were still able to exchange information 4X faster than the speed of light.
Or am i missing something and no one actually understands any of this?
Two events A and B can occur "A first, then B" in one reference frame, but "B first, then A" in another reference frame.
The answer is in the question.
When you measure your atom on earth, you will get results with certain probabilities. When I measure my atom on the moon, I will get results with certain probabilities. My probabilities don't depend on how you performed your measurement - I can't tell what measurement you performed from just the results of my measurement.
However, our results will be correlated in unusual ways depending on our measurements. Maybe we both get a spin up result, or both get a spin down result.
These correlations go beyond the amount of correlation it's possible for classical objects to have, which is the essence of the Bell experiment, and specifically the CHSH inequality.
So entanglement can't be used to tell someone that the British are coming faster than normal classical communication. But it could be use to coordinate our responses (maybe both attack or both retreat, or one of each, with certain probabilities) in ways that can't be reproduced classically without using communication.
Hope that helps.
It's only possible to achieve correlations that are stronger than are classically possible without communication. Communication itself still cannot be achieved, by any sequence of measurements.
How so? How would you define information and its communication?
But nonlocality is different from the capacity for communication. To make accurate predictions about the measurements, one would need information from the earth and from mars. However, the results of the measurements cannot be used to transmit information.
These two concepts - what is necessary for prediction and what the results can accomplish - often line up in the classical world, but in the quantum world they are quite distinct. So your intuition has merit, but it reveals quantum nonlocality, not quantum superluminal communication.
So it's like syncing up the state of of two PRNGs!
They are not really contradictory.
> - Altering one atom affects the other instantaniously
It kind-of does, if by "affecting the other" you mean changing the physical state of the pair from "the A/B system is in state S0" to "A is in state SA and B is in state SB".
> - There's no FTL communication going on,
There isn't unless you want to define as "communication" the change in the physical state described above.
> Basically, never take seriously anything written about quantum physics in pop-science magazines.
That's unfortunately a wise advice.
Is this faster than light communication?
I thought that principles of physics do not actually allow information to transfer in this process?
The writer knows less than you.