6 ms·

Record-setting quantum entanglement connects two atoms across 20 miles

73 points by elorant 4y ago

rootw0rm 4y ago
"This is a record distance for this kind of communication and marks a breakthrough towards a fast and secure quantum internet." I'm no expert, but the following link suggests otherwise? https://www.scientificamerican.com/article/china-shatters-ldquo-spooky-action-at-a-distance-rdquo-record-preps-for-quantum-internet/ 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.
- SkyMarshal 4y ago
  I am not a quantum scientist, so in what way would 20mi over fiber be more impressive than 1200mi from space to earth?
  - bowsamic 4y ago
    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
- beloch 4y ago
  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.
- jobigoud 4y ago
  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.
tjpnz 4y ago
Can anyone clarify a few points for a layman? - 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?
- vlmutolo 4y ago
  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.
  - collaborative 4y ago
    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
    - nerdponx 4y ago
      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.
      - affgrff 4y ago
        Funny how similar our comments are, I swear I believe I have not conciously read your comment before writing mine.
      - eloff 4y ago
        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.
hsn915 4y ago
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.
- saargrin 4y ago
  so is this legit or not?
  - deleted 4y ago
    [deleted]
- TrapLord_Rhodo 4y ago
  Paul Revere entangled: 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?
  - netcraft 4y ago
    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)
    - oneoff786 4y ago
      That could be agreed upon ahead of time.
dsq 4y ago
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 :)
- mocha_nate 4y ago
  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
- AvSaba 4y ago
  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.
brrrrrm 4y ago
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. Is this faster than light communication?
- seibelj 4y ago
  I would say it’s logical deduction. Communication is when you send information, including something I can’t possibly know ahead of time.
  - binbag 4y ago
    No info was sent.
- qwertyuiop_ 4y ago
  If I understood the article correctly, you could also change the property of one card and see the effect instantaneously in the other.
  - devwastaken 4y ago
    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.
    - throwoutway 4y ago
      is “the quantum internet” not grounded in reality then?
      - layer8 4y ago
        No, it is fine in the sense of https://en.m.wikipedia.org/wiki/Quantum_network#Applications https://en.m.wikipedia.org/wiki/Quantum_network#Applications. But it doesn’t allow FTL communication, in case you heard someone claim that.
jb1991 4y ago
> That leads to the unsettling implication that information is being “teleported” faster than the speed of light I thought that principles of physics do not actually allow information to transfer in this process?
- nabla9 4y ago
  They are and there is no such implication. The writer knows less than you.

73 points by elorant 4y ago

"This is a record distance for this kind of communication and marks a breakthrough towards a fast and secure quantum internet." I'm no expert, but the following link suggests otherwise? https://www.scientificamerican.com/article/china-shatters-ldquo-spooky-action-at-a-distance-rdquo-record-preps-for-quantum-internet/ 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.

SkyMarshal 4y ago
I am not a quantum scientist, so in what way would 20mi over fiber be more impressive than 1200mi from space to earth?
- bowsamic 4y ago
  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
beloch 4y ago
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.
jobigoud 4y ago
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.

tjpnz 4y ago

Can anyone clarify a few points for a layman? - 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?

vlmutolo 4y ago
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.
- collaborative 4y ago
  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
  - nerdponx 4y ago
    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.
    - affgrff 4y ago
      Funny how similar our comments are, I swear I believe I have not conciously read your comment before writing mine.
    - eloff 4y ago
      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.

hsn915 4y ago

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.

saargrin 4y ago
so is this legit or not?
- deleted 4y ago
  [deleted]
TrapLord_Rhodo 4y ago
Paul Revere entangled: 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?
- netcraft 4y ago
  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)
  - oneoff786 4y ago
    That could be agreed upon ahead of time.

dsq 4y ago

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 :)

mocha_nate 4y ago
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
AvSaba 4y ago
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.

brrrrrm 4y ago

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. Is this faster than light communication?

seibelj 4y ago
I would say it’s logical deduction. Communication is when you send information, including something I can’t possibly know ahead of time.
- binbag 4y ago
  No info was sent.
qwertyuiop_ 4y ago
If I understood the article correctly, you could also change the property of one card and see the effect instantaneously in the other.
- devwastaken 4y ago
  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.
  - throwoutway 4y ago
    is “the quantum internet” not grounded in reality then?
    - layer8 4y ago
      No, it is fine in the sense of https://en.m.wikipedia.org/wiki/Quantum_network#Applications https://en.m.wikipedia.org/wiki/Quantum_network#Applications. But it doesn’t allow FTL communication, in case you heard someone claim that.

jb1991 4y ago

> That leads to the unsettling implication that information is being “teleported” faster than the speed of light I thought that principles of physics do not actually allow information to transfer in this process?

nabla9 4y ago
They are and there is no such implication. The writer knows less than you.