Record-setting quantum entanglement connects two atoms across 20 miles
newatlas.com
newatlas.com
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.
I'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.
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.