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-   -   Why can't quantum entanglement be used for speed of light communication or slower? (https://www.mersenneforum.org/showthread.php?t=24537)

 jasong 2019-06-20 18:51

Why can't quantum entanglement be used for speed of light communication or slower?

Why can't quantum entanglement be used for speed of light communication or slower?

I tried to ask Google, but it appears to be obsessed with FTL communication, which I know is impossible.

What if we use double quantum entanglement, one over the long distance and one entanglement locally, where we are, and don't check the information until a photon would have had time to make the trip?

 xilman 2019-06-20 19:41

[QUOTE=jasong;519666]Why can't quantum entanglement be used for speed of light communication or slower?

I tried to ask Google, but it appears to be obsessed with FTL communication, which I know is impossible.

What if we use double quantum entanglement, one over the long distance and one entanglement locally, where we are, and don't check the information until a photon would have had time to make the trip?[/QUOTE]It can, and it has been. Search on [URL="https://en.wikipedia.org/wiki/Quantum_key_distribution"]quantum key distribution.
[/URL]

 ewmayer 2019-06-21 23:20

"...or slower"? Last time I checked, comms at speed <= c was a solved problem, no entanglement needed.

 xilman 2019-06-22 08:29

[QUOTE=ewmayer;519768]"...or slower"? Last time I checked, comms at speed <= c was a solved problem, no entanglement needed.[/QUOTE]True, but alternative approaches can have their own benefits. I gave a specific example of a useful form of communication: quantum key distribution. It's been demonstrated [I]in vacuo[/I] between spacecraft and [I]in vitro[/I] terrestrially. Light travels in glass at substantially less than c. Secure key distribution is a seriously hard problem without making use of entanglement.

 ewmayer 2019-06-22 20:10

[QUOTE=xilman;519797]True, but alternative approaches can have their own benefits. I gave a specific example of a useful form of communication: quantum key distribution. It's been demonstrated [I]in vacuo[/I] between spacecraft and [I]in vitro[/I] terrestrially. Light travels in glass at substantially less than c. Secure key distribution is a seriously hard problem without making use of entanglement.[/QUOTE]

I was mainly puzzled by the OP's "slower than light speed" emphasis, since the most common misunderstanding re. quantum entanglement is that is allows faster-than-light communication.

QKD is theoretically useful by way of being eavesdropping-secure, but per Wikipedia:

"The main drawback of Quantum Key Distribution is that it usually relies on having an authenticated classical channel of communications. In modern cryptography, having an authenticated classical channel means that one have [sic] either already exchanged a symmetric key of sufficient length or public keys of sufficient security level. With such information already available, one can achieve authenticated and secure communications without using QKD, such as by using the Galois Counter Mode of the Advanced Encryption Standard. Thus it is sometimes jokingly said that QKD does the work of a Stream Cipher at a million times the cost.
...
[In QKD] the sender (traditionally referred to as Alice) and the receiver (Bob) are connected by a quantum communication channel which allows quantum states to be transmitted. In the case of photons this channel is generally either an optical fibre or simply free space."

Do you know of a way around the 'usually' in the above?

 xilman 2019-06-22 20:53

[QUOTE=ewmayer;519847]I was mainly puzzled by the OP's "slower than light speed" emphasis, since the most common misunderstanding re. quantum entanglement is that is allows faster-than-light communication.

QKD is theoretically useful by way of being eavesdropping-secure, but per Wikipedia:

"The main drawback of Quantum Key Distribution is that it usually relies on having an authenticated classical channel of communications.

Do you know of a way around the 'usually' in the above?[/QUOTE]Also from that article:
"The major difference of quantum key distribution is the ability to detect any interception of the key, whereas with courier the key security cannot be proven or tested." as you note. In my view, this important. For instance, Alice and Bob may have met in person and, each encased within the same Faraday cage, exchanged an initial key. Subsequent key exchanges are at least as resistant to eavesdroppers. A common dictum is that keys should be treated like teethbreesh: change them every now and again and don't share them with strangers.

I need to think about the "usually" caveat but I'm too far under the affluence of incahol right now.

 retina 2019-06-22 22:38

[QUOTE=xilman;519849]A common dictum is that keys should be treated like teethbreesh: change them every now and again and don't share them with strangers.[/QUOTE]I think teethbreesh [sic] would be under more restrictive rules; don't share them with anyone, especially not the family dog.

 CRGreathouse 2019-06-23 06:40

[QUOTE=jasong;519666]Why can't quantum entanglement be used for speed of light communication or slower?

I tried to ask Google, but it appears to be obsessed with FTL communication, which I know is impossible.

What if we use double quantum entanglement, one over the long distance and one entanglement locally, where we are, and don't check the information until a photon would have had time to make the trip?[/QUOTE]

Fundamentally, when you and a partner have a collection of entangled particles, you don't communicate information when you both measure them. Rather, you both get (in essence) the same copy of noise/static/etc. So it's as instant as you like, but it doesn't convey information, thus circumventing the ban on speed-of-light transmission. Waiting longer doesn't somehow make the particles transmit information: they'll still just show noise, like they always do. (Indeed, if you measure them without entangling them you'll get the same sort of noise.)

 chris2be8 2019-06-23 15:50

[QUOTE=ewmayer;519847] QKD is theoretically useful by way of being eavesdropping-secure, but per Wikipedia:

Do you know of a way around the 'usually' in the above?[/QUOTE]

As I understand it you need a classical channel that can't be undetectably *altered* but it doesn't matter if it can be intercepted. And it doesn't need a public key that can't be broken by a quantum computer.

Chris

 xilman 2019-06-23 17:26

[QUOTE=retina;519851]I think teethbreesh [sic] would be under more restrictive rules; don't share them with anyone, especially not the family dog.[/QUOTE]Which advice in the cryptography regime is sensible for authentication or archival protection.

AFAIK, secure communication between two parties, assumed to be friends for the purpose of expanding on the present analogy, requires that they share a key and an algorithm.

Note that the any initial shared key and subsequently transmitted keys can be of arbitrary length and their security against eavesdroppers do not rely on the security of any particular conventional cryptographic algorithm such as AES.

 jasong 2019-07-12 00:08

[QUOTE=ewmayer;519768]"...or slower"? Last time I checked, comms at speed <= c was a solved problem, no entanglement needed.[/QUOTE]
Technically, yes, but two quick points.

One, as far as I know, most communication involves electrons, which I believe travel at a third the speed of light.

Secondly, the farther you go, the more switching stations you need, which causes lag.

If quantum entanglement could be used for commuication, you could have line-of-sight, literally through the earth distance, almost as fast as light communication.

The only people that would suffer would be crappy MMO players who couldn't blame lag anymore. ;)

 jasong 2019-07-12 00:10

[QUOTE=xilman;519671]It can, and it has been. Search on [URL="https://en.wikipedia.org/wiki/Quantum_key_distribution"]quantum key distribution.
[/URL][/QUOTE]
I know about that, but I'm talking about coherent communication. As in being in control of what gets transmitted.

 jasong 2019-07-12 00:18

[QUOTE=CRGreathouse;519861]Fundamentally, when you and a partner have a collection of entangled particles, you don't communicate information when you both measure them. Rather, you both get (in essence) the same copy of noise/static/etc. So it's as instant as you like, but it doesn't convey information, thus circumventing the ban on speed-of-light transmission. Waiting longer doesn't somehow make the particles transmit information: they'll still just show noise, like they always do. (Indeed, if you measure them without entangling them you'll get the same sort of noise.)[/QUOTE]
I was becoming psychotic right around the time I started my physics class in high school, so I didn't learn a whole lot. But the reason I said slower than light was because I know faster than light communication is theoretically impossible. I'm assuming that if deterministic commuication is possible, something will happen to slow it down. Or maybe it will need to be slowed down for a new, deterministic method to work. If one exists.

 VBCurtis 2019-07-12 00:42

[QUOTE=jasong;521358]One, as far as I know, most communication involves electrons, which I believe travel at a third the speed of light.[/QUOTE]

Huh? An electron is a physical object. Why would one travel at any particular speed? That's like saying "baseballs travel at 95 MPH." They travel as fast as they're thrown/shot/fired/whatever, and the same is true of electrons.

You should also learn about the distinction between the speed electric current travels in a wire and the speed the individual electrons travel within the material.

For example: [url]https://www.scienceabc.com/nature/what-is-the-speed-of-electricity.html[/url]

 xilman 2019-07-12 07:34

[QUOTE=VBCurtis;521369]Huh? An electron is a physical object. Why would one travel at any particular speed? That's like saying "baseballs travel at 95 MPH." They travel as fast as they're thrown/shot/fired/whatever, and the same is true of electrons.

You should also learn about the distinction between the speed electric current travels in a wire and the speed the individual electrons travel within the material.

For example: [url]https://www.scienceabc.com/nature/what-is-the-speed-of-electricity.html[/url][/QUOTE]Yup. The rest mass of an electron is roughly 0.5MeV. Before it was shut down, the LEP collider at CERN reached energies in excess of 200,000 MeV, so the relativistic mass increase factor was over 400 thousand. Exercise: convert that into a velocity with respect to the lab. The electrons within quasar jets are travelling much faster.

To be fair, neither of those are used for point-to-point communication.

 retina 2019-07-12 07:41

Relative to the most distant parts of the Universe that are beyond our reach we are travelling at FTL, hence the reason we can't get there. Although, perhaps I am stretching the meaning of "travelling". Many people might suggest that space moving apart is not the same as travelling (through space).

 xilman 2019-07-12 07:50

[QUOTE=jasong;521358]Technically, yes, but two quick points.

One, as far as I know, most communication involves electrons, which I believe travel at a third the speed of light.[/QUOTE]
[code]traceroute to www.mersenne.org (162.212.57.131), 30 hops max, 60 byte packets
1 adsl (192.168.1.1) 2.210 ms 3.126 ms 3.699 ms
2 111.red-80-58-67.staticip.rima-tde.net (80.58.67.111) 40.612 ms 41.851 ms 43.312 ms
3 * * *
4 121.red-81-46-0.customer.static.ccgg.telefonica.net (81.46.0.121) 73.058 ms 74.828 ms 78.087 ms
5 117.red-80-58-96.staticip.rima-tde.net (80.58.96.117) 75.596 ms 78.166 ms 78.271 ms
6 ae0-400-grtmadno2.net.telefonicaglobalsolutions.com (213.140.51.56) 84.640 ms 64.441 ms 64.643 ms
7 5.53.6.64 (5.53.6.64) 65.959 ms 176.52.248.174 (176.52.248.174) 62.541 ms 5.53.6.64 (5.53.6.64) 65.891 ms
8 176.52.248.178 (176.52.248.178) 66.971 ms 68.125 ms 69.507 ms
9 94.142.107.37 (94.142.107.37) 70.287 ms 71.241 ms 72.988 ms
10 89.149.139.201 (89.149.139.201) 196.000 ms 197.222 ms 198.378 ms
11 ip4.gtt.net (173.205.47.98) 210.433 ms 211.312 ms 195.014 ms
12 * * *
13 Dallas-TX.r1.Public.Pwr-2xPDU-2xUPS-2N-100SLA.incero.com (144.168.34.10) 192.131 ms 192.399 ms 192.595 ms
14 * * *
15 * * *
[/code]
I am communicating with you right now through a WiFi network in my house (the first in the traceroute output above). That link goes at within 0.1% of the speed of light. From there links 2-4 and possibly 5 go over metallic wires. Link between 5 and 6 is fibre from the Canaries to mainland Spain. Communications on that link travel at roughly 3/4 of the speed of light. Thereafter it's hard to interpret but the transatlantic link almost certainly goes on a fibre and I would expect the US infrastructure to use fibre in large part. How you connect to [url]www.mersenne,org[/url] is also unknown to me but would be a surprise if it is exclusively over copper.

 CRGreathouse 2019-07-13 02:54

[QUOTE=xilman;521392]Link between 5 and 6 is fibre from the Canaries to mainland Spain. Communications on that link travel at roughly 3/4 of the speed of light.[/QUOTE]

Makes sense, since the speed of light in fiber optic is about 31% slower than "the" speed of light (in a vacuum, that is).

 CRGreathouse 2019-07-13 03:33

[QUOTE=jasong;521363]But the reason I said slower than light was because I know faster than light communication is theoretically impossible. I'm assuming that if deterministic commuication is possible, something will happen to slow it down.[/QUOTE]

Your current understanding is that quantum entanglement would allow communication, except that FTL isn't possible, so something happens to stop that. If the communication isn't FTL, then nothing stops that, then quantum entanglement allows communication!

This isn't how it works. There's no point at which something steps in, this just isn't what entanglement means. Reread my last post for an idea of what entanglement does mean. Or look here for a lighter take:
[url]https://www.smbc-comics.com/comic/the-talk-3[/url]

But there are things you can do if you have the ability to communicate already. First, you could just ignore the entanglement and communicate. Bingo, lightspeed or slower communication!

Second, if you have N bits entangled and a message of length M you could use superdense coding to transmit it using only max(ceil(M/2), M - N) bits/qubits.

 xilman 2019-07-13 08:48

[QUOTE=CRGreathouse;521509]This isn't how it works. There's no point at which something steps in, this just isn't what entanglement means. Reread my last post for an idea of what entanglement does mean. Or look here for a lighter take:
[url]https://www.smbc-comics.com/comic/the-talk-3[/url][/QUOTE]:tu:

A magnificent exposition and thanks for drawing it to my attention. Now I know of its existence I'll doubtless be pointing others in that direction.

 LaurV 2019-07-14 14:09

[QUOTE=CRGreathouse;521509] Or look here for a lighter take:
[URL]https://www.smbc-comics.com/comic/the-talk-3[/URL]
[/QUOTE]
:davar55: thanks for sharing, nice one. Not to mention that I spent half hour navigating back and forth on that site (which I didn't know). Most of the comics are junk, but few are really-really good! (of course humor is subjective, other people may think funny what we don't and viceversa).

 CRGreathouse 2019-07-15 08:09

[QUOTE=LaurV;521601]:davar55: thanks for sharing, nice one. Not to mention that I spent half hour navigating back and forth on that site (which I didn't know). Most of the comics are junk, but few are really-really good! (of course humor is subjective, other people may think funny what we don't and viceversa).[/QUOTE]

Alternately, navigate back and forth on
[url]https://www.scottaaronson.com/blog/[/url]
where you will find more of the quantum computing stuff rather than more of the comics stuff. (You can also search for [url=https://www.scottaaronson.com/blog/?cat=4]just quantum[/url] if you don't want his other blogging.)

 jwaltos 2019-07-16 03:25

"Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric (electrically polarizable) medium with a speed greater than that at which light propagates in the same medium."
This is the only `technically` correct assertion where `faster than light` propagation exists within a `relative` context.

Some questions of my own...where is the center of the universe relative to earth (using appropriate space-time coordinates)? Prior to decoupling, when light did not exist, what other "things" would travel at that velocity? Is there a theoretical limitation on acceleration and other time derivatives (ie. jerk, snap, crackle and pop [no joking!])? What is creating space-time..something inside or outside of this known universe?
Regarding the mathematics describing these concepts, what are the fundamental concepts (those which cannot be broken down) without which none of the above could be expressed? An understanding of enumeration, an appreciation of position and experiencing transformation [interactively] as a passage of time in my opinion still needs work. For example, factorization is an aspect of entanglement yet integer factorization is not yet completely tamed.

 jwaltos 2019-07-16 05:00

[url]https://www.sciencenews.org/article/entanglement-gravitys-long-distance-connection[/url]

This is a relevant link to the initial post and again is a `jumping-off point.`

 retina 2019-07-16 05:01

[QUOTE=jwaltos;521702]Some questions of my own...where is the center of the universe relative to earth (using appropriate space-time coordinates)?[/QUOTE]Exactly wherever you are. Since what you can observe is a sphere of radius r extending in all directions. Another observer in a another part of the universe can see a different (possibly overlapping) sphere where they are the centre of their observational extents.

 LaurV 2019-07-16 07:07

Well, the questions make sense. My understanding of these things is extremely limited, there are a lot of "universe-related" and "space-time" related things which I can not imagine and I can not conceive in my head. But from classical geometry point of view, if I observe 3 objects running away from each-other, I can see how fast A runs away from B, how fast B runs away from C, and how fast A runs away from C, and I can calculate exactly where in space and time they where when they were a single object. This is independent of where I am and if I see objects D, E, and F. Now the real question is, if I take objects A, B and D, do I get the same point as before? I know that "space dilation" is tricky... and I never studied much in this direction, that is why I can not really grasp these things, but there are few people here with a better understanding, who could shed some "faster then light"..

 xilman 2019-07-16 07:53

[QUOTE=jwaltos;521702]
Some questions of my own...where is the center of the universe relative to earth (using appropriate space-time coordinates)?[/QUOTE]You hit the nail on the head with the word "appropriate".

Many people find visualization in two dimensions easier than in three so I'll restrict my arguments to the simpler case.

If the universe is open, and observation suggests it is, there is no center. Where is the center of an infinite plane? As retina notes, any point is as good as any other.

If the universe is closed, like a sphere, for instance, which point [I]on the surface of a sphere[/I], is the center of that sphere? There is no such point and retina's prescription is once more as good as any. The key word is indeed "appropriate".

I'll address some of your other questions when the round tuit supply improves.

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