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Old 2019-06-20, 18:51   #1
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Default 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?
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Old 2019-06-20, 19:41   #2
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Quote:
Originally Posted by jasong View Post
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?
It can, and it has been. Search on quantum key distribution.
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Old 2019-06-21, 23:20   #3
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"...or slower"? Last time I checked, comms at speed <= c was a solved problem, no entanglement needed.

Last fiddled with by ewmayer on 2019-06-21 at 23:21
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Old 2019-06-22, 08:29   #4
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"...or slower"? Last time I checked, comms at speed <= c was a solved problem, no entanglement needed.
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 in vacuo between spacecraft and in vitro terrestrially. Light travels in glass at substantially less than c. Secure key distribution is a seriously hard problem without making use of entanglement.

Last fiddled with by xilman on 2019-06-22 at 08:31 Reason: Fix speeling misteak
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Old 2019-06-22, 20:10   #5
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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 in vacuo between spacecraft and in vitro terrestrially. Light travels in glass at substantially less than c. Secure key distribution is a seriously hard problem without making use of entanglement.
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?
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Old 2019-06-22, 20:53   #6
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Quote:
Originally Posted by ewmayer View Post
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?
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.
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Old 2019-06-22, 22:38   #7
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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 think teethbreesh [sic] would be under more restrictive rules; don't share them with anyone, especially not the family dog.
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Old 2019-06-23, 06:40   #8
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Quote:
Originally Posted by jasong View Post
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?
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.)
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Old 2019-06-23, 15:50   #9
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Quote:
Originally Posted by ewmayer View Post
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?
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.

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Old 2019-06-23, 17:26   #10
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Originally Posted by retina View Post
I think teethbreesh [sic] would be under more restrictive rules; don't share them with anyone, especially not the family dog.
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.

Last fiddled with by xilman on 2019-06-23 at 17:30 Reason: Clarification
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Old 2019-07-12, 00:08   #11
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Quote:
Originally Posted by ewmayer View Post
"...or slower"? Last time I checked, comms at speed <= c was a solved problem, no entanglement needed.
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. ;)
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