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Quantum cryptography is hopeless
(Fig.1) Quantum information (= fragile photons or very weak light ) are easily lost by atmosphere, and a lot of background light causes information errors in satellite-based quantum communication.
Quantum communication, key distribution (= QKD ) sending quantum information (= very weak light or fictitious photons ) by using satellites is impractical ( this-lower-Challenges ).
Because the weak light or photons (= quantum information ) are easily lost by the atmosphere ( this 3rd-last-paragraph ), and irrelevant background light (= from stars or city light ) always causes errors or wrong quantum information ( this-challenges- 2nd-paragraph ).
Quantum internet or communication that needs to send or receive a lot of continuous errorless information is definitely impossible due to severe loss of photons or quantum information in today's impractical quantum network, contrary to hypes.
Satellite-based quantum key distribution experiments susceptible to weather condition ( this p.1-left-1st-paragraph ) were conducted only at night or in places without lights (= bright urban cities cause a lot of irrelevant background photons and errors ), but errors by irrelevant background light or photons can never be avoided ( this p.9-2nd-paragraph ).
↑ Today's erroneous quantum key distribution needs ordinary classical communication's error correction (= so it is far better to use ordinary practical cheap classical communication from the beginning ) and makes the purpose of quantum communication meaningless.
This p.2-2nd-paragarph says
"Satellite communication, however, poses significant challenges, as they are costly to build, send to
space and maintain. Their availability depends on weather and atmospheric conditions that are difficult
to control."
This p.1-introduction-2nd-paragraph says
"The main challenge of long-distance free-space quantum communication are the high
photon losses in the channel, caused mostly by absorption, diffraction and turbulence in
the air"
This p.1-abstract says
"the number of secret bits that can be generated in a
single satellite pass over the ground station is severely restricted by the pass duration and the free-space
optical channel loss. High channel loss may decrease the signal-to-noise ratio due to background noise,
reduce the number of generated raw key bits, and increase the quantum bit error rate (QBER), all of
which have detrimental effects on the output secret key length"
This 17th-paragraph says
"The satellite cannot yet establish a real quantum key distribution link."
↑ Unlike ordinary strong classical information that can be freely amplified, sending each fragile quantum information (= very weak light or a single photon that is not allowed to be amplified ) to the exact pinpoint location by adjusting satellite's position and orientation is unrealistic and impossible.
(Fig.2) Only one photon (= one bit information ) per 5.9 million photons reached the destination, which massive photon loss makes quantum internet impossible forever.
In the recent experiment using Chinese satellite, about 5.9 million pairs of photons or weak lights per second (= 5.9 MHz, one light is horizontally-polarized and the other light is vertically-polarized in each pair ) were emitted from a satellite toward two separate ground stations which were about 500 ~ 1600 km away from the satellite.
And only about one pair of photons (= 1.1 photons or 1.1 Hertz, this-7~8th-paragraphs ) per second were successfully detected by two ground stations, which rate is too low to be practical quantum communication
↑ When one ground station detected a horizontally-polarized light, the other ground station detected a vertically-polarized light, which they called "entanglement" that has nothing to do with superluminal spooky link. ← quantum entanglement is a meaningless, useless concept.
↑ One photon pair out of 5.9 million pairs of photons emitted from the satellite reached the destination = the success rate is only 0.00000017 ← This was too low success rate to be useful.
↑ This original paper ↓
p.2-left-2nd-paragraph says "the distance between the satellite and two ground stations at an altitude of about 3000m" is 500~2000km.
p.2-left-3rd-paragraph (or this p.3-last-paragraph ) says "we established entanglement between two single photons.. with an average two-photon count rate of 1.1 Hz and state fidelity of 0.869 (= which means even the small numbers of sent photons that reached ground stations at a rate of one photon per second showed the error rate of 0.131 (= 1 - 0.869 ) or 13.1% error, which cannot be used as practical key )"
p.2-left-last says "The source emits 5.9 million entangled photon pairs per second (= entangle means just a pair of photons; one light is vertically-polarized V and the other light is horizontally-polarized H, ← meaningless entanglement, No superluminal spooky link )"
p.3-right-1st-paragraph this same paper says "the background noise ranged from 500 to 2000 counts/s in each detector (= which caused errors that must be corrected by ordinary classical communication, so quantum information is meaningless )"
↑ The total success probability of sending pairs of photons to two ground stations from the satellite is only 1.1/5.9 million = 0.00000017 (= 99.99999% photons or quantum information were lost ), which is too bad and too slow to use as practical communication or internet.
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