(Fig.1) Useless quantum repeater can only measure a pair of weak polarized lights (= fictitious photons ) with very low success rate, which cannot amplify nor send information over practically-long distance.
Quantum internet, communication, quantum key distribution (= QKD ) are useless forever, because they just repeat failed attempts to send fragile information (= very weak light or fictional photon), which is forbidden from being amplified, over long distance ( this 1st-paragraph ).
This 1st-paragraph says
"Distributing quantum resources such as entanglement and qubits over long distance fibre optic networks represents an enormous challenge. If we send single photons over 1000km, even at rates of 10GHz, we would need to wait hundreds of years to detect just one (= one photon or information ), due to loss in the fibre. Not very practical !"
This p.1-left-1st-paragraph (12/17/2024) says
"In QKD systems,
single-photon serves as the carriers of quantum keys,
which cannot be amplified and are easily scattered or
absorbed by the transmission channel"
Sending photons via expensive satellites is also useless, susceptible to a lot of background noise causing errors.
The only way of sending such a fragile quantum information over long distance is said to be a quantum repeater, but there are still No practical quantum repeaters despite extremely long years of fruitless researches.
This p.1-introduction-2nd-paragraph, p.2-2nd-paragraph (12/4/2024) say
"Long-distance (quantum) communication remains a significant challenge... As photons traverse fiber optic cables, losses become exponentially detrimental,.... these repeaters have yet to attain the
required technological maturity"
"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 quantum repeater is also useless ( forever ), because the quantum repeater can Not amplify the weak light or photon information to send it over long distance.
The quantum repeater can just measure two polarized lights (= B,C in upper figure ) emitted from two different light sources (= between a sender and a receiver ) at the middle photodetectors (+ beam splitter ) called Bell state measurement (= BSM ).
In the upper figure, each of two different light sources 1,2 sends a pair of lights (= one is horizontally, the other is vertically-polarized, which is called entanglement = No quantum spooky action ) into the middle photo detectors (= B,C ) and the sender A (or receiver D ).
↑ When the middle photodetectors measure two lights B,C to be vertically-polarized, it means both the sender and receiver obtain the same horizontally-polarized lights, which is equal to sending the horizontally-polarized light (= photon quantum information ) from sender to receiver, which quantum repeater mechanism is called entanglement swapping ( this 5th-paragraph ).
The problem is the quantum repeaters have to rely on very-low success rate measurement of two polarized lights B.C at the middle photodetectors simultaneously (= called coincidence detection, this p.4-1st-paragraph, this p.4-introduction ) to exclude irrelevant background lights or photons.
This p.2-left-2nd-paragraph says
"entanglement swapping (= quantum repeater ) relies on fourfold coincidence (= detecting 4 photons simultaneously, this p.2-1.2 )"
The probability of detecting two weak polarized lights (= two fictional entangled photons ) at photodetectors coincidentally is extremely low, less than only 1/1000 (= which becomes much lower and worse over longer distance ).
The 6th-paragraph of this news about the recent (useless) quantum repeater (2023) says
"The obtained fidelity was 0.72 (= error rate was 28% = useless ), with nodes A and B obtaining entanglement with a success rate of 9.2 Hz (= very slow ) and a success probability of 9.2 × 10−4 per attempt"
↑ Even the recent (useless) quantum repeater could connect only very short = 50km (= 25km × 2 ) distance with very low success rate of just 9.2 × 10-4 = about 1/1000 (= 9.2Hz = sending only 9.2 photons per second = too slow to be practical network ), whose sent information or photons contained 28% errors, which cannot be used as reliable quantum key.
This other recent hyped research showed much lower two-photon coincidence probability of only 8.5 × 10-6 ( this p.10(or p.2)-1st-paragraph ) over just 36km distance.
To send the fragile quantum information (= photon or weak light ) over practically-long distance, they have to use many quantum repeaters and measure many pairs of weak polarized lights emitted from many light sources simultaneously (= with too low success rate ) by using many photodetectors on the way from the sender to the receiver.
The probability of sending the quantum information or photon over long distance by connecting multiple repeaters is almost zero = 0 = 1/1000 (= probability of measuring each pair of polarized lights ) × 1/1000 × 1/1000 ...
↑ Connecting two repeaters needs detecting at least 4 photons simultaneously, this 4-photon coincidence probability is just less than 400 counts per hour ( this p.2-1st-paragraph ), which rate of sending fragile quantum information or photons is too low and too slow to be a practical network due to massive photon loss.
This is why quantum repeaters cannot send quantum information over long distance, and practical quantum repeaters will never be realized, as long as they try to use the fragile very weak light or photon as quantum information (= which is Not allowed to be amplified ) in vain.
Even in the latest researches in 2023 ~ 2024, the (still-impractical) quantum repeaters could connect points over only 35km ~ 50km due to severe photon (= information ) loss and a lot of errors, which was far from useful internet ( this last-paragraph ).
Quantum memory easily losing stored information is also impractical forever.
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