(Fig.1) Weak classical light wave surpassing the detection threshold of a photodetector is measured to be a (fictional) photon.
Quantum key distribution (= QKD ) is impractical, because its quantum information (= key ) is very weak (classical) light or a fictional photon, which is easily lost before reaching the receiver ( this p.1-left-1st-paragraph ).
The present QKD or quantum cryptography is based on BB84 protocol in which when the sender sends vertically-polarized 0o (or horizontally-polarized 90o ) weak light or photon, and the receiver chooses the polarizing filter 0o (= called rectilinear basis ), vertically-polarized 0o light passes (or horizontally-polarized 90o light reflects from ) the filter, measured to be a (quantum) key of 0 = pass (or 1 = reflect ), as shown in Fig.1-upper.
When the sender sends 45o-polarized (or 135o-polarized ) weak light or photon, and the receiver chooses the polarizing filter 45o (= called diagonal basis ), 45o-polarized light passes (or 135o-polarized light reflects from ) the filter, measured to be a quantum key of 0 = pass (or 1 = reflect ), as shown in the middle of this Fig.1.
But when the sender sends 45o-polarized light, and the receiver chooses the (wrong) polarizing filter 0o, the 45o-polarized light splits into the pass and reflect sides of the filter, and is measured to be 0 (= pass ) or 1 (= reflect ) randomly, as shown in of this Fig.1-lower ( this-Figure.3, this 9th-paragraph, this p.3-1st~2nd-paragraphs ) obeying Malus law of the pass light intensity or probability proportional to I cos245o = 1/2 I (= half of the original light intensity I ). ← This case causes errors, because whethre quantum key becomes 0 or 1 is unclear, random.
When the intensity of the very weak (classical) light narrowly surpasses the detection threshold of the photodetector, it is detected as a (fictional) photon.
Even when the weak light with 45o polarization splits into pass and reflect sides of 0o polarizing filter, only one photon on one pass or reflect side can be detected, because the very weak light intensity further weakens by the filter (= a part of light is lost ), and it is unlikely that the split very weak light intensity can surpass two photodetector's detection thresholds in both sides of the filter simultaneously.
In QKD, the sender and receiver use only the cases when they choose the right combination of the polarized light and filter (= a case of sending 0o or 90o-polarized light and the receiver choosing 0o-filter, or a case of sending 45o or 135o-polarized light and the receiver choosing 45o-filter ) where the sender's quantum information = light of polarization of 0 is detected as 0 by the receiver, as shown in Fig.1's upper and middle.
↑ They discard a case of sending 0o or 90o-polarized light and the receiver choosing the wrong 45o-filter, and a case of sending 45o or 135o-polarized light and the receiver choosing the wrong 0o-filter where the sender's quantum information = light of polarization of 0 may be detected as randomly 0 or 1 (= error ), as shown in Fig.1-lower.
Even if eavesdroppers intercepts the light or photon, the eavesdroppers don't know what polarizing filters or bases are chosen by the receiver, so the eavesdropper who may measure the polarized light (= ex. 0o-polarized light ) by the wrong different filter (= 45o-polarizing filter ) is likely to get and re-send the wrong quantum key toward the receiver (= 0 or 1 randomly ), which can be detected as a sign of eavesdropping ( this 11~12th-paragraphs, this middle ).
As shown here, the classical weak light can naturally explain the present quantum key distribution, and quantum mechanics or a fictional photon is unnecessary.
QKD is said to use the impossibility of copying quantum photon (= this 2. No cloning theorem of quantum mechanics ) as a reason for the secure quantum key, but in fact, this quantum mechanical no cloning theorem is unnecessary in the QKD.
Even if the eavesdropper intercepts (= measures ) the light (or photon ), copies and resends it towards the receiver, there is always a time lag or delay in the eavesdropper copying and resending the same light (= which takes extra time ), which time lag can be easily detected as an evidence of eavesdropping. ← Quantum mechanical no-cloning theorem is unnecessary in QKD.
So quantum mechanics or no-cloning theorem is unnecessary in the current (useless) quantum key distribution. Classical weak light can naturally explain the current quantum key distribution, so quantum mechanics is useless.
(Fig.2) 99.995% of all fragile quantum information or photons are lost over just 130km. ←quantum network is completely useless.
Quantum communication or quantum key distribution (= QKD ) is impractical, deadend, still unable to send the fragile quantum information or photons over 80km stably as shown in the recent hopeless QKD research in 2024 ( this 7th-paragraph ).
↑ This research paper in the recent QKD ↓
p.1-abstract says "Utilising the 79 km long.... secret key bits per pulse of 4.8 × 10−5 " ←The probability of a photon (= quantum information bit ) reaching the destination is extremely low = just 0.000048 (= 99.995% of all information or photon pulses are lost over just 79km )
p.7-Fig.4 shows this current quantum key distribution cannot send the fragile quantum information over 150km where bits (= quantum key generation ) per pulse is zero.
↑ Quantum key distribution sending information key over practically-long distance
is impossible (forever).
Quantum internet or communication needing the zero-loss of information is much more impossible.
(Fig.3) The probability of each fragile quantum information or a photon transmitted over 500km (× 2 ) is hopelessly low = just 0.000000000003 (= almost all photons are lost ), which is unrealistic.
The 3rd paragraph of this recent quantum key distribution news (in 2023 ) says
The recent achievement by Chinese scientists has pushed this limit to 1,002 kilometres (= two persons sent each photon over 500km towards the center point between them ), with a secure key rate of 0.0034 bits per second (= too few and too slow )."
↑ This rate of getting quantum key is too slow (= only one bit or one photon reaching the destination per 300 seconds = 0.0034 bits/second ) to be a practical quantum key distribution that needs a key-generation rate of at least megabits per seconds ( this-middle-Challenge of QKD ).
↑ This research paper ↓
p.1-abstract says "The secure key rate is 3.11 × 10–12 per pulse" ← The probability of a fragile quantum information or a photon reaching the destination over 1000km per one sent photon pulse is just 0.00000000000311 (= 99.9999999997% of all photons were lost ).
p.3-Fig.1 and p.4-right says "The fibre distances between Alice-Charlie and Bob-Charlie are measured to be 500 km and 502 km" ← Alice and Bob sent their photons over about 500km towards the center Charlie.
p.5-left-3rd-paragraph says "The quantum bit error rate (QBER)... is measured to be 4.20%" ← Even photons reaching the destination showed the error rate of 4.20%, which cannot be used as reliable quantum keys after all.
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