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Quantum network is useless
Quantum supremacy or advantage is fake.
Quantum teleportation is meaningless.
(Fig.1) The largest numbers factored so far by (still-useless) quantum computers is only 21 = 3 × 7 using fake (slower) Shor algorithm based on a single-world calculation
The current encryption system and keys such as RSA are based on two prime numbers or semi-primes, which are said to be cracked by (still-unrealized) quantum computers which may implement multiple simultaneous calculations of Shor's algorithm at once using (fictional) superposition = a dead-and-alive cat states or parallel worlds ( this p.4 ).
The point is all the current quantum computers are unable to correct errors, so they can Not calculate any meaningful values nor give right answers. ← Useful factoring by quantum computers or executing Shor algorithm is impossible (forever). ← There is No evidence of quantum superposition or parallel worlds causing quantum advantage.
So far, the largest numbers factored by the (still-useless) quantum computers are only 21 = 3 × 7 and 15 = 3 × 5 ( this 6th-paragraph, this 3rd-paragraph, this 4~5th-paragraphs ) using the fake modified version of Shor's algorithm (= called "semi-classical or Kitaev's (fake) Shor's algorithm" this 9, this 2-3rd paragraphs, this p.6 ).
↑ This fake Shor algorithm can carry out only one task at once using only one single world (= implement one task, reset, and recycle each quantum bit or qubit, this 14th-paragraph, this p.2 6th-paragraph,Fig.1.B-D this Figure.3,4,6,7 ) instead of implementing the dreamlike simultaneous quantum calculations which are required for faster factoring in true Shor's algorithm.
So the so-called post-quantum cryptography anticipating such an illusory quantum computers' threat or speed-up is completely meaningless ( this 4th-last~ 2nd-last paragraph, this 4~5th-paragraphs ).
Actually such dubious post-quantum encryption keys were easily cracked even by ordinary laptops or classical computers. ← The post-quantum encryption is useless, meaningless and just a waste of time.
As shown in the upper figure, when we try to factor 15 = 3 × 5 using Shor's algorithm, we first choose an arbitrary number (= for example, choosing "2" here ).
And then, we conduct multiple calculations like 21 divided by 15 gives the remainder of 2 (= 21 mod 15 = 2 ), 22 divided by 15 gives the remainder of 4, 23 divided by 15 gives the remainder of 8, 24 divided by 15 gives the remainder of 1,.. and find the period number (= 4, in this case ) which means the number of terms periodically giving and repeating the same remainders ( this p.7-12, this p.96 ).
↑ These simultaneous calculations giving multiple different remainders in Shor's algorithm need to be done by using (still-unrealized) quantum computer's fictional superposition ( this Step.6, this p.9, this p.2-last-paragraph ) or parallel worlds ( this last-paragraph, this 19-20th paragraphs, this last-paragraph ) in order to factor large numbers faster and break the current encryption key system ( this 10th-paragraph ).
↑ In order to unrealistically carry out these multiple different simultaneous calculations for factoring large numbers using Shor algorithm on the same common quantum bits or qubits, not only qubits but also the external devices emitting microwave pulses for manipulating qubits have to be unrealistically split into multiple different parallel worlds ( = one microwave pulse must split into multiple microwave pulses existing in different parallel worlds to simultaneously calculate different parallel-world numbers by manipulating qubits existing in different parallel worlds ), which is impossible also in the original quantum mechanical rule.
Using the period "4" allegedly found by performing multiple (parallel-world) calculations, we can obtain two prime numbers (= 24/2 - 1 = 3, and 24/2 + 1 = 5 ) composing 15 = 3 × 5 ( this p.13-14 ).
↑ Contrary to the media-hype baselessly saying like "Quantum computers will be exponentially faster", the current useless quantum computers can Not carry out these simultaneous quantum mechanical calculations (= giving multiple different remainders using different parallel worlds simultaneously by Shor algorithm is impossible in the current useless quantum computers ), instead, they can only do one single simple task using fake slower Shor's algorithm needing the help of traditional classical computer or algorithm ( this p.3-first-paragraph,p.23-2(b) ).
It means thinking about the post-quantum encryption prepared for the fictional quantum computers' power of simultaneous calculations using imaginary parallel worlds is nonsense, and just a waste of time and money.
(Fig.2) Using fragile photons or very weak classical lights as quantum information, which is easily destroyed by eavesdropper, is impractical
Quantum network, information, cryptography and key distribution (= QKD ) use a very fragile photon or very weak light as a carrier of quantum information, which is completely useless ( this p.2-left-last-paragraph, this-lower-challenges ).
↑ Because fragile photons or weak light is easily lost and cannot be sent over long distance ( this p.4 ).
This p.1-left-introduction says
"Despite recent experimental demonstrations of
entanglement distribution in quantum networks with fiber,
and free-space communications, the fragility
of quantum information in the face of noise remains as the
major barrier to the physical realization of scalable, useful
quantum networks."
The 2-3rd paragraphs and lower challenges of QKD on this site say
"In a Quantum Key Distribution system, the transmitter, usually called Alice, encodes the key into.. photon (= weak classical light ) polarization or phase...'0' or '1'. Alice then sends the quantum states (= of fragile photons ) through an optical fiber or free-space transmission channel to the receiver, usually called Bob."
"This disturbance means that an eavesdropper, usually called Eve, cannot gain any information about the key without introducing errors that Alice and Bob will detect" ← Classical weak light can explain this quantum key mechanism (= classical weak light disturbed by eavesdropper also causes detectable errors ). Quantum mechanics is unnecessary.
Current QKD systems have limitations to operate reliably beyond a few hundred kilometers due to channel loss and noise over long distances. ← sending fragile photons or quantum key over long distance is impossible. ← Quantum network or internet is far from reality, contrary to hypes.
In quantum information, light's vertical V or horizontal H polarization is used as a bit's information of "0", or "1" ( this p.2-Fig.1 ).
In other methods of encoding quantum information into the light wave phases, the photon or weak light pulse arriving earlier is used as a bit's information "0". and a photon or weak light pulse arriving later is used as "1" ( this p.2 ).
There is No such thing as a photon.
Physicists just use the attenuated laser light as fictitious photons. ← Generating a single (fictitious) photon at the designated time is impossible ( this p.2-4th-paragraph, this p.6 ).
When some eavesdropper tries to steal this fragile photon's information, the photon or weak light is easily destroyed or changed by the eavesdropper, which destroyed photon or light can be detected as a sign of eavesdropper ( this 2nd-paragraph, this 1st-paragraph, this 4th-paragraph ).
↑ According to this quantum cryptography mechanism, very weak classical light instead of (fictional) photons also can be used as a means to detect eavesdroppers (= very weak light affected or changed by eavesdropper can be detected as a sign of eavesdropping ), so quantum mechanics is unnecessary for quantum cryptography or quantum key distribution's overhyped "unhackable security" .
(Fig.2') Secrete key rate = probability of sending quantum information or photon to destination drastically decreases with distance (= sending photons over 100 ~ 200km is almost impossible due to massive photon loss ).
This fake secure quantum cryptography is useless, because if some eavesdroppers or other noises constantly intercept or influence this fragile quantum information or weak photons (= weak light ), the photons or weak lights are easily and constantly destroyed, and sending quantum information itself is impossible.
This quantum key distribution (= QKD ) sending very fragile photons or weak light is impractical, unable to send information stably over long distance ( this p.1-last-paragraph, this 1 distance limitation ).
This 5th-paragraph says
"quantum theory.. ensures that any attempt at eavesdropping on the channel results in a modification of the qubits (= destruction of fragile photons or weak light by eavesdroppers ), which will be detected by Alice and Bob. Therefore, QKD offers a secure way to exchange cryptographic keys. However, because qubits cannot be amplified, there are physical limitations on the distances between links. Most commercially available systems have a range of around 100km"
So quantum cryptography or quantum key distribution is completely useless, trying to get meaningless security by using easily-broken photons or weak light as information carriers, sacrificing transmitting distance and practicality.
Quantum information comprised of very faint photons or light is easily destroyed and lost.
But quantum mechanical stupid rule unreasonably forbids such fragile photons or weak light from being copied and amplified ( this p.11 ).
So it is impossible to send quantum information over long distance (= successful key generation rate over 200km is extremely low, and almost all photons easily get lost, this p.34 ).
This 2nd-paragraph says
"One of the most significant challenges is to extend the distance of quantum communication to a practically useful scale. Unlike classical signals that can be noiselessly amplified, quantum states in superposition cannot be amplified"
Due to this extremely low success rate of sending fragile photons, it takes extremely long time for a receiver to get information ( this 1st-paragraph ).
This p.5-right-3rd-paragraph says
"A central challenge in implementing QKD over
long distances and at high communications rates
is the ‘transmission loss in optical fibres: approximately nine out of ten photons are lost for every
50km of fibre they travel"
".. We saw that quantum information cannot be copied, which makes it challenging to develop repeaters for quantum information. Although it is an active area of research, currently QKD cannot rely on quantum repeaters to send quantum information"
This p.1-introduction-3rd-paragraph says "Current distance record in fiber for QKD (= quantum key distribution ) is 404 km. At a distance of 1000 km fiber, one would detect only 0.3 photons per century (= due to massive photon loss ) even with 10 GHz ideal single photon source (= even when 1010 photons per second or 10 GHz are generated, it takes a century for only 0.3 photons out of them to reach destination over 1000 km, and almost all other photons are lost ),
".. the key rate, however, will still drop down dramatically in long distance. One solution to this challenge is quantum repeater.. its real application still suffers from the limited performance of quantum memory."
(Fig.E) Quantum information is an error-prone, unreliable meaningless technology
The problem is the quantum information or fragile photons sent to the receiver always includes errors (= there is always disagreement between a sender's and a receiver's quantum information ), which can Not be used as legitimate reliable quantum key ( this-lower 4. this 2nd-paragraph ).
This Quantum noise and
error rates say
"Quantum cryptography uses quantum particles, which are sensitive to noise and errors. For example, photons can be affected by temperature changes or electromagnetic fields. This can cause errors in quantum communication, making information less secure."
In order to correct these errors, sending errorless information via the ordinary classical communication (= ex. telephone, ordinary internet.. ) is indispensable after all, which is called "reconciliation ( this p.1-last-paragraph )."
This p.6-last-paragraph says "there are typically still errors in the raw secret key that need to be identified, and the affected bits need to be corrected (or discarded). Error reconciliation is performed to correct any such errors and to minimise the amount of information leaked to eavesdroppers on the classical communication"
This middle says
"While in practice, bit error rate (BER) is not necessarily associated with vicious eavesdropping. Detection inefficiency, transmission loss, imperfect entanglement sources and many other factors contribute as false alarms. Error correction is an essential post-processing procedure."
↑ The fact that sending information by classical channel is necessary to correct errors ( this p.2-left-2~3rd-paragraph ) means the alleged "secure" quantum cryptography or quantum key distribution relying exclusively on quantum channel is just a pipe dream.
Sending fragile photons or quantum information cannot use the ordinary telephone lines, because noise easily causes errors in quantum information.
They need to prepare different independent fiber lines for different senders and receivers, one-to-one, which needs infinite numbers of lines for many users, hence impractical ( this p.6-left-1., this middle-1. ).
This-middle Can QKD be deployed on a large scale ? says
"its point-to-point nature,.. make its large-scale deployment extremely complex and costly
The alternative of directly linking all nodes that need to communicate is not feasible"
This 5th-paragraph says
"The problem is that the sites that wish to engage in quantum cryptographic communication must be directly connected by a quantum key distribution system.
Widespread real-world deployment of quantum cryptographic communications would require the installation of a prodigious number of quantum key distribution systems directly connecting every site. This is simply not practical."
This 1. says
"because at the moment QKD supports point-to-point connections only." ← quantum internet sending information to many receivers is impossible.
As a result, all the quantum cryptography, key distribution, internet and network are unrealistic, impossible forever, contrary to the hype.
This recent summary (= 2024 ) about the impractical quantum key distribution (= QKD ) says ↓
p.1-2nd-paragraph says "For the vast majority of use cases where classical key agreement schemes are currently used it is not possible to use QKD in practice. Furthermore, QKD is not yet sufficiently mature from a security perspective" ← QKD is useless, Not secure, contrary to hypes.
p.2-4th-paragraph says "In a QKD protocol, quantum states (for example as polarized photons) are exchanged or distributed and measured; then after post-processing, using classical communication over the authenticated channel (= ordinary classical communication is also necessary ),.. Alice and Bob may detect an eavesdropper by comparing parts of their measurement results since a quantum state changes if there is any non-trivial interaction with it"
p.3-2nd-paragraph says " Claims about "absolute" or "unconditional" security allegedly offered by QKD can never apply to actual implementations" ← ultra-secure quantum information is just the unfounded media hype.
p.4-1st-paragraph says "Signal losses in fibre-optic cables grow exponentially as a function of distance. Therefore, it is currently not possible to reliably transmit quantum states via fibre-optic cables over longer distances. QKD demonstrations at present can reach at most a few hundred kilometres".
p.4-2nd-paragraph says "One possible solution to reach longer distances is the use of quantum repeaters based on quantum entanglement (= useless ). Quantum repeaters are still the subject of fundamental research and not practical at present. An alternative is to use satellite-based QKD. However, current implementations mostly target nongeostationary orbits so that the availability of these satellites, which is also sensitive to weather conditions, is limited to a short timeframe per day" ← satellite-based quantum key distribution is also useless, hopeless.
p.4-4th-paragraph says "Even in cases where QKD might be identified as a good fit, a lot more work is required to have confidence in the security of concrete QKD devices" ← still Not secure.
p.5-2nd-paragraph says "no security proof for a practically relevant protocol has been written up"
(Fig.3) The allegedly secure quantum key distribution can be explained by classical very weak polarized light. No quantum mechanics is necessary.
The upper figure shows the original quantum key distribution (= QKD ) protocol called BB84 ( this-middle-lower ).
In this QKD protocol, Alice (= sender ) is supposed to send four kinds of (classical) very weak lights or photons with polarizations of ① vertical (= 0 ), ② horizontal (= 1 ), ③ 45o (= 0 ) or ④ 135o (= 1 ) angles, randomly and repeatedly to Bob (= Alice records what polarized lights are sent ).
Bob chooses one polarization filter or a polarizer (= basis ) of rectilinear (= parallel to vertical or horizontal polarization ) axis or diagonal axis (= parallel to 45o or 135o ).
The probability of a (fictitious) photon or weak light passing or reflecting from the polarizing filter is said to obey the classical Malus law where as the angle between light polarization and the polarizer's axis is smaller, the light (or photon ) can more likely pass the polarizing filter.
When Bob chooses the rectilinear polarizing filter, the ① vertically-polarized light ( or ② horizontally-polarized light ) can 100% pass (or is 100% reflected from ) this rectilinear polarizing filter and is detected by the photodetector as "0 (= pass)" or "1 (= reflect )" without being split at the polarizing filter.
But if Bob chooses the diagonal polarizing filter, the ① vertically-polarized light (= 0 ) may split into two weaker lights into "pass (= 1 )" and "reflect (= 0 )" sides, which can give wrong answers or errors ( this p.1-2 ), because Alice sends quantum information of "0" as vertically-polarized light, but Bob may mistake it for "1" due to the wrongly-chosen polarizer or basis.
↑ In the classical Malus law, when the vertically (or horizontally )-polarized light splits at 45o diagonal polarizing filter into two weaker lights into pass and reflect sides, only one of which barely surpasses the detection threshold of the photodetector (= in this case of choosing wrong base or polarizing filter, only one photon is detected at the pass or reflect side of the polarizing filter randomly → error ), or no photon is detected.
↑ In this case (ex. vertically-polarized light enters the 45o polarizing filter ), the originally very weak light or photon is split and far weaker by the polarizer, so only one (or zero ) weaker light on the side of "pass" or "reflect" can narrowly exceed the detection threshold of the photodetector and is detected as a (fictitious) photon.
Later, Alice and Bob have a talk with each other through ordinary classical channel, and choose only the cases of Alice and Bob choosing the same measurement polarizing filters or the same basis (= the case where Alice sends vertically or horizontally- polarized lights, and Bob chooses rectilinear polarizing filter, or the case where Alice sends 45o or 135o-polarized lights, and Bob chooses diagonal polarizing filter ).
They ignore the cases such as Alice sending vertically-polarized lights and Bob choosing the diagonal 45o polarizing filters, which causes the erroneous measurement result of the pass (= 1 ) or reflect (= 0 ) randomly.
↑ Eavesdroppers, who don't know which polarizing filter Bob will choose, may send wrongly-polarized lights (= when eavesdroppers try to hide its stealing information, copy and send the same polarized light ), after the eavesdroppers detect lights or photons with some randomly-chosen polarizing filters ( this p.1-2.BB84 protocol ).
↑ This is the mechanism of finding eavesdropping.
As seen here, this mechanism of secure quantum key distribution or cryptography can be explained by classical weak light (= which can be split at the polarizing filter and detected by photodetector only when it's light intensity exceeds the detection threshold ), and quantum mechanics and photons are unnecessary.
As I said, this quantum key distribution based on very fragile photons or very weak light is impractical, error-prone, unable to send information stably and correctly.
(Fig.3') classical light wave's polarization (= horizontal = 1, vertical = 0 ) or wave phase (= late-arriving = 1, early-arriving = 0 ) is used as quantum information bit. ↓
In actual experiments, encoding quantum information of 0 or 1 in the polarization of weak light or a photon is difficult.
So they often use the time-bin or light phase-encoding where the early-arriving light pulse (= corresponding to a bit = 0 ) and the late-arriving light pulse (= a bit = 1 ) are used as carriers of quantum information.
In this experiment ( = p.2-Figure 1 ), they encoded quantum information 0 or 1 in four different weak lights or photons with different phases (= different arriving time ) of 0 phase (= bit 0, basis-1 = polarizing filter-1 ), π/2 phase (= bit 0, basis-2 = polarizing filter-2 ), π phase (= bit 1, basis-1 = polarizing filter-1 ) and 3π/2 phase (= bit 1, basis-2 = polarizing filter-2 ), and could not send this fragile quantum information or weak light over 200 km (= p.5-Figure 5 ), useless.
(Fig.F) Quantum key distribution (= QKD ) is useless ↓
In the first quantum key distribution experiment, very weak light (= or fictional photons ) with rectilinear or circular polarization (= left-handed or right-handed-circularly-polarized light instead of 45o or 135o polarization ) was sent over only 32cm with success rate of just 0.75% (= less than one photon could reach the detector 32 cm away, when 100 photons were sent ), and it included errors. ← Far from a practical communication.
This p.10-4th-paragraph says "The quantum channel itself is a free air path of approximately 32cm (= too short traveling distance )"
↑ This original paper ↓
p.4-lower says "Realistic detectors have some noise, which cause errors.. Producing a single photon is impossible (= so the attenuated laser light was used as a unseen fictitious photon )."
p.6-last-paragraph says "circularly-polarized light was used instead of diagonal polarization (= classical weak light was used, quantum mechanical photon was unnecessary )"
p.10-4th-paragraph of this same paper says "640 light or photons out of 85000 generated light pulses were received by Bob with error rate of 4.375% (= even the 640 sent photons included 4.37% errors that must be fixed by classical communication after all, quantum information was meaningless )"
↑ So the success probability of sending fragile photons or quantum information over only 32cm was just 640/85000 = 0.0075 = 0.75% (= 99.25% photons were lost. Very inefficient ).
Even in the latest quantum key distribution research, almost all the fragile photon information was lost over only 79km, which is impractical.
(Fig.D) Quantum information is too easily lost to send over long distance. ↓
The recent quantum key distribution experiments showed very low success rate of sending quantum key = only 0.25 photon or quantum key bit per second could reach the receiver's detector over 421km, even when 2.5 × 109 photons per second (= 2.5 GHz = 2.5 × 109 Herz, 1 Herz or 1 Hz means one photon per second ) were generated and sent, which is too low success rate to be practical.
This 2nd-paragraph says
"Since the first QKD transmission across 32 cm of an optical bench... A major obstacle is posed by the fact that most photons are scattered or absorbed before getting to the receiver. In a standard optical fiber, a photon’s chances of survival are 10% after 50 km and fall to only 0.01% after 200 km (= 99.99% of the sent photons or quantum information was lost over only 200 km )"
"This is devastating considering that standard optical repeaters can’t faithfully regenerate a quantum signal, and quantum repeaters are still beyond today’s technological reach"
"Even with large signal loss, the count rate in Bob’s detectors can exceed a thousand counts per second if billions of photons are transmitted every second (= this success probability of sending photons is only 1000/1000000000 = 0.000001 = only 0.0001 %, which is impractical and too slow to send information )"
This-middle-challenges of QKD-key rate constraints say
"Key generation rates in existing QKD systems are quite low, in the order of kilobits per second, which is insufficient for many applications. Improving key rates to megabits per second level is required.", which is impossible to realize in today's impractical quantum key distribution.
↑ This original paper on this news research ↓
p.1-right-3rd-paragraph (or p.1-left-3~4rd-paragraphs ) says "Alice uses a phase-randomized diode laser pulsed at 2.5 GHz (= 2.5 × 109 Herz )". ← Alice sent 2.5 billion photons (whose phase was used as quantum information ) per second to Bob
p.3-Table I (or p.3-right-last-paragraph) shows SKR (= secret key rate, this p.6-3 ) over 421 km was only 0.25 bps (= bit per second ), which means only 0.25 bits or 0.25 photons could reach the receiver's detector per second, which is too low success rate to be practical.
↑ It means the success rate of sending photons or quantum information over 421km is only 0.25/(2.5 × 109 = 2.5 GHz ) = 10-10 = 0.0000000001. ← Too low success rate, and quantum key distribution and quantum internet are impractical forever, contrary to hypes.
↑ The (imaginary) practical quantum internet or communication needing 100% chance of sending photons or information to receiver is impossible forever.
(Fig.T) Sending fragile quantum information (= photon ), which gets easily lost, over long distance is impossible. ↓
The so-called twin-field quantum key distribution (= TF-QKD ) means two different persons A and B send two independent information or photons to the middle-point between A and B with very low success rate, which means the actual distance they send information is just half (= in 830km twin-field quantum key distribution, each A or B sends photons or information over only 415km = 1/2 × 830km, ← Not 830km, this Long range-3rd-paragraph ).
This latest (impractical) research on twin-field quantum key distribution ↓
p.3-last says "The signals (= laser lights or photons ) are then attenuated to the predetermined intensities and subsequently transmitted to Charlie (= middle point between senders of Alice and Bob ) via the quantum channels"
p.4-right-2nd-paragraph-Result says "The fibre distances between Alice-Charlie and Bob-Charlie are measured to be 500 km and 502 km (= actual distance of the sent photons is 500km = half of the overall 1000 km )"
p.5-left-3rd-paragraph says "Owing to the significant optical attenuation (= photon loss ) experienced over long-distance fibre, it is necessary to send a larger number of quantum signals.."
"..the QBER (= quantum bit error rate ) in X basis is measured to be 4.20% (= too high error rate to be practical ).
The final secure key is 3.11 × 10-12 (= success rate of sending photon information is only 0.00000000000311, too bad, p.4-Figure.3 ),
which equates to 0.0011 bps (= they could send only 0.0011 photon bits per second, or only one photon bit information per 1000 seconds, too slow )"
See Quantum network by satellite
(Fig.5) Quantum repeater just tries to detect two photons or weak lights simultaneously (= Bell state measurement BSM ), which is very low success rate, and impractical
Quantum network, quantum key distribution (= QKD ) are unable to send fragile quantum information or photons (= weak light ) over long distance due to massive photon loss.
Unlike ordinary practical classical repeaters, quantum mechanical stupid rule forbids amplifying and copying a photon or weak light carrying quantum information ( this 1st-paragraph, this 2~3rd-paragraphs, this p.1-right-3rd-paragraph ). ← This is why sending fragile quantum information over long distance is impossible.
In order to send such a fragile quantum information or photons over long distance, they have to rely on quantum repeaters that are still impractical.
In quantum repeaters, physicists divide the overall route into multiple segments where multiple light sources are set to emit pairs of polarized light (= called entangled photons ).
↑ Each light source is set to emit a pair of light: one light is horizontally-polarized, the other light is vertically-polarized, which they call "entangled photons", but entanglement is a meaningless, unnecessary concept.
As shown in the upper figure, when the central photodetector measures both of two lights B (= emitted from light source-1 ) and C (= emitted from light source-2 ) having vertical polarization, it means both Alice and Bob received lights A and D with the same horizontal polarization.
↑ This quantum repeater can indirectly send quantum information or light (with horizontal polarization ) from Alice to Bob by giving the same horizontally-polarized lights to Alice and Bob using two light sources ( this p.10-Fig.4 ).
This measurement of polarizations of two lights at each photodetector is called Bell state measurement (= BSM, this 3rd-paragraph, this Figure 1 ), and connecting two distant places by this simultaneous measurement of light pairs is called entanglement swapping ( this p.2-Fig.1 ).
This p.13-right-2nd-paragraph says "To achieve entanglement swapping the repeater carries out a Bell State Measurement (BSM)" ← So quantum repeater based on entanglement swapping is just the measurement of pairs of lights, Not amplifying fragile photons ( this-lower-figure Can you repeat ? ).
↑ This Bell state measurement (= BSM ) for quantum repeater and entanglement swapping consists of ( polarizing ) beam splitters (= PBS or BS where two polarized lights interfere with each other ) and phorodetectors (= D, this p.2-Figure.2, this p.7-Figure 1, this p.2-Fig.1 ).
The problem of this quantum repeater is it has to measure two photons or two lights (= two lights B and C in the upper figure ) simultaneously at the central photodetector (= coincidence count or simultaneous measurements of two, four.. photons at two, four.. phototedetectors is necessary in this Bell state measurement. this p.3-Fig.1, this p.2-Fig.2, this p.3-left-3rd-paragraph ).
Basically, the probability of successfully measuring two lights simultaneously at detectors (= two-fold-coincidence ) is extremely low = less than 1/1000.
Connecting many light sources needs to measure many pairs of photons or lights simultaneously, whose success rate is almost zero (= 1/1000 × 1/1000 ×: 1/1000 × .. = 0 ).
This p.3-Fig.2 shows 2-fold coincidence count (= simultaneous detection of two photons ) was only 100000 photon pairs per second, and p.3-Fig.3 shows 4-fold coincidence count (= simultaneous detection of 4 photons, this p.2-Figure 1 & p.3-entanglement swapping ) drastically decreased to only 3000 counts per 30 seconds (= only 100 counts per second ).
↑ The probability of detecting 2 pairs of photons (= 4 photons ) is 1/1000 smaller than that of detecting a pair of photons (= 2 photons ).
↑ Probability of simultaneous detection of photon pairs decreases by 1/1000 in each count of a pair of photons,
Connecting multiple light pairs by multiple repeaters' simultaneous measurements over long distance has almost zero success rate of 1/1000 × 1/1000 × 1/1000 ... = 0.
As a result, even quantum repeater cannot send quantum information or photon over long distance.
Even in the latest repeater experiment, physicists could only connect two positions separated by just 50km.
The total success probability of detecting two photons emitted from the central light source (= ions ) after traveling 25km is only 9.2 × 10-4 ( this p.3-left, p.2-Fig.2, or this 6th-paragraph ) = about 1/1000 (= 99.9% photons or lights emitted from ions got lost ).
↑ Probability of simultaneous detection of two photons is only less than 1/1000 (= the success probability of one quantum repeater connecting two photons by the simultaneous Bell state measurement of two photons is only less than 1/1000 ).
↑ This means the success probability of sending fragile photons over longer distance connecting more repeaters becomes 1/1000 × 1/1000 × 1/1000 ... = 0.
As a result, even quantum repeaters cannot send fragile quantum information or photons over long distance.
↑ As this 6th-paragraph says, this latest quantum repeater's error rate or fidelity was 0.72 (= error rate = 1 - 0.72 = 0.28 = 28% ).
↑ This error rate is too high to send precise quantum key or information, and these errors caused by quantum repeaters must be corrected by ordinary classical communication, so the original goal of sending quantum information using only secure quantum channel was meaningless.
See Quantum memory is hopeless.
The 3rd-last ~ last paragraphs of this hyped news (3/13/2024) say
"Excess noise erodes the rate of the key that can be distributed. Too much noise,.." ← Quantum key distribution suffering too much noise is impractical.
"By using a narrow energy laser as your local oscillator, it acts as a filter for the background noise" ← Classical laser light was used as quantum key.
"Future efforts will (= just speculation, still useless quantum key distribution ) focus on reproducing the experiment's results.."
↑ This research paper ↓
p.5-Fig.7 shows this research just sent weak classical laser light or photons over only 10.4 km optical fiber, which is too short to be practical.
p.6-Fig.10 estimates the secret key rate is only 10-6 (= only one photon or one bit can reach the destination per 1000000 sent photons due to massive photon loss ) over only 200km (= which is just guess, actually more information would be lost ), which is too short distance for practical quantum communication.
(Fig.M) Sending only small numbers of fragile photons or weak lights over only 35 km with massive photon loss and very high error rate (= 30 % ) is completely impractical quantum internet. ↓
The 4th, 10th, last paragraphs of this hyped news (5/15/2024) say
"The Harvard team established the practical makings of the first quantum internet (= hype ) by entangling two quantum memory nodes separated by optical fiber link deployed over a roughly 22-mile (= just 35 km, far from practical internet ) loop"
"A quantum network cannot use standard optical-fiber signal repeaters because copying of arbitrary quantum information is impossible —making the information secure, but also very hard to transport over long distances."
"A two-node quantum network is only the beginning" ← Just sending fragile photon or weak light over short distance (= 35km ) is far from practical internet.
↑ This research paper ↓
p.2-Fig.1 connected two atoms (= nodes A and B ) called silicon-vacancy center in diamond (= SiV ) where different energy levels labeled as ↑e (= up-spin-electron ? ), ↓e (= down-spin electron ? ), ↑n (= up nuclear spin ? ), ↓n (= down nuclear spin ? ) are used as a bit state that can be changed by light (or micro ) wave with different frequencies (= Fig.1b ). ← Fictional electron spin itself is unmeasurable, all they could measure was just atomic energy levels through light wave.
p.2-right says researchers utilized a photon (= just weak light wave ) to adjust and make two distant atomic energy levels (= in nodes A and B ) the same (= when one atomic energy level was spin-up state, the distant atomic energy level was also spin-up state, which they called entanglement that has nothing to do with quantum spooky link ).
p.4-Fig.4 shows they sent a photon or weak light over (only) 35km, so that two distant atomic energy levels (= in nodes A and B ) became the same (= spins are up-up or down-down ), which error rate was impractically high = 31% (= fidelity was 0.69, ).
p.14 The success probability of sending a photon over 35km was only 2.0 × 10-5 (= only 2 photons out of 100000 sent photons could successfully arrive at the distant node, and all other photons were lost ).
↑ The success rate of sending photons (= bit information ) over only 35km was only 6mHz (= only 6 photon bits could be sent per 1000 seconds due to massive photon loss, which is too slow and completely useless internet ).
This recent research (4/2/2024) sent fragile photons or quantum information over only 100km with a lot of errors, which is far from a practical network.
↑ This research paper ↓
p.4-Fig.4A shows sending fragile photons or quantum information over 130km was impossible (= SKR or secret key rate became zero in the sending distance 100km ~ 120km ).
p.3-left-2nd-paragraph mentions reconciliation which means classical channel's error correction of quantum information errors (= reconciliation efficiency β = 92.5%, frame error rate or FER = 0.59, this p.2-right-last-paragraph, which was far from errorless quantum information ).
↑ Sending fragile photons or weak lights as quantum information always causes serious errors (= so quantum key is useless ), which must be corrected by ordinary classical communication called reconciliation, so quantum communication is meaningless.
The 7th, 13th, last paragraphs of this hyped new (7/3/2024) says
"deployed fiber of ∼79 km length" ← sending information only 79km is far from practical quantum key distribution.
"Quantum communication uses the quantum characteristics of light" ← just classical (weak) light wave was used as quantum information.
"we are thrilled to demonstrate their potential for many more fascinating experiments and applications in the future (= uncertain future, still unrealized, useless research ),"
↑ This research paper ↓
p.1-abstract says "Utilising the 79 km long link (= shorter than experiments in the past, so No progress ) secret key bits per pulse of 4.8 × 10−5 (= success rate of sending quantum key photon is only 4.8 × 10-5 due to massive photon loss )
Despite longtime research, it is still impossible to send fragile quantum information (= photon or weak light ) over long distance, which means quantum key distribution research is already deadend and hopeless.
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