Computer Science. The English Language Perspective - Беликова (1176925), страница 34
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Quantum physics has provided a way around thisproblem. By harnessing the unpredictable nature of matter atthe quantum level, physicists have figured out a way toexchange information on secret keys.Quantum cryptography uses photons to transmit a key. Oncethe key is transmitted, coding and encoding using the normalsecret-key method can take place. But how does a photonbecome a key? How do you attach information to a photon'sspin?This is where binary code comes into play.
Each type of aphoton's spin represents one piece of information - usually a 1or a 0, for binary code. This code uses strings of 1s and 0s tocreate a coherent message. So a binary code can be assigned toeach photon. Alice can send her photons through randomly200chosen filters and record the polarization of each photon. Shewill then know what photon polarizations Bob should receive.Bob has no idea what filter to use for each photon, he's guessingfor each one.
After the entire transmission, Bob and Alice havea non-encrypted discussion about the transmission.The reason this conversation can be public is because of the wayit's carried out. Bob calls Alice and tells her which filter he usedfor each photon, and she tells him whether it was the correct orincorrect filter to use. Since Bob isn't saying what hismeasurements are - only the type of filter he used - a third partylistening in on their conversation can't determine what theactual photon sequence is.In modern cryptology, Eve (E – an eavesdropper) canpassively intercept Alice and Bob's encrypted message - shecan get her hands on the encrypted message and work todecode it without Bob and Alice knowing she has theirmessage. Eve can accomplish this in different ways, such aswiretapping Bob or Alice's phone or reading their secure emails.Quantum cryptology is the first cryptology that safeguardsagainst passive interception. Here's an example.
If Alice sendsBob a series of polarized photons, and Eve has set up a filter ofher own to intercept the photons, Eve is in the same boat asBob: Neither has any idea what the polarizations of the photonsAlice sent are. Like Bob, Eve can only guess which filterorientation she should use to measure the photons.After Eve has measured the photons by randomly selectingfilters to determine their spin, she will pass them down the lineto Bob.
She does to cover up her presence and the fact that sheintercepted the photon message. But Eve's presence will bedetected. By measuring the photons, Eve inevitably alteredsome of them.Alice and Bob can further protect their transmission bydiscussing some of the exact correct results after they'vediscarded the incorrect measurements. This is called a paritycheck.
If the chosen examples of Bob's measurements are all201correct - meaning the pairs of Alice's transmitted photons andBob's received photons all match up - then their message issecure.Bob and Alice can then discard these discussed measurementsand use the remaining secret measurements as their key. Ifdiscrepancies are found, they should occur in 50 percent of theparity checks. Since Eve will have altered about 25 percent ofthe photons through her measurements, Bob and Alice canreduce the likelihood that Eve has the remaining correctinformation down to a one-in-a-million chance by conducting20 parity checks.Quantum Cryptology ProblemsDespite all of the security it offers, quantum cryptology also hasa few fundamental flaws. Chief among these flaws is the lengthunder which the system will work: It’s too short.The original quantum cryptography system, built in 1989 byCharles Bennett, Gilles Brassard and John Smolin, sent a keyover a distance of 36 centimeters.
Since then, newer modelshave reached a distance of 150 kilometers (about 93 miles). Butthis is still far short of the distance requirements needed totransmitinformationwithmoderncomputerandtelecommunication systems.The reason why the length of quantum cryptology capability isso short is because of interference. A photon’s spin can bechanged when it bounces off other particles, and so when it'sreceived, it may no longer be polarized the way it wasoriginally intended to be. As the distance a photon must travelto carry its binary message is increased, so, too, is the chancethat it will meet other particles and be influenced by them.One group of Austrian researchers may have solved thisproblem. This team used what Albert Einstein called “spookyaction at a distance.” This observation of quantum physics isbased on the entanglement of photons. At the quantum level,photons can come to depend on one another after undergoingsome particle reactions, and their states become entangled.
This202entanglement doesn’t mean that the two photons are physicallyconnected, but they become connected in a way that physicistsstill don't understand. In entangled pairs, each photon has theopposite spin of the other. If the spin of one is measured, thespin of the other can be deduced. What’s strange (or “spooky”)about the entangled pairs is that they remain entangled, evenwhen they’re separated at a distance.The Austrian team put a photon from an entangled pair at eachend of a fiber optic cable. When one photon was measured inone polarization, its entangled counterpart took the oppositepolarization, meaning the polarization the other photon wouldtake could be predicted.
It transmitted its information to itsentangled partner. This could solve the distance problem ofquantum cryptography, since there is now a method to helppredict the actions of entangled photons.Even though it’s existed just a few years so far, quantumcryptography may have already been cracked. A group ofresearchers from Massachusetts Institute of Technology tookadvantage of another property of entanglement. In this form,two states of a single photon become related, rather than theproperties of two separate photons. By entangling the photonsthe team intercepted, they were able to measure one property ofthe photon and make an educated guess of what themeasurement of another property - like its spin - would be.
Bynot measuring the photon’s spin, they were able to identify itsdirection without affecting it. So the photon traveled down theline to its intended recipient none the wiser.The MIT researchers admit that their eavesdropping methodmay not hold up to other systems, but that with a little moreresearch, it could be perfected. Hopefully, quantum cryptologywill be able to stay one step ahead as decoding methodscontinue to advance.203Assignments1. Translate the sentences from the texts into Russian inwriting paying attention to the underlined words andphrases:1. By harnessing the unpredictable nature of matter at thequantum level, physicists have figured out a way toexchange information on secret keys.2. After the entire transmission, Bob and Alice have a nonencrypted discussion about the transmission.3.
Since Bob isn't saying what his measurements are -- onlythe type of filter he used -- a third party listening in ontheir conversation can't determine what the actualphoton sequence is.4. One of the great challenges of cryptology is to keepunwanted parties – or eavesdroppers -- from learning ofsensitive information.5. In modern cryptology, Eve (E – an eavesdropper) canpassively intercept Alice and Bob's encrypted message -she can get her hands on the encrypted message andwork to decode it without Bob and Alice knowing shehas their message.6.
After Eve has measured the photons by randomlyselecting filters to determine their spin, she will passthem down the line to Bob.7. If discrepancies are found, they should occur in 50percent of the parity checks. Since Eve will have alteredabout 25 percent of the photons through hermeasurements, Bob and Alice can reduce the likelihoodthat Eve has the remaining correct information down toa one-in-a-million chance by conducting 20 paritychecks.8. As the distance a photon must travel to carry its binarymessage is increased, so, too, is the chance that it willmeet other particles and be influenced by them.2049.
At the quantum level, photons can come to depend onone another after undergoing some particle reactions,and their states become entangled.2. Answer the following questions:1. How can quantum physics help users to exchangeinformation securely?2. How does a photon become a key?3. Can the users communicate openly using photons forencryption?4. How can quantum cryptology safeguard against passiveinterception?5. How does the parity check work?6.
What are the main flaws of quantum cryptology?7. Is it possible to increase the quantum cryptologycapability?3. Translate into English:Наибольшее практическое применение КК находитсегодня в сфере защиты информации, передаваемой поволоконно-оптическим линиям связи (ВОЛС). Это объясняетсятем, что оптические волокна ВОЛС позволяют обеспечитьпередачуфотоновнабольшиерасстояниясминимальными искажениями.
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