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1、18.1QuantummeasurementsandencryptionSlides:Video18.1.4QuantumcryptographyTextreference:QuantumMechanicsforScientistsandEngineersSection18.2subsection“Asimplequantumencryptionscheme”QuantummeasurementsandencryptionQuantumcryptographyQuantummechanicsforscientistsandengineers
2、DavidMillerSimplequantumencryptionAlicewantstocommunicatesecurelywithBobbysendingsinglephotonsofspecificpolarizationsAliceBobpolarizingvertically“vertical”singlebeamsplitterpolarizeddetectorphotonsphotonssourceofsinglehorizontallypolarizedphotonswithphotonscontrolledpolari
3、zation“horizontal”detectorSimplequantumencryptionButtheremaybean“eavesdropper”,EvetryingtosecretlyreadtheirmessagesAliceEve?Bobpolarizingvertically“vertical”singlebeamsplitterpolarizeddetectorphotonsphotonssourceofsinglehorizontallypolarizedphotonswithphotonscontrolledpola
4、rization“horizontal”detectorSimplequantumencryptionSuppose,first,fora“1”AlicesendsaverticallypolarizedAlicephotonaphotoninthestateV?"1"VH?"0"Fora“0”shesendsahorizontallypolarizedphotonaphotoninthestateHSimplequantumencryptionBob’sapparatusseparatesthetwopolarizationsAliceB
5、obtodifferentsingle-photondetectorsV?"1"V?"1"H?"0"H?"0"verticalfor“1”horizontalfor“0”SimplequantumencryptionThisschemeisnotsecureEvecouldinsertadetectionAliceBobsystemlikeBob’sintothepathreceivethephotonfromAliceV?"1"V?"1"H?"0"H?"0"writedowntheanswerthen,usingatransmission
6、systemlikeAlice’sretransmitthephotonontoBobwithAliceandBobbeingunawareofherinterceptionSimplequantumencryptionNowrotateby45otheapparatusAliceusesfortransmissionAliceBobNowAlicetransmitsa“1”usingthestateand+45+45?"1"V?"1"a“0”usingthestate-4545oH?"0"45oIfBobleaveshisapparatu
7、sunchanged-45?"0"hewillreceivenoinformationNote+45????1/2??HVand-45????1/2??HVSimplequantumencryptionSince+45????1/2??HVand-45????1/2??HVAliceBobnomatterwhichstateAlice+45?"1"V?"1"sends45oH?"0"Bob’sapparatuswillgivethe45oanswer-45?"0"HhalfthetimeandtheanswerVhalfthetimeSim
8、plequantumencryptionNowAliceandBobeachrotatetheirapparatusby45oAliceBobThentheycansendinf