18.1.4_Quantum_cryptography

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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