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1、Experimental Demonstration of Counterfactual Quantum Communication,University of Science and Technology of China,第十五届全国量子光学会议广州,2012.07.15,Kai Chen,In collaboration with Yang Liu,Lei Ju,Xiao-Lei Liang,Shi-Biao Tang,Guo-Liang Shen Tu,Lei Zhou,Cheng-Zhi Peng,Teng-Yun Chen,Zeng-Bing Chen,and Jian-Wei P
2、an,Motivations and Background,1,Counterfactual Quantum Communication,Secure Communication,Alice,Bob,Eve,Classical CryptographyModern CryptographyRSA,Rabin,Diffie-Hellman,ElGamal etc.AES,MD5 etc.Quantum Key Distribution(Quantum Cryptography)BB84E91Continuous VariableDPS based(Differential Phase Shift
3、 Quantum Key Distribution),Classical and Quantum Cryptography,Applications of Counterfactual Quantum Phenomena,The presence of a non-transmitting object is ascertained seemingly without interacting with it,i.e.,with no photon absorbed or scattered by the object.,Theory:A.C.Elitzur,L.Vaidman,Found.Ph
4、ys.23,987(1993).Experiment:P.G.Kwiat et al.,Phys.Rev.Lett.83,4725(1999).,“Interaction-free”Quantum Interrogation Measurements,Applications of Counterfactual Quantum Phenomena,Counterfactual quantum computation through quantum interrogation,Theory:G.Mitchison,R.Jozsa,Proc.R.Soc.Lond.A 457,1175(2001).
5、Experiment:O.Hosten,M.T.Rakher,J.T.Barreiro,N.A.Peters,P.G.Kwiat,Nature 439,949(2006).,Counterfactual computation is accomplished by putting the computer in a superposition of running and not running states,and then interfering the two histories.Conditional on the as-yet-unknown outcome of the compu
6、tation,it is sometimes possible to counterfactually infer information about the solution.,CQC Scheme,2,Counterfactual Quantum Communication,A novel new scheme!Based on orthogonal quantum stateIn normal scheme,the security is based on non-orthogonal quantum states,e.g.,BB84 protocolEnable to generate
7、 keys when“no photon”travelsIn normal paradigm for communication,keys can only be generated when photons travel across two stations,Counterfactual Quantum Communication,T.G.Noh,Phys.Rev.Lett.103,230501(2009),T.G.Noh,Phys.Rev.Lett.103,230501(2009),CQC Scheme,T.G.Noh,Phys.Rev.Lett.103,230501(2009),CQC
8、 Scheme,Protocol:,1.A triggering single-photon source S,which emits a short optical pulse containing a single photon.The polarization is chosen at random to have either horizontal polarization H representing the bit value 0,or vertical polarization V representing 1.2.Bob also randomly chooses one of
9、 the two polarizations representing his bit value.Bob blocks the optical path b of the single-photon pulse if the polarization of the pulse is identical to his polarization.The blocking of optical path b in such a polarization-selective way can be suitably accomplished,for instance,using the setup d
10、epicted in Bobs site.3.On the other hand,if the single-photon pulse has a polarization orthogonal to Bobs,its optical path b is not affected by the SW.Hence,a split pulse travelling through path b may be reflected by the FM in Bobs site and is returned back to the BS.4.The interferometer can be stab
11、ilized using feedback control.,CQC Scheme,Analysis:,If Alices and Bobs bit values differ,the photon leaves the interferometer going toward detector D2 with certainty owing to the interference effect.If,however,Alices and Bobs bit values are equal,the split pulse in path b is blocked by detector D3 a
12、nd the interference is destroyed.In this case,there are three possibilities for a single photon:(i)the photon travels through path a and is detected at detector D1 with probability RT;(ii)the photon travels through path a and is detected at detector D2 with probability R2;(iii)the photon goes to Bob
13、 through path b and is detected at detector D3 with probability T.After the detection of a photon is completed,Alice and Bob tell each other whether or not each of the detectors clicked.If D1 clicks alone,Alice compares the detected polarization state to her initial polarization state:if they are co
14、nsistent,she does not reveal any information about the polarization states;otherwise,she also announces her measurement results.Alice and Bob can then establish an identical bit string(a sifted key)by selecting only the events for which D1 alone detects a photon with a correct final polarization sta
15、te.They disregard all other events.Monitoring:If D2 or D3 clicks,they also announce both the detected polarization state and the initial polarization states that were chosen.This is intended to detect Eves intervention by monitoring the correct operation of the interferometer.,No-cloning principle o
16、f orthogonal states in a composite system which consists of two subsystems.If reduced density matrices of an available system are non orthogonal and if the other subsystem is not allowed access,it is impossible to distinguish two orthogonal quantum states without disturbing them.,CQC-why is it secur
17、e?,T.G.Noh,Phys.Rev.Lett.103,230501(2009),According to the chosen bit value,the initial quantum state after the BS is given by one of the two orthogonal states in our CQC scheme,Quantum states utilized,A sifted key is created by selecting only the events during which a single photon is detected at D
18、1.In ideal cases,the photons used to create a sifted key have not travelled through path b but only through path a(if the photons have traveled through the path b,they must have been detected at D3).The task of a secret key distribution,therefore,can be accomplished without any photon carrying secre
19、t information being sent through the quantum channel(path b).A photon that carries secret information has been confined from its birth to death within Alices secure station,and Eve can never access the photon.Bob in fact extracts a secret key from the non-detection events.,CQC-why counterfactual?,Ex
20、perimental CQC,3,Counterfactual Quantum Communication,Full implementation with table and 1 km fiber spool,Experimental Setup,Single mode fiber up to 1 km lengthUsing on-shelf componentsSimilar condition with real world environmentPolarization adjustment is capableOriginal experiment setup and implem
21、entationMonitor all detect events,Implementation of experiment,Random data generationModulation sequential logic,Massive Data and Logical Control,Special designed FPGA,Fast analog signal(from FPGA)High speed switch(100 ns)Proper tie sequences,Fast Pulse Switch,Bob needs to choose fastly“which pulse
22、to detect”Key to enhance system working frequency,Basis for successful realization,Fiber length estimation&adjusting(10cm)Optical delay line(0.1 mm)Fiber stretcher(0.01 m),Accurate Optical Delay Adjustment,Accurate optical delay adjustment(1mm)is a must for Interferometer with high visibility,High S
23、peed,Wide Range Fiber Stretcher,Fast Feedback loop,Phase of fiber is very sensitive to environment,which will change even with voice fluctuation!Stabilizing phase for long distance fiber interferometer,Fast PI board and amplify boardAll-Analog circuitsAble to response as fast as 1 s!,Fast Feedback l
24、oop,Timing analysis:Strong pulse as feedback referencePI“lock”strong pulseWith 200 ns,strong pulse is off for signal workingHigh distinct ratio,keep noise awayBeing able to enhance working frequency!,Fast Feedback loop,Keep environment noise off!Keep temperature stableKeep quietKeep fiber stable,Env
25、ironment Control,1 km fiber spool in laboratoryStable working for about 15 minutes Phase lock visibility up to 98%and stable113k bit raw keys total,5.8%raw quantum bit error rateMore strict security analysis for imperfect devices and sources,Experiment results,Interferometer visibility(98%)Optical m
26、isalignment of about 1%Contributing about 3.8%error rateDetector events for keys are much smaller than that of othersExtinction ratio of optical switch(17 dB)Contributing about 2%error rateDetector dark count rate(1e-5)Contributing to about 0.5%error rate5.8%total 3.8%+2%+0.5%,Error analysis,Outlook
27、,4,Counterfactual Quantum Communication,Unconditional security with single photon sourceZ.Q.Yin et al.,PRA 82,042335(2010)Method to enhance efficiencyY.Sun et al.,PRA 82,052318(2010)An experiment to modified schemeM.Ren et al.,Laser Physics 21,755(2011)A simple desktop experimentG.Brida et al.,Laser
28、 Physics Letters 9,247(2012)Direct counterfactual quantum communicationH.Salih et al.,http:/arxiv.org/abs/1206.2042v2(2012),Current Status of CQC,Novel scheme of secure key distribution:Counterfactual quantum communication realizationLiu et al.,to appear in PRL(2012),Demonstrations implemented with
29、a desktop setup,and with 1 km fiber cable,respectively.Developments of high-precision active feed-back control technology for maintaining real-time stabilization of Michelson-type interferometer with high visibilities of greater than 98%,which ensures high-speed and steady generation of keys,If Alic
30、es and Bobs choices for bit values does not destroy interference,only detector D2 will click due to interference effect.If detector D1 clicks,Alices and Bobs choices for polarizations will destroy interference.Particularly,the photon does not travel along path b(counterfactual).,Summary and Outlook,
31、We have achieved the first faithfully proof-in-principle demonstrations realization of counterfactual quantum communication,in which process information carriers are seemingly not traveled in the quantum channel.From a desktop test to a setup with 1 km fiber cable,we have given a confirming answer f
32、or feasibility of CQCOne can infer that the mere possibility for signal particles to be transmitted is sufficient to create a secret key.We remark that,to ensure such possibility,partial signal particles still need to randomly travel along quantum channel for detection of possible eavesdropping.,Sum
33、mary and Outlook,Such a implementation by exploiting counterfactual effect has revealed new surprising physics behind quantum mechanics,in addition to existing experimental demonstrations of“interaction-free”measurements,and counterfactual quantum computation.Active feed-back control technology for
34、maintaining real-time stabilization of Michelson-type interferometer.Tailored optical,controlling electronics designs,and special optical switch are made and chosen for attaining high visibilities.Long-distance realization are ahead with by utilizing state-of-the-art technology.Other CQC schemes dem
35、onstrations,Summary and Outlook,Thank you!,陈凯中国科学技术大学Mobile:18601798188Tel.0551-3607951,A complete analysis of the QKD security,including various experimental imperfections,is very challenging and open for future studyWeak coherent pulses realizations:Eve cannot determine the number of photons in ea
36、ch pulse because she is not allowed to access path a.It is impossible for Eve to measure even the number of photons travelling through the quantum channel(path b),provided that she does not disturb the states.Eve obtains which-path information through the photon number measurement in path b,and she
37、destroys the interference.Hence,Eve may cause detection errors,and she may be detected due to the photon number measurement itself.,CQC Scheme-Security Advantages,T.G.Noh,Phys.Rev.Lett.103,230501(2009),A complete analysis of the QKD security,including various experimental imperfections,is very chall
38、enging and open for future studyWeak coherent pulses realizations:Thus,the present protocol is inherently robust against the so-called photon number-splitting attack.Eve cannot split a photon when all of the photons in the pulse travel through path a.That is,if all of the photons are detected at D1
39、after traveling through path a,the bit information is not revealed to Eve,even when a multiphoton pulse is used.Eve cannot obtain a copy of the initial quantum state even when she succeeds in splitting a photon.That is,Eve only obtains a collapsed state whenever she knows that she has a photon,and she remains limited by the no-cloning theorem.,CQC Scheme-Security Advantages,T.G.Noh,Phys.Rev.Lett.103,230501(2009),
