1、1Experimental Demonstration of Counterfactual Quantum CommunicationUniversity of Science and Technology of China第十五屆全國量子光學會議, 2012.07.15Kai ChenIn 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-

2、Wei Pan2Motivations and BackgroundCQC SchemeExperimental CQCCounterfactual Quantum CommunicationOutlook3Secure CommunicationAliceBobEve4 Classical Cryptography Modern Cryptography RSA, Rabin, Diffie-Hellman, ElGamal etc. AES, MD5 etc. Quantum Key Distribution (Quantum Cryptography) BB84 E91 Continuo

3、us Variable DPS based (Differential Phase Shift Quantum Key Distribution )Classical and Quantum Cryptography5Applications of Counterfactual Quantum PhenomenaThe presence of a non-t r a n s m i t t i n g o b j e c t i s ascertained seemingly without interacting with it, i.e., with no photon absorbed

4、or scattered by the object. Theory: A.C. Elitzur, L. Vaidman, Found. Phys. 23, 987 (1993). Experiment: P. G. Kwiat et al., Phys. Rev. Lett. 83, 4725 (1999).“Interaction-free” Quantum Interrogation Measurements 6Applications of Counterfactual Quantum PhenomenaCounterfactual quantum computation throug

5、h quantum interrogationTheory: G. Mitchison, R. Jozsa, Proc. R. Soc. Lond. A 457, 1175 (2001).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 ru

6、nning states, and then interfering the two histories. Conditional on the as-yet-unknown outcome of the computation, it is sometimes possible to counterfactually infer information about the solution.7CQC SchemeExperimental CQCCounterfactual Quantum CommunicationOutlookMotivations and Background8 A no

7、vel new scheme! Based on orthogonal quantum state In normal scheme, the security is based on non- orthogonal quantum states, e.g., BB84 protocol Enable to generate keys when “no photon” travels In normal paradigm for communication, keys can only be generated when photons travel across two stationsCo

8、unterfactual Quantum CommunicationT.G. Noh, Phys. Rev. Lett. 103, 230501 (2009)9T.G. Noh, Phys. Rev. Lett. 103, 230501 (2009)CQC Scheme10T.G. Noh, Phys. Rev. Lett. 103, 230501 (2009)CQC SchemeProtocol:1. A triggering single-photon source S, which emits a short optical pulse containing a single photo

9、n. 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 the two polarizations representing his bit value. Bob blocks the optical path b of the single-photon pulse if t

10、he 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 depicted in Bobs site.3. On the other hand, if the single-photon pulse has a polarization orthogonal to Bobs,

11、 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 stabilized using feedback control.11CQC SchemeAnalysis:If Alices and Bobs bit values differ, the photon le

12、aves 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 and the interference is destroyed. In this case, there are three possibilities for a single photon:

13、 (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 through path b and is detected at detector D3 with probability T. After the detection of

14、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 consistent, she does not reveal any information about the polarization states; otherwis

15、e, 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 state. They disregard all other events. Monitoring: If D2 or D3 clicks, they also

16、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. 12No-cloning principle of orthogonal states in a composite system which consists of two subsyste

17、ms.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 secure?T.G. Noh, Phys. Rev. Lett. 103, 230501 (2009)13According to the chosen

18、 bit value, the initial quantum state after the BS is given by one of the two orthogonal states in our CQC schemeQuantum states utilized14 A sifted key is created by selecting only the events during which a single photon is detected at D1. In ideal cases, the photons used to create a sifted key have

19、 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 secret information being sent through the quantum channel (pa

20、th 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?15Experimental CQCCQC SchemeCounterfactual Quantum Commu

21、nicationOutlookMotivations and Background16 Full implementation with table and 1 km fiber spoolExperimental Setup17 Single mode fiber up to 1 km length Using on-shelf components Similar condition with real world environment Polarization adjustment is capable Original experiment setup and implementat

22、ion Monitor all detect eventsImplementation of experiment18 Random data generation Modulation sequential logicMassive Data and Logical ControlSpecial designed FPGA19Fast analog signal (from FPGA)High speed switch (100 ns)Proper tie sequencesFast Pulse Switch Bob needs to choose fastly “which pulse t

23、o detect” Key to enhance system working frequencyBasis for successful realization20Fiber length estimation & adjusting (10cm)Optical delay line (0.1 mm)Fiber stretcher (0.01 m)Accurate Optical Delay Adjustment Accurate optical delay adjustment(98%)Optical misalignment of about 1%Contributing abo

24、ut 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 analysis27OutlookCQC SchemeCounterfactual Quantum

25、CommunicationExperimental CQCMotivations and Background28 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 deskt

26、op experimentG. Brida et al., Laser Physics Letters 9, 247 (2012) Direct counterfactual quantum communicationH. Salih et al., (2012) Current Status of CQC 29n Novel scheme of secure key distribution: Counterfactual quantum communication realizationLiu et al., to appear in PRL (2012)R Demonstrations

27、implemented with a desktop setup, and with 1 km fiber cable, respectively.R 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 gene

28、ration of keys If Alices 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 (counterfa

29、ctual).Summary and Outlook30We 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 h

30、ave given a confirming answer for 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 detect

31、ion of possible eavesdropping. Summary and Outlook31Such 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.A

32、ctive feed-back control technology for 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-o

33、f-the-art technology.Other CQC schemes demonstrationsSummary and Outlook32陳凱中國科學技術大學Mobile: Tel. 33A 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 each 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 obta