Quantum Cryptography by alicejenny

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									    Quantum Cryptography
                 Berk Akinci




April 12, 2006      Berk Akinci   1
                     Overview
    Classical Cryptography
    Quantum Random Number Generation
    Quantum Cryptography
         Using Entanglement
         Using Uncertainty
    Devices
         Single-Photon Emitter
         Single-Photon Detector

April 12, 2006            Berk Akinci   2
                 Classical Cryptography
    Computational security
         Practical; widely used
         Examples: AES, DES, RC-4, RSA, DH…
    Unconditional security
         Breaking is impossible
         Not practical for most applications
         Example: One-time pad
         Problem: Key Distribution

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                              One-time pad
                                                  Eve

                                  Insecure communication channel

   Alice         Encryption                                           Decryption   Bob


                                                   ?
                    Key                                                  Key




                                 Plaintext:    0000 1111 0000 1111…




                               Random Key:     0110 0010 0110 1110…




                               Ciphertext:     0110 1101 0110 0001…




April 12, 2006                                Berk Akinci                                4
   Q. Random Number Generator
       True Random Numbers are critical!
       Quantum processes are fundamentally random

                                                             Semi-transparent mirror
idQuantique - Quantis    Photon source

                                                                               Single-photon detector
                                                         ~50%
                                                                           1

       2”
                                                      ~50%



                        Single-photon detector

                                                                                       0100111011…
                                                  0                    Unbiasing




  April 12, 2006                           Berk Akinci                                                  5
             Quantum Cryptography
    Quantum Key Distribution
    Uses laws of quantum mechanics
    Provides unconditional security
    One of two fundamentals
         Uncertainty
         Entanglement




April 12, 2006           Berk Akinci   6
                 Using Entanglement
    Create pairs of entangled photons
    Transmit them to Alice and Bob
    Alice and Bob get ‘complementary’
    photons
    Difficult to keep states entangled for
    long time/distances
    No commercial application yet


April 12, 2006          Berk Akinci          7
                 Using Uncertainty
    Measuring a quantum system disturbs it
         Alice sends individual quanta
         If Eve makes measurements, Bob can’t;
          that’s tamper-evident
         Eve can’t reproduce the original
         Neither Eve nor Bob can ever detect the
          entire state
    Devices by idQuantique and MagiQ

April 12, 2006            Berk Akinci               8
 Using Uncertainty – Principles
    Practical approach uses photons
         Photons can be transmitted over long
          distances
         Photons exhibit the required quantum
          mechanical properties
    Quantum properties exploited
         Photons can not be divided or duplicated
         Single measurement is not sufficient to
          describe state fully
April 12, 2006             Berk Akinci               9
   Polarized Photons and Filters




                 Source: id Quantix – Vectis…


April 12, 2006           Berk Akinci            10
                 BB84 Protocol




                   Source: id Quantix – Vectis…



April 12, 2006             Berk Akinci            11
      Using Uncertainty – Reality
    Photon polarization is transformed
    through fiber
         Autocompensation – Faraday
          orthoconjugation
    No good single-photon emitter
    No good single-photon detector
    Quantum Error Correction
    Privacy Amplification

April 12, 2006           Berk Akinci     12
         Faraday orthoconjugation




                  Source: Risk – Bethune


April 12, 2006         Berk Akinci         13
             Single-Photon Detector
    Avalanche Photodiode (APD)
    InGaAs APD used in ‘Geiger’ mode
         Reverse biased just below breakdown idle
         Reverse biased just above breakdown for
          1ns
         Kept cool (e.g. 140K) to prevent thermally-
          induced avalanche



April 12, 2006             Berk Akinci                  14
                 Single-Photon Emitter
    ‘Approximated’ by attenuating a train of
    laser pulses
         If attenuating to average power matching a
          single photon
                 37% 0 photon – no information
                 37% 1 photon
                 26% 2+ photons – security risk!




April 12, 2006                    Berk Akinci      15
  Single-Photon Emitter (Cont.)
         Practical systems attenuate to 0.1 photon
                 89.5% 0 photon
                 10% 1 photon
                 0.5% 2+ photons




April 12, 2006                     Berk Akinci        16
                    Bibliography
    Risk, W. P.; Bethune, D. S. – “Quantum Cryptography – Using
    Autocompensating Fiber-Optic Interferometers.” Optics and
    Photonics News. July 2002, pp 26-32
    id Quantique – “Quantis-OEM Datasheet.” v1.3, July 2004,
    http://www.idquantique.com
    id Quantique – “White Paper – Random Numbers Generation
    using Quantum.” Version 2.0, August 2004,
    http://www.idquantique.com
    id Quantique – “White Paper – Understanding Quantum
    Cryptography.” Version 1.0, April 2005,
    http://www.idquantique.com
    Wikipedia community – “Quantum Cryptography.” Wikipedia –
    The Free Encyclopedia. Viewed on April 12, 2006.
    http://en.wikipedia.org/wiki/Quantum_cryptography


April 12, 2006                Berk Akinci                         17

								
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