April 12, 2006 Berk Akinci 1 Quantum Cryptography Berk Akinci

2April 12, 2006Berk Akinci Overview Classical Cryptography Quantum Random Number Generation Quantum Cryptography Using Entanglement Using Entanglement Using Uncertainty Using UncertaintyDevices Single-Photon Emitter Single-Photon Emitter Single-Photon Detector Single-Photon Detector

3April 12, 2006Berk Akinci Classical Cryptography Computational security Practical; widely used Practical; widely used Examples: AES, DES, RC-4, RSA, DH… Examples: AES, DES, RC-4, RSA, DH… Unconditional security Breaking is impossible Breaking is impossible Not practical for most applications Not practical for most applications Example: One-time pad Example: One-time pad Problem: Key Distribution Problem: Key Distribution

4April 12, 2006Berk Akinci Insecure communication channel One-time pad … Plaintext: … Random Key: … Ciphertext: AliceEncryptionDecryptionBob Eve KeyKey ?

5April 12, 2006Berk Akinci Q. Random Number Generator True Random Numbers are critical! Quantum processes are fundamentally random Semi-transparent mirror Photon source 1 0 Single-photon detector ~50% ~50% 2” idQuantique - Quantis Unbiasing …

6April 12, 2006Berk Akinci Quantum Cryptography Quantum Key Distribution Uses laws of quantum mechanics Provides unconditional security One of two fundamentals Uncertainty Uncertainty Entanglement Entanglement

7April 12, 2006Berk Akinci 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

8April 12, 2006Berk Akinci Using Uncertainty Measuring a quantum system disturbs it Alice sends individual quanta Alice sends individual quanta If Eve makes measurements, Bob can’t; that’s tamper-evident If Eve makes measurements, Bob can’t; that’s tamper-evident Eve can’t reproduce the original Eve can’t reproduce the original Neither Eve nor Bob can ever detect the entire state Neither Eve nor Bob can ever detect the entire state Devices by idQuantique and MagiQ

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

10April 12, 2006Berk Akinci Polarized Photons and Filters Source: id Quantix – Vectis…

11April 12, 2006Berk Akinci BB84 Protocol Source: id Quantix – Vectis…

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

13April 12, 2006Berk Akinci Faraday orthoconjugation Source: Risk – Bethune

14April 12, 2006Berk Akinci Single-Photon Detector Avalanche Photodiode (APD) InGaAs APD used in ‘Geiger’ mode Reverse biased just below breakdown idle Reverse biased just below breakdown idle Reverse biased just above breakdown for 1ns Reverse biased just above breakdown for 1ns Kept cool (e.g. 140K) to prevent thermally- induced avalanche Kept cool (e.g. 140K) to prevent thermally- induced avalanche

15April 12, 2006Berk Akinci Single-Photon Emitter ‘Approximated’ by attenuating a train of laser pulses If attenuating to average power matching a single photon If attenuating to average power matching a single photon 37% 0 photon – no information 37% 1 photon 26% 2+ photons – security risk!

16April 12, 2006Berk Akinci Single-Photon Emitter (Cont.) Practical systems attenuate to 0.1 photon Practical systems attenuate to 0.1 photon 89.5% 0 photon 10% 1 photon 0.5% 2+ photons

17April 12, 2006Berk Akinci Bibliography Risk, W. P.; Bethune, D. S. – “Quantum Cryptography – Using Autocompensating Fiber-Optic Interferometers.” Optics and Photonics News. July 2002, pp id Quantique – “Quantis-OEM Datasheet.” v1.3, July 2004, id Quantique – “White Paper – Random Numbers Generation using Quantum.” Version 2.0, August 2004, id Quantique – “White Paper – Understanding Quantum Cryptography.” Version 1.0, April 2005, Wikipedia community – “Quantum Cryptography.” Wikipedia – The Free Encyclopedia. Viewed on April 12,