This document provides an overview of quantum cryptography, including: - The BB84 protocol allows Alice and Bob to generate a shared secret key by Alice sending encoded photons and Bob measuring them to discard mismatched results. - Quantum cryptography derives security from photons being altered when read, preventing interception without detection. - Milestones include the first bank transfer using quantum cryptography in 2004 and distribution over 148.7 km in 2007. - Quantum cryptography has applications in secure broadcast, network, banking and email communications.

2. Contents
• Cryptography
• Photon Polarization
• Quantum Key Distribution
• BB84 Protocol
• Security of Quantum Cryptography
• Milestones
• Applications
• Advantages
• Disadvantages
• Conclusion
• References

3. Introduction
Cryptography
•Transmitting information with access restricted to the intended
recipient even if the message is intercepted by others.
Quantum
•An indivisible elementary particle, usually a photon.
Quantum Cryptography
•The use of quantum mechanics to guarantee secure
communication.

4. Cryptography
Two types of Cryptography
• Symmetric / Private Key
• Asymmetric / Public Key
Two types of Encryption
•Transposition
•Substitution

6. Photon Polarization

7. Quantum Key Distribution
BB84 protocol
Charles H. Bennett and Gilles Brassard (1984)
•Alice sends Bob a string of encoded photons.
•Bob measures the string of encoded photons using random basis
(rectilinear or diagonal).
•Alice and Bob publically compare the basis they encoded and
measured in, and discard all results where they do not match.
The result is the Shared Secret Key.

8. Channels

9. Sender-receiver of photons
• Suppose Alice uses 0 deg/90 deg polarizer sending photons to
Bob. But she does not reveal which.
• Bob can determine photons by using filter aligned to the same
basis.
• But if he uses 45 deg/135 deg polarizer to measure the photon
he will not be able to determine any information about the
initial polarization of the photon.
• The result of his measurement will be completely random.

10. Eavesdropper Eve
• If Eve uses the filter aligned with Alice’s she can recover the
original polarization of the photon.
• If she uses the misaligned filter she will receive no
information about the photon.
• Also she will influence the original photon and be unable to
retransmit it with the original polarization.
• Bob will be able to deduce Eve’s presence.

12. Example of key distribution

13. Security of quantum key distribution
• Quantum cryptography obtains its fundamental security from
the fact that each qubit is carried by a single photon, and each
photon will be altered as soon as it is read.
• This makes impossible to intercept message without being
detected.

14. Milestones
2004 - World's first bank transfer using quantum cryptography in
Vienna, Austria.
2004 - DARPA Quantum Cryptographic Network in Massachusetts,
USA.
Mar 2007 - BB84 implementation along 148.7 km fibre optic cable in
Canary Islands.
Oct 2007 - Quantum Cryptography used in Geneva for Swiss
elections.
Oct 2008 - World's first computer network protected by quantum
cryptography implemented in Vienna.
2010 - Japanese researchers succeed in quantum cryptographic key
distribution from single-photon emitter at 50 km

15. Applications
Cryptography is of increasing importance in our technological
age using :
•broadcast
•network communications
•Internet Banking
•e-mail
•cell phones
which may transmit sensitive information related to finances,
politics, business and private confidential matters.

16. ADVANTAGES
• Based on natural quantum laws
• Perfect for public communication
• Easy to detect an eavesdropper
• Security provided by QKD is future proofed

17. DISADVANTAGES
• Practical systems are limited by distance
• Photon emitters and detectors are far from perfect ,
causing errors
• Detecting eavesdropper in the presence of noise is
difficult.

18. Conclusion
Quantum cryptography is a major achievement in security engineering.

19. References
• Ekert, A. 1995. What is quantum cryptography.
http://www.qubit.org/index.html
• Ford, J. Quantum cryptography tutorial.
http://www.cs.dartmouth.edu/~jford/crypto.html.
• Wikipedia
http://en.wikipedia.org/wiki/Quantum_cryptography
• Quantum Key Distribution Protocols and Applications –
Sheila Cobourne

20. Thank You