1. By Jeff Hinson CS691, Summer 2009
Quantum Cryptography
By Jeff Hinson
CS691, Summer 2009

2. Brief History
Idea born in late 60’s with Stephan Wiesner’s “Conjugate Coding”
1st successful quantum exchange: October 1989
Charles Bennet and Gilles Brassard expand idea to “Quantum Key Distribution Channel”
Has been a growing topic, especially in the past decade
CS Jeff Hinson Summer 2009

3. Quantum Channels Needed equipment: The idea: Fiber-optic cable
photon cannons on each end
filters for polarization
The idea:
Two computers: Alice and Bob
Alice polarizes a photon using a filter, sends to Bob
Bob receives photon, tries to read using a filter; guesses the polarization
CS Jeff Hinson Summer 2009

4. Quantum Channels (cont.)
Problem: unrealistic for regular use
unreliable, Bob must guess Alice’s polarization
normal internet connection still needed to verify accuracy of transmission on quantum channel
Photon-reading is slow, especially over large distances (~10kb/s)
Advantages:
difficult to hack; can tell if someone is eavesdropping on the line...
CS Jeff Hinson Summer 2009

5. Quantum Key Distribution
Makes quantum channels more realistic
The Idea:
use a quantum channel to send photons from Alice to Bob
Bob tells Alice what he thinks Alice’s polarizations were
Alice confirms/denies. Wrong bits are discarded, remaining bits are the new key
Key is used to encrypt/decrypt data send over a regular channel
CS Jeff Hinson Summer 2009

6. Quantum Key Distribution (cont.)
Cons
still expensive
slow connection over short distances
Pros
very secure; is difficult to guess the key
can tell if someone is eavesdropping:
measuring the photos will change their state
if Bob gets X number of bits wrong, eavesdropping is assumed; key discarded and a new channel is used to distribute the key
CS Jeff Hinson Summer 2009

7. Eavesdropping Intercepting/Resend Beam-splitting
traditional approach; easily detectable
ideas to read momentum without measuring; study still shows it to be detectable
Beam-splitting
Photons difficult to send one at a time; if multiple photons are sent, eavesdropper can split beam to read one and pass others on without being detected
Is difficult to do, highly unreliable, and chance of detection still exists
CS Jeff Hinson Summer 2009

8. In the News...
Cambridge team found way to transmit up to 10mb/s over large distance (20km) on lower-cost components
Spain plans to release 1st metropolitan quantum cryptography network by 2010
Austrian scientists made successful quantum connection over 90mi distance
CS Jeff Hinson Summer 2009

9. Conclusions
Quantum channel not likely to be used for regular connections anytime soon
Quantum Key Distribution is a good idea, and is very quickly becoming realistic
As scientific advancements are made, cost will decrease, and reliability and efficiency will increase
It sounds like a lot of fun, and makes for good dinner conversation....
CS Jeff Hinson Summer 2009

10. Questions/Comments?