Quantum Cryptography

1. 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 CS691 Jeff Hinson Summer 2009

3. Quantum Channels • Needed equipment: • 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 CS691 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... CS691 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 CS691 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 CS691 Jeff Hinson Summer 2009

7. Eavesdropping • Intercepting/Resend • 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 CS691 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 CS691 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.... CS691 Jeff Hinson Summer 2009

10. Questions/Comments?