1. Seung Hwan An University of Washington October 18, 2016 PHYS 494
Quantum Cryptography
Seung Hwan An
University of Washington
October 18, 2016
PHYS 494

2. Introduction Background Quantum Key Distribution Protocols Experiments
Limitations

3. Background
Cryptography is the practice of techniques used for secure communication
Prevents a third party from reading private messages
First machine assisted system was the rotor cipher machine

4. Classical Encryption Example
Pretty Good Privacy (PGP) is a classical encryption program that provides privacy for data communication in s.
“wUwDPglyJu9LOnkBAf4vxSpQgQZltcz7LWwEquhdm5kSQIkQlZtfxtSTsmaw q6gVH8SimlC3W6TDOhhL2FdgvdIC7sDv7G1Z7pCNzFLp0lgB9ACm8r5RZOBi N5ske9cBVjlVfgmQ9VpFzSwzLLODhCU7/2THg2iDrW3NGQZfz3SSWviwCe7G mNIvp5jEkGPCGcla4Fgdp/xuyewPk6NDlBewftLtHJVf =PAb3”
Lock with public key, unlock with private key
“Come on over for hot dogs and soda!”

5. Quantum Key Distribution
Guarantee secure communication
Produce shared random key
BB84 – Photon Polarization
E91 – Entangled Pairs

6. Terminologies
Qubit – Unit of quantum information. The classical counterpart is the “bit”.
Basis – State of an isolated quantum system.

7. BB84 Alice encodes qubits randomly into one of the two basis
Bob measures qubits with random basis
Half of sent qubits are used to create a private key

8. Alice and Bob
Both figures:

9. The Eavesdropper “Eve”
Alice and Bob can determine the presence of an eavesdropper
If Eve did Eavesdrop, Alice and Bob can abort the key
P number

10. E91
Utilizes an entangled pairs of photons. Each one goes to Alice and Bob
Only 2 out of 9 measurements are used to create a key, but provide a more secure establishment

11. Gigahertz Decoy University of Cambridge
1.02 Megabits per second over 20 kilometers with BB84
Practical and low cost QKD systems for broadband communication
A. R. Dixon, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields, "Gigahertz decoy quantum key distribution with 1 Mbit/s secure key rate," Opt. Express 16, (2008)

12. E91 QKD University of Singapore
Reached 200 bits per second over 1.5 kilometer
Ling, Alexander, Matt Peloso, Ivan Marcikic, and Christian Kurtsiefer. "Experimental E91 Quantum Key Distribution." ResearchGate (2008): n. pag. Web. 18 Oct

13. Satellite Quantum Communications
University of Padua, Italy
Satellite to Ground
Feasibility of using BB84 Protocol in space
Qubits are sent at 100 Megahertz and photon reflected back
Vallone et al. Phys. Rev. Lett. 115, (2015)

14. Limitations
Computers capable of transmitting quantum cryptography are custom made and expensive
High attenuation over 50 kilometers
Non-quantum cryptography is still very safe
Human error