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

Introduction Background Quantum Key Distribution Protocols Experiments Limitations

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

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

Quantum Key Distribution Guarantee secure communication Produce shared random key BB84 – Photon Polarization E91 – Entangled Pairs http://ercim-news.ercim.eu/en85/special/high-speed-quantum-key-distribution-and-beyond

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

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

Alice and Bob Both figures: http://www.cse.wustl.edu/~jain/cse571-07/ftp/quantum/

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

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

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, 18790-18797 (2008)

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. 2016.

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, 040502 (2015)

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