Quantum Cryptography for Secure Communications

EECS Joint Colloquium Distinguished Lecture Series

Dr. Richard Hughes
Physics Division, Los Alamos National Laboratory

September 29, 1999
Hewlett Packard Auditorium, 306 Soda Hall
4:00-5:00 p.m.


Quantum cryptography, or more accurately, quantum key distribution (QKD), is a new technique that uses single-photon transmissions to generate the shared, secret random number sequences, known as cryptographic keys, which are used to encrypt and decrypt secret communications. Appealing features of QKD are that its security is based on principles of quantum physics and attempted eavesdropping can be detected. (Heisenberg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection because eavesdropping raises the key error rate above a threshold value). QKD could enable on demand re-keying of secure communications systems.

In my talk I shall describe experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over a 48-kilometer optical fiber path at Los Alamos and over line-of-sight links. In both cases, key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender.

A particularly attractive potential use of QKD is in free-space communications, such as surface to satellite transmissions. At Los Alamos, we have developed and tested an experimental free-space QKD system over an outdoor optical path of 1 km at night, and over a 0.5-km path in daylight. I will describe the implementation of QKD in our system, and present an analysis of the system's efficiency and error rate. I will discuss several eavesdropping strategies on our system and present a feasibility analysis of surface to satellite free-space QKD.

For further information see: http://p23.lanl.gov/Quantum/quantum.html

Practical free-space quantum key distribution over 1 km, Physical Review Letters 81, 3283 (1998).

Practical quantum cryptography for secure free-space communications, quant-ph/9905009.

Quantum cryptography takes to the air, Physics World, May 1999, 31 (1999).

Practical quantum key distribution over a 48-km optical fiber network, to be published in Journal of Modern Optics. (quant-ph/9904038)