## 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.

### Abstract:

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)

231cory@EECS.Berkeley.EDU