Cryptography Reference
In-Depth Information
principle, absolutely secure against any computational improvements. In this
chapter we review the state of the art in free-space quantum cryptography.
We describe a semiportable free-space quantum cryptography system that
has been tested in a key exchange experiment between two mountain tops,
Karwendelspitze (2244 m) and Zugspitze (2960 m), in southern Germany [12].
The distance between the two locations is 23.4 km. The elevated beam path
dramatically reduced the air turbulence effects experienced in previous low-
altitude tests [11] but also caused unprecedented requirements on stability
against temperature changes, reliability under extreme weather conditions,
and ease of alignment. In future high-altitude experiments we plan to extend
this range more than 100 kilometers.
We go on to describe how such a system combined with sophisticated au-
tomatic pointing and tracking hardware could exchange keys with low Earth
orbit satellites. If we engineer a satellite to be a secure relay station, we may
see secure key exchange between any two arbitrary locations on the globe.
The advantage of the space environment for communications is the loss-free
(and distortion-free) optical path provided by the vacuum. Conventional op-
tical free-space laser communication systems have been under development
for some time. The recent success of the ARTEMIS-SPOT4 satellite-to-satellite
(GEO-to-LEO) link [14] has increased confidence in these technologies. The
question remains whether one can exchange a key to a low earth orbit satel-
lite. Preliminary studies suggest this will be possible [15,16] with lightweight
launch optics of
125 mm aperture. In this chapter we discuss some of the
detailed designs for such a system and remaining technical challenges to be
overcome.
We also extend the scope of our study to introduce entangled state key
exchange methods [17-22]. Such systems are intrinsically more secure than
the faint pulse techniques that have predominated to date.
∼
9.2 Quantum Coding
In quantum communications, the primary carrier of the information is the
particle of light, the photon. The general qubit is represented by
|
>
=
α
|
0
>
+
β
|
1
>
(9.1)
2
2
with probability amplitudes normalized to
1. The implicit as-
sumption is that a single two-state system is involved. This generic notation
can stand for any of the properties of various two-state systems, for exam-
ple for ground
|
α
|
+|
β
|
=
|
g> and excited
|
e> state of an atom, for horizontal
|
H> and
vertical
V> polarization of a photon, or for path 0 and path 1 around an inter-
ferometer. The probability of detection of either state is the square modulus
of the state amplitudes
|
2
. The key to quantum communications
is the principle of superposition, where the probability amplitudes are both
nonzero; the photon then exhibits wavelike and particlelike properties.
Another key concept for quantum communications is the phenomenon of
entanglement. Entanglement describes the strong correlations that can exist
2
|
α
|
and
|
β
|
