Cryptography Reference
In-Depth Information
ALICE
LEO Satellite
BOB
London
BOB
O. Teide
4000km
Figure 9.10
Satellite-to-ground quantum cryptography using entangled photons.
as in the ARTEMIS-SPOT4 classical optical communications experiment [14].
Such a satellite would be able to relay instantaneously keys between any
two points on a hemisphere, and fixed pointing ground stations could be
used. However, present technology on ARTEMIS produces a 7 µR divergence
beam at a distance of 35,000 km. This then produces a ground spot 250 m in
diameter on Earth. Even with a 2.5 m diameter telescope, diffraction losses
will exceed 40 dB. Losses in a GEO-to-LEO link are much higher, as LEO
launched telescopes would be limited in size to 25-30 cm in diameter.
9.5
Experimental Feasibility of Key
Exchange to Space
9.5.1 Link Budgets for the Various Systems
Losses arise from various sources:
Diffraction spreading of the beams
Beam wandering due to atmospheric turbulence
Pointing wander
Atmospheric absorption (for ground-to-satellite experiments)
Lumped receiver efficiency
Diffraction losses are set by the dimensions of the output telescope and by
the standard equations of Gaussian optics. The divergence half-angle (1/
e
2
)
for Gaussian beams with 1/
e
2
radius
W
0
is given by
λ
π.
θ
=
(9.3)
W
0
