Cryptography Reference
In-Depth Information
be between 14 and 16 dB in clear weather with total lumped losses of 28 to
32 dB. Present experiments produce around 20,000 detected pairs per second
in the laboratory using compact diode lasers with 18 mW of power. Future
systems will easily achieve an order-of-magnitude improvement leading to
ground-based coincidence rates
200 per second with an effective raw key
rate of 100 per second. Errors due to background count rates up to 10,000 per
second are negligible because the background coincidence rate is around 10
-
1
per second if gates of 1 ns can be used. The pointing requirements for such
an experiment are quite challenging. Two separate closed loop pointing and
tracking systems are required with an accuracy of better than 4 µR. Down to
7 µR diffraction and beam wander is the state of the art in GEO-to-LEO clas-
sical communications experiments [14]. However, the mass of typical optical
pointing and tracking terminals (
∼
25 kg) mean that minimum experiment
plus satellite mass would be in the 100 to 200 kg range.
∼
9.6 Conclusions
As an important step toward satellite-based quantum cryptography we have
demonstrated a secure key exchange over a free-space distance of 23.4 km.
Operation down to 0.08 photons per bit has been demonstrated with optical
losses of about 18 dB. A large fraction of errors arose from background counts
but was still below 6%. Improved performance including daylight operation
is expected with improved spatial filtering at the receiver and a narrow band-
pass filter set to the correct wavelength together with accurate temperature
control of the transmitter lasers. The apparatus showed high stability with
the ambient temperatures in these experiments ranging from
25
◦
C.
The polarization preparation and analysis modules developed in this work
were stable and required no adjustments over the whole temperature range.
In fact this was quite a relief, as system alignment is not a very pleasant task
at 4:00
A
.
M
.,
5
◦
Cto
+
−
20
◦
C, and 2960 m altitude. Extension of entangled state key ex-
change experiments from the laboratory out to similar multikilometer ranges
is already underway.
We have looked at space applications of quantum key distribution. We
identified two future experiments/applications that could be achieved in the
coming years. The first is global key distribution using satellite-to-ground
faint pulse quantum cryptography (Figure 9.4) with secure bit rates greater
than 1000 per second. The second is simultaneous key generation between
two ground stations using entangled state quantum cryptography (Figure 9.5)
with key generation at bit rates greater than 100 per second.
Obviously the nearest to implementation is the faint pulse quantum cryp-
tography scheme, since a low-cost (but still greater than $20 million) mi-
crosatellite experiment is possible. A key problem yet to be solved is the
extreme pointing and tracking requirements, although these are being ad-
dressed in classical optical communications experiments. Further work on
brighter and lighter sources of pair photons will bring entangled state sys-
tems to space readiness. Similar improvements to sources will be required
−
