Cryptography Reference
In-Depth Information
ALICE
BOB
classical
communication
010011
secret
key
110010
secret
key
entangled pair
EPR-source
Figure 9.8
Schematic entangled pair key exchange system. Alice and Bob measure
the arriving photons on 0 or 45 degree bases using receivers described in Figure 9.4.
Keeping only those coincident detections measured on the same basis, they are able
to establish identical keys.
coincidence rates of 2.10
4
per second were measured in a laboratory environ-
ment using only 18 mW of diode laser to pump a BBO nonlinear crystal. This
led to long-range coincidence rates of 20 per second (or a raw key rate of 20
bits per second). Recently a full key exchange has been implemented over
several kilometers [22].
In the next generation of portable equipments, pair brightness could be
increased by at least an order of magnitude so that similar key rates to faint
pulse systems could be achieved.
9.4
Space Applications
9.4.1 Key Upload to Satellites
Satellites storing and distributing large amounts of data such as digital TV
transmitters and Earth observation satellites need to scramble data before
transmission to licensed users on the ground. The scrambling and unscram-
bling process is done using symmetric key encoding data at the satellite and
decoding them at the ground. New keys are thus required regularly to ensure
the security of the transmissions. Key upload to satellites is thus a possible
first application of quantum communications in space. The feasibility of key
exchange to low Earth orbit is now being studied [15, 16]. The system will
have to work between a range of 600 and 2000 km with fully automated ac-
quisition and maintenance of the link during the satellite pass. Optical losses
(see Section 9.5) in a typical pass could be as low as 20 dB for a close approach,
rising to 35 dB at the longest range, which is around the maximum loss toler-
ance of a faint pulse system. Higher losses could be tolerated if one used true
single-photon sources or accepted the reduced security associated with high
average photon numbers per bit (< 0.5 photons/pulse).
