Cryptography Reference
In-Depth Information
averaged
detection events
per block
200
150
100
50
0
0°
0°
45°
45°
transmitter
diode
90°
90°
receiver
diode
135°
135°
Figure 9.7
The detector matrix showing, on the diagonal, the number of bits trans-
mitted and measured on the same basis. The suppressed measurements are the errors,
bits encoded and measured on the same basis, but turning up in orthogonal channels
(e.g., 0
◦
and 90
◦
). Measurements where different bases were used show roughly half
the height of the diagonal, but constitute roughly half of all measurements.
keys because of the need to estimate the error rate sacrificing some of the key
before correction.
Sifted and corrected keys showed a high degree of randomness with little
or no bias. An online check of the balance of received bits compared to sent bits
could be performed. A typical matrix of sent versus measured polarizations
is shown in Figure 9.7. Care was taken to keep the diagonal values as close as
possible.
9.3.3 Entangled State Key Exchange
Entangled state quantum cryptography is schematically illustrated in
Figure 9.8. A source of (polarization) entangled photon pairs is configured
to send one photon to Alice and one photon to Bob. Using receiver units
identical to the one used in faint pulse cryptography (Figure 9.4) and se-
lecting coincident detections measured on the same polarization basis, they
establish identical random strings to use as cryptographic keys. There is no
encoding of a random number to form the basis of the key, as the randomness
comes from the superposition in the entangled state. This gives key security
advantages over the faint pulse systems.
The first attempts at free-space entangled-state distribution have already
begun. In [21], strong polarization correlations at 600 m range (across the
river Danube) were demonstrated. These experiments could be easily ex-
tended to demonstrate entangled state key exchange [20]. As small telescopes
were used, the lumped optical losses were in the 30 dB region. Pair photon
