Cryptography Reference
In-Depth Information
by sender and receiver. Any remaining differences (errors) signal the inter-
ception of an eavesdropper! If an eavesdropper measures the polarization of
one pulse, that pulse, being a single photon, is destroyed and does not reach
Bob and thus is not incorporated in the key. The eavesdropper could choose
a basis, measure the pulses, and then reinject copies. However, this strategy
has to fail because half the time the eavesdropper will have chosen the wrong
measurement basis and the reinjected pulses will induce an error rate of 25%.
Of course a certain level of error could be caused by imperfections in the
equipment used, but in order to guarantee absolute security any error should
be attributed to (partial) interception. Below a certain threshold the error can
be corrected and potential knowledge of the key by any eavesdropper can be
erased by privacy amplification protocols [23,24].
9.3.1.2 The Tools
Compared to the original experiment using polarization rotations performed
by high-voltage Pockels cells, it is by far advantageous to use separate laser
diodes for every polarization at the transmitter. An additional simplification
of the equipment can be achieved by randomly splitting the light in the re-
ceiver between the analyzers for two bases by a nonpolarizing beam splitter.
This allowed us to design a long-range free-space key exchange apparatus
capable of exchanging keys over free-space ranges greater than 20 km, where
diffraction/turbulence and absorption losses reach up to 20 dB.
Transmitter
The transmitter (Figure 9.1) is designed around an 80-mm-
diameter transmit telescope. A novel miniature source of polarization coded
faint pulses approximating single photons is used. The task of this transmit-
ter module in a BB84 kit is to launch a faint light pulse with one out of 4
linear polarizations into a quantum channel, containing an average photon
number of approximately 0.1. This ensures that the information leakage to
laser diode
to telescope
mirror
relay lens
spatial filter
xy adjustment stage
Figure 9.1
Optical configuration for a free-space transmitter module. The light of
four laser diodes is combined with a mirror; here, only the diode emitting V polarized
light is shown. Spatial indistinguishability is achieved by using a spatial filter. The
module can be attached directly to a telescope.
