Cryptography Reference
In-Depth Information
an eavesdropper due to the possibility of the light field containing more than
one photon is small [25].
A significant reduction in system complexity compared to former ap-
proaches was achieved by using four independent sources for the individual
polarizations rather than optical elements selecting or preparing the polariza-
tions as in earlier implementations of the BB84 protocol [7]. We have chosen
laser diodes as light sources, because of the technological simplicity in achiev-
ing switching times of fractions of a nanosecond.
Another reduction of the necessary optical elements is enabled by the fact
that light emitted by laser diodes shows a very high degree of polarization.
Therefore not even passive optical polarization preparation elements like po-
larizers are needed. Together with an appropriate combination scheme for the
light of the four laser diodes, the four necessary polarizations for the BB84
protocol are obtained by geometrical orientation of the laser diodes.
In principle, the combination of the light from the different sources could
be achieved by beam splitters [26], but it turns out that a simple beam overlap
in front of a spatial mode filter is sufficient [27]. A spatial filter is necessary
to ensure that information is only encoded in the polarization degree of free-
dom and that the parasitic channel of emission direction is closed for the
information encoded into the faint pulse.
Another important simplification arises from the fact that the laser diodes
have to be attenuated strongly in order to come down to the 0.1 photon-per-
pulse level on the quantum channel. This can be looked at as a very relaxed
specification of the coupling efficiency into the quantum channel, which in
our case was either a single-mode optical fiber or the spatial mode filter for a
free-space optical link. Therefore, all the coupling into the quantum channel
can be achieved using partial mode overlap between the laser diodes and the
target mode of the optical fiber or the spatial filter.
All of this was elegantly achieved in a novel miniature source of polar-
ization coded faint pulses (Figure 9.1). It consists of four laser diodes (850 nm
wavelength) arranged on a ring around a conical mirror. Each laser is rotated
to produce one of the four polarizations 0
◦
,90
◦
,45
◦
, or 135
◦
and illuminates
a spatial filter consisting of two pinholes with a diameter of 100 µm spaced
at a distance of 9 mm. Since the overlap of the emission modes of the four
laser diodes with the filter mode is rather poor, the initially very bright laser
pulses are attenuated to about the required “one photon per pulse” level. The
actual attenuation can be fine tuned by manipulating the diode current and
precisely calibrated by optionally shining the light transmitting the spatial
filter onto a single-photon detector. The filter erases all spatial information
about which laser diode fired. Spectral information is also not attainable by
an eavesdropper, as the spectra of the four laser diodes well overlap with a
width of about 3 nm in pulsed mode. A continuous wave alignment laser
was also fed through the spatial filter in order to ease optimizing the fo-
cusing of the receiver. The complete optical setup (Figure 9.2) was confined
into an aluminum block of size 35
35 mm to maintain rigid alignment,
demonstrating the ability to integrate the source module, e.g., into a PC-based
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