Cryptography Reference
In-Depth Information
and
S
3
were modulated randomly with either positive or negative patterns
simultaneously. Bob's measurement thus has to discriminate between positive
and negative modulation in either of the two polarization bases. To achieve
this, Bob records the detector signal for each event and does a baseline restora-
tion using the start and stop parts of the modulation pattern. The central part
of his measurements then corresponds to Alice's modulation.
5.8 Bob: Measurement Systems
Bob performs the polarization measurements on the states he receives from
Alice (Figures 5.10, 5.11, 5.12). To choose between measurements in
S
2
or
S
3
,
Bob can introduce an EOM in his receiver, described in Section 5.6, switching
between no retardation for
S
2
and quarter-wave retardation for
S
3
measure-
ments (see Figure 5.11). Although the EOM has the advantage of introducing
little extra loss, it is very slow, owing to its large clear aperture, needed for the
free-space transmission line. Thus only low switching speeds (approximately
200 Hz) could be achieved with this type of basis choice.
To circumvent the problem of slow switching speed with the EOM,
one can use the loss tolerance advantage of postselection to build a mea-
surement system without active switching. Instead of choosing a basis, both
the
S
2
and
S
3
parameters are measured simultaneously (Figure 5.12).
The resulting decrease in accuracy due to the quantum penalty in measure-
ment of two conjugate variables must be encountered by postselection. A
polarization-independent beamsplitter is inserted into the beam behind the
300mm
EOM
BS
PBS
λ
/2
λ
/4
balanced
detector
PBS
balanced
detector
Figure 5.10
Schematic view of Bob's setup, including electro-optical modulator and
double detector scheme.
