Cryptography Reference
In-Depth Information
show a linear behavior up to 1 mW incident cw light power. The peak in the fre-
quency response results from the single pole low pass filtering in the detector.
5.6 Alice: Preparation of Polarization States
The experimental apparatus consists of independent setups for Alice and
Bob, which are separated by roughly 30 cm. Alice controls the laser source
and the state preparation. As a light source a commercial diode laser is used
(Figure 5.8). The diode used (TOPTICA DL100) delivers up to 40 mW of power
and is wavelength stabilized by an external grating to 810 nm. The laser is
decoupled from the rest of the experiment by a tunable optical isolator. A
subsequent half-wave plate and polarizing beam splitter combination serves
as a variable optical attenuator. The attenuation of roughly 10 dB is used to
suppress intensity fluctuations induced by the laser power supply. A mode
cleaning of the laser radiation is achieved by coupling the light into a standard
telecom fiber. The light that is coupled out of the fiber is then polarized using
two polarizing beam splitters, increasing the purity of
S
1
polarization. The
overall attenuation of the sender setup was adjusted to give an output power
of 0.5 mW of
S
1
polarized cw light without any modulation (
p
-polarized
light). To induce small
S
2
and
S
3
components in the beam for signal encoding,
two modulators were used.
optical
isolator
4.6mm
PBS
λ
/2
λ
/2
laser
diode
silica fiber
4.6mm
PBS
EOM
MOM
PBS
λ
/2
λ
/2
Figure 5.8
Schematic view of Alice's setup.
