Cryptography Reference
In-Depth Information
intensities to ensure a sufficient overlap of the signal states. Our scheme can
be viewed as the 4
2 protocol but with homodyne detection as the receiver's
measurement system and with postselection of data to enhance the security
beyond the 3-dB loss limit.
+
5.5 Sub Shot Noise Polarization
Measurement
To detect reliably the polarization with high speed and good efficiency, a
homodyne setup as in Figure 5.2 is used. For the measurement of the S 3 pa-
rameter, the incoming light polarization is rotated by the appropriate 2 and
4 retarders. A subsequent lens is used to focus the incoming beam on the two
photodiodes of the homodyne detector. A high-quality calcite Wollaston po-
larizer is used to separate the two orthogonal polarization components. It pro-
duces a contrast of better than 10 6 . Each of two resulting beams is reflected by a
mirror onto the photodetector diode. The balanced detection systems use two
silicon PIN photodiodes (Hamamatsu S3883), which have a large active area
(1 mm diameter), low dark noise, fast response, and a high quantum efficiency
(
9 electrons per photon at 810 nm wavelength). The photocurrents are sub-
tracted directly before the net current is converted into a symmetric voltage by
a Philips NE5211 transimpedance amplifier, which has a high transimpedance
of 28 k
>
0
.
and low dark noise. The low dark noise of diodes and electronics
is important to resolve the low quantum noise of the polarization signal.
Figure 5.5 shows the electronic noise of the detector compared to the signal
noise at an input power of 250
µ
W. At the sampling frequency of Bob'sre-
ceiver, 100 kHz, the electronic noise is more than 10 dB below the signal noise.
Frequency in kHz
Figure 5.5
Comparision of detector dark noise (lower trace) and detector signal for
µ
500
W input power (upper trace).
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