Cryptography Reference
In-Depth Information
Figure 2.5
Self-aligned plug & play system (L: laser diode, APD: avalanche pho-
todiode, BS: beam splitter, C: circulator, PM
j
: phase modulator, PBS: polarizing
beam splitter, DL: optical delay line, FM: Faraday mirror, D: classical detector).
well as the latest free space experiments over more than 20 km [24]. For long
distance QKD through optical fibers, however, polarization is probably not
the appropriate choice. Polarization is not maintained in optical fibers [87].
Therefore, polarization-based QKD through a long optical fiber link asks for a
permanent compensation for the rapidly changing evolution of the polariza-
tion state in the fiber. In contrast, as seen in Section 2.2, any arbitrary time-bin
qubit can be created and measured using interferometers. Indeed, one of
the first QKD experiments in optical fibers used essentially the setup shown
in Figure 2.3 [25,88]. In such a setup, one difficulty is to stabilize the arm
length difference in both interferometers down to a fraction of a wavelength
during the whole key exchange. However, by folding the interferometer of
Figure 2.3 and sending light pulses back and forth, it is possible to design
an auto-compensating interferometer [4,26]. Since no continuous alignment
is required this design is also called plug & play configuration.
Let us have a closer look at the plug & play setup (see Figure 2.5) [27]. A
strong laser pulse (@1550 nm) emitted at Bob's (L) is separated at a first 50/50
beam splitter (BS). The two pulses impinge on the input ports of a polarization
beam splitter (PBS) after having traveled through a short arm and a long arm,
including a phase modulator (PM
B
) and a 50 ns delay line (DL), respectively.
All fibers and optical elements at Bob's are polarization maintaining. The lin-
ear polarization is turned by 90 degrees in the short arm, so the two pulses
exit Bob's setup by the same port of the PBS. The pulses travel down to Alice,
are reflected on a Faraday mirror (FM), are attenuated well below an average
photon number per pulse of 1 (VA), and come back orthogonally polarized
[89]. In turn, both pulses now take the other path at Bob's and arrive at the
same time at the BS, where they interfere. Since the two pulses take the same
path, inside Bob's apparatus in reversed order, this interferometer is autocom-
pensated. Finally, the photon is detected either in D1 or after passing through
