Cryptography Reference
In-Depth Information
Figure 3.9
Sketch of the experimental setup from [71]. Our entangled state source
produces polarization-entangled photon pairs. One of the photons is locally analyzed
in Alice's detection module, while the other is sent over a 1.45 km long single-mode
optical fiber (SMF) to the remote site (Bob). Polarization measurement is done in one
of two bases (0
◦
and 45
◦
) by using a beam splitter (BS) that randomly sends incident
photons to one of two polarizing beamsplitters (PBS). One of the PBS is defined for
measurement in the 0 basis, and the other in the 45 basis as the half-wave plate (HWP)
rotates the polarization by 45
◦
. The final detection of the photons is done in passively
quenched silicon avalanche photodiodes (APD). When a photon is detected in one
of Alice's four APDs, an optical trigger pulse is created (sync. laser) and sent over
a second fiber to Bob to establish a common time basis. At both sites, this trigger
pulses and the detection events from the APDs are fed into a dedicated quantum key
generation (QKG) device for further processing.
applied in a real-world scenario in April 2004. This was the first time that
a quantum cryptography system was used for the encryption of an Internet
bank transfer [71]. The system was installed at the headquarters of a large
bank (Alice) and at the Vienna City Hall (Bob), and a key was distributed
over the 1.45 km optical single-mode fiber connecting the parties.
The quantum cryptography system (Figure 3.9) consists of a port-
able source for polarization-entangled photons (Figure 3.10) and two sets
of fourfold single-photon detection modules with integrated polarization an-
alyzers and embedded hardware devices that are capable of handling the
complete software protocol needed to extract a secure and private key out
of raw detection events. The quantum channel between Alice and Bob con-
sisted of an optical fiber that has been installed between the two experimental
sites in the Vienna sewage system. The classical protocol in that experiment is
performed via a standard TCP/IP connection. The exposure of the fibers to re-
alistic environmental conditions, such as stress and strain during installation
as well as temperature changes, was an important feature of this experiment;
the successful operation of the system shows that laboratory conditions are
not necessary for its operation.
