Cryptography Reference
In-Depth Information
still possible to select secret information from the measurements. As the post-
selection is done by Bob alone, no data has to be transmitted over the public
channel during this process. Only after Bob has chosen his events with high
information advantage does he reveal their measurement basis to Alice. Thus
the effort in authentication and the bandwidth requirements on the public
channel decrease substantially.
5.10 Conclusions and Outlook
We have presented an experimental quantum cryptography that uses coher-
ent states, homodyne detection, and postselection to generate a shared key
between Alice and Bob. The states are polarization encoded, ensuring fast
modulation compared to quadrature encoding and giving a perfect mode
overlap in Bob's homodyne detector. There is no need to send a separate local
oscillator along with the quantum channel. The system is robust against losses
of more than 50% of the states and does not need special reconciliation tech-
niques. Its implementation is very simple, and it could be used in free-space
communication with high efficiency and speed. When using a polarization
dispersion compensation, the scheme can also be adapted to fiber transmis-
sion lines by exchanging the 810 nm coherent laser source with a 1.5 µm laser.
Further improvements could be made with the receiver design. First, the
homodyne detection setup of Figure 5.11 can be modified. If one uses the
setup described in Section 5.8, used to generate Figure 5.4 (see Figure 5.12),
Bob's detector would not require active basis switching, as in some single-
photon systems [34]. The theoretical upper bound for detection speed is then
the bandwidth of the balanced detector, which can be very high compared
to photon counters in single-photon experiments. Second, the photon count-
ing and homodyne systems are not the only possible detection systems. It is
worthwhile to look for a system that might be more capable of discriminating
nonorthogonal quantum states preserving the advantages of the homodyne
technique. For example, one might think about implementation of a modifi-
cation of the so-called Kennedy receiver [35].
The most important issue is the security of the scheme. Extension of the
security analysis to more general kinds of eavesdropping attacks is required.
An interesting issue here is the role of squeezing and entanglement in the
protection of the system against eavesdroppers. These issues were recently
considered by several authors [13,28,29,36].
Acknowledgments
This work was supported by the German Association of Engineers (VDI) and
the Federal Ministry of Education and Research (BMBF) under FKZ: 13N8016.
The authors would like to thank Ch. Silberhorn for discussions related to
postselection procedure and Ulrik Andresen, Jessica Schneider, and Andreas
Berger for technical assistance.
