Cryptography Reference
In-Depth Information
obtained after the swapping process was high enough to infer a violation of
Bell inequalities between the remaining photons.
2.5.4 Conclusions and Outlook
To summarize this review, we presented experimental and theoretical achieve-
ments of GAP Optique in the domains of fundamental research and quantum
communication. In contrast to work done in other groups, our experiments
take advantage of photons at telecommunication wavelengths, standard op-
tical fibers and time-bin qubits, enabling implementations over distances be-
tween a few kilometers and several tens of kilometers, depending on the ap-
plication. All work is closely connected to quantum key distribution and can
be summarized under the common aspect of increasing the performance of
quantum key distribution in spite of the lack of true single photon sources, of
lossy quantum channels and imperfect detectors. Starting with an auto align-
ing plug & play system for quantum cryptography based on faint laser pulses,
we turned to experiments with pairs of time-bin entangled photons that ren-
der eavesdropping attacks based on photon number splitting ineffective, and
that make it possible (in principle) to increase the maximum transmission
span as engendered by the probabilistic arrival of photons at Bob's receiver
station. Extending our work to three photons and pursuing the idea to reduce
the effect of loss in the quantum channel, we then demonstrated quantum
teleportation in a quantum relay configuration. Finally, adding a fourth pho-
ton, we could recently demonstrate the fundamental concept of entanglement
swapping (or teleportation of entanglement), again in the spirit of a quantum
relay. In parallel to these experimental issues, we proposed a new protocol
that also makes it possible to attenuate the effect of photon number splitting
attacks, and we pointed out a general link between quantum entanglement
and classical information.
Quantum key distribution is now commercialized by different compa-
nies. In the short term, improvements should mainly be related to detectors:
this could considerably increase the actual key creation rates and sightly in-
crease the range. However, if we want to be able to implement QKD on scales
of several hundreds of kilometers, we need quantum repeaters. Therefore,
mid- and long-term research efforts are focused on long-distance entangle-
ment swapping and quantum memories.
The future of quantum communication appears bright with still plenty
of opportunities for both experimental and theoretical physicists addicted to
conceptual issues and for those who are more engineering oriented.
Acknowledgments
Financial support by the Swiss OFES within the European projects RESQ
and RAMBOQ and by the Swiss NCCR
Quantum Photonics
is acknowledged.
We thank all collaborators from GAP Optique for their contributions to the
various experimental and theoretical investigations mentioned in this review.
