Cryptography Reference
In-Depth Information
devise means to extend the maximum transmission distance in spite of tech-
nical imperfections like lack of single-photon sources, lossy quantum chan-
nels and non-perfect detectors. In detail, we present an auto-aligning “plug
& play” system for quantum key distribution based on faint laser pulses,
two entanglement-based systems, teleportation in quantum relay configura-
tion and finally entanglement swapping. All experiments take advantage of
photons at telecommunication wavelengths and optical fibers, and use time-
bin encoding which enables us to demonstrate the different protocols over
distances of a few kilometers to several tens of kilometers. In addition, we
developed a new protocol for quantum key distribution which also enables
extending the maximum transmission distance in spite of so-called photon
number splitting eavesdropper attacks and non-ideal faint laser pulses in-
stead of true single photons.
2.1 A Geneva-Biased Introduction
Quantum communication is the natural follow-up of classical communication
into the era of quantum technologies. Accordingly, its natural wavelengths are
the same as those used in today's fiber optics communication. However, most
groups opted for shorter wavelengths, because they originate from academic
research in quantum optics, which traditionally uses visible or near-infrared
light. Additionally, until recently, detectors sensitive to single photons existed
only at those shorter wavelengths, roughly below 1µm. In contrast to most
groups, in Geneva we originate from a research group dealing with classical
telecom physics. Hence, when we started our activities in quantum commu-
nication, we decided to go for the telecom wavelengths (see Figure 2.1). This
implied from the very beginning the development of single-photon detectors
at these (at the time) exotic wavelengths. The next step was to choose the
appropriate degree of freedom to encode quantum information. We opted for
a solution that we believe is better adapted to optical fibers though at first
sight less obvious than polarization: the time-bins (see Section 2.2).
Our group is also known for having introduced Faraday mirrors into the
field of quantum communication. It should be stressed that such mirrors have
been invented by Professor M. Martinelli from Milan [1]. In Geneva, we first
used them for a fiber-based sensor [2], a development that turned out to be
much less successful than our activity on quantum cryptography. Actually,
the main outcome of this sensor project has been the idea of using Faraday
mirrors in quantum communication [82]. The nonreciprocal Faraday effect is
also used in many other crucial components in quantum optics experiments:
isolators and circulators are based on this effect.
In the following pages, we review first the concept of time-bins, in Sec-
tion 2.2. Next, pseudo-single photon quantum cryptography is presented in
Section 2.3, followed by two-photon quantum cryptography (which includes
tests of Bell inequalities as a natural child), in Section 2.4. Finally, in Section
2.5.3 we briefly comment on three- and four-photon applications: quantum
teleportation, entanglement swapping and quantum relays.
