We present a new concept for birefringence compensation in Sagnac single mode fiber (SMF) interferometer,
which has opened the door to secure multiuser quantum communications. The concept has been tested in both
quantum key distribution (QKD) and secret sharing applications at total fiber loop lengths up to 150 km. Now,
we present a Jones matrix based model of the concept as well as the visibility measurements confirming its
practical validity.
Sagnac based circular SMF network architecture is an attractive solution for both commercial telecom Intranets
and backbone networks as well as for secure quantum information channels exploring QKD and secret
sharing applications. Due to the difference in birefringence between the clockwise and counterclockwise transmission
directions, Sagnac has not been so much explored by the quantum information community as other network
architectures, especially the "plug & play".
Our SMF Sagnac birefringence compensation uses polarization maintaining, horizontally aligned components
only, except for the SMF fiber link. It converts the arbitrary elliptically polarized clockwise and counterclockwise
light signals, transmitted over the SMF fiber, into the horizontal polarization. Thus, both the sent and
received light signals have the same horizontal polarization. In order to avoid unnecessary power losses in the
birefringence compensator, we have included a simple proportional-integral-derivative controller into our setup.
The compensator provides a perfect birefringence compensation for the destructive interference, which leads to
a high system visibility. The presented SMF Sagnac birefringence compensation could also be use in Sagnac
metrology, for instance in very low power level sensing, requiring high measurement accuracy.
We present the first Sagnac quantum secret sharing (in three- and four-party implementations) as well as Sagnac
two-user quantum key distribution (QKD) over 1550 nm single mode fiber (SMF) networks, using the BB84-
protocol with phase encoding. The secret sharing experiment has implemented a single qubit protocol, which
allows for a practical secret sharing implementation over fiber telecom channels and in free-space. The previous
quantum secret sharing proposals were based on multiparticle entangled states, not scalable and diffcult in the
experimental implementations. Our experimental data show stable, in regards to birefringence drift, quantum
secret sharing transmissions at the total Sagnac transmission loop distances of 45-55 km with the quantum bit
error rates (QBER) of 3.0-3.7 % for the mean photon number μ = 0.1. In the QKD experiment we have achieved
the total Sagnac transmission loop distances of 100-150 km with quantum bit error rates (QBER) of 5.84-9.79
% for μ = 0.1. The distances were much longer and rates much higher than in any other published Sagnac
QKD experiments. The stability of quantum transmission in both secret sharing and QKD experiments has
been achieved thanks to our new concept for compensation of SMF birefringence effects in Sagnac, based on
a polarization control system and a polarization insensitive phase modulator. The measurement results have
showed feasibility of quantum secret sharing and QKD over telecom fiber networks in Sagnac confi;guration,
using standard fiber telecom components. Our birefringence compensation in SMF Sagnac open the door to
other Sagnac-based applications over SMF links such as precise optical sensing, dispersion characteristics of
optical fibers, acoustic and strain sensing, and generally sensing of any time varying phenomenon.
Unconditionally secure multiuser quantum key distribution (QKD) over long distance switched telecom fiber networks
will have a revolutionary impact on people's, companies', and governmental exchange of confidential information. We
report five-user QKD over switched fiber networks in both star and tree configurations, using the BB84-protocol with
phase encoding. Both setups implement polarization insensitive phase modulators, necessary for birefringent single
mode fiber networks. In both configurations, for the mean photon number μ=0.1, we have achieved raw rates between
50Hz and 300Hz at distances between 25km to 50km with quantum bit error rates between 1.24% to 5.56%. The
measurements have showed feasibility of multiuser QKD over switched telecom fiber networks, using standard fiber
telecom components.
Multiphoton entanglement is the basis of many quantum
communication schemes, quantum cryptographic protocols, and
fundamental tests of quantum theory. Spontaneous parametric
down-conversion is the most effective source for polarization
entangled photon pairs. Here we show, that a entangled 4-photon
state can be directly created by parametric down-conversion. This
state exhibit perfect quantum correlations and a high robustness
of entanglement against photon loss. We have used this state for
four-particle test of local realistic theories. Therefore this
state can be used for new types of quantum communication. We also
report on possibilities for the experimentally realization of a
3-photon entangled state, the so called W-state, and discuss some
of its properties.
We discuss single-photon counting technologies, in particular at the optical wavelength of 1550 nm and their application in quantum communication. In particular we discuss the case of quantum cryptography illustrating with experiments performed in our lab, supplemented by recent experiments by other groups. We furthermore discuss briefly more general protocols for quantum communication and address some experimental changes.
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