Efficient generation of photonic entanglement and multiparty quantum communication

Entangled photons are at the heart of experimental quantum physics.
They were used for the first fundamental tests of quantum theory,
and became a basic building block for many novel quantum protocols,
such as quantum cryptography, dense coding or teleportation.
Therefore, the efficient generation of entangled photons, as well
as their distribution and accurate analysis are of paramount
importance, particularly with regard to the practicability of many
applications of quantum communication. This thesis deals largely
with the problem of efficient generation of photonic entanglement
with the principal aim of developing a bright source of
polarization-entangled photon pairs, which meets the requirements
for reliable and economic operation of quantum communication
prototypes and demonstrators. Our approach uses a correlated
photon-pair emission in nonlinear process of spontaneous parametric
down-conversion pumped by light coming from a compact and cheap
blue laser diode. Two alternative source configurations are
examined within the thesis. The first makes use of a well
established concept of degenerate non-collinear emission from a
single type-II nonlinear crystal and the second relies on a novel
method where the emissions from two adjacent type-I phase-matched
nonlinear crystals operated in collinear non-degenerate regime are
coherently overlapped. The latter approach showed to be more
effective, yielding a total detected rate of almost 10^6 pairs/s at
>98 % quantum interference visibility of polarization
correlations. This performance, together with the almost free of
alignment operation of the system, suggest that it is an especially
promising candidate for many future practical applications,
including quantum cryptography, detector calibration or use in
undergraduate lab courses. The second issue addressed within the
thesis is the simplification and practical implementation of
quantum-assisted solutions to multiparty communication tasks. While
the recent rapid progress in the development of bright entangled
photon-pair sources has been followed with ample experimental
reports on two-party quantum communication tasks, the practical
implementations of tasks for more than two parties have been held
back, so far. This is mainly due to the requirement of multiparty
entangled states, which are very difficult to be produced with
current methods and moreover suffer from a high noise. We show that
entanglement is not the only non-classical resource endowing the
quantum multiparty information processing its power. Instead, only
the sequential communication and transformation of a single qubit
can be sufficient to accomplish certain tasks. This we prove for
two distinct communication tasks, secret sharing and communication
complexity. Whereas the goal of the first is to split a
cryptographic key among several parties in a way that its
reconstruction requires their collaboration, the latter aims at
reducing the amount of communication during distributed
computational tasks. Importantly, our qubit-assisted solutions to
the problems are feasible with state-of-the-art technology. This we
clearly demonstrate in the laboratory implementation for 6 and 5
parties, respectively, which is to the best of our knowledge the
highest number of actively performing parties in a quantum protocol
ever implemented. Thus, by successfully solving and implementing a
cryptographic task as well as a task originating in computer
science, we clearly illustrate the potential to introduce
multiparty communication problems into real life.