Integrated Quantum Key Distribution sender unit for hand-held platforms

Mastering the generation, propagation and detection of
electro-magnetic waves has enabled a technological breakthrough
that has changed our entire society. World-wide communication
through the telephone and the internet has become an integral part
of our daily-life, which is expected to grow even further with the
emergence of the internet of things. While secure communication was
of concern mostly for governmental and financial institutions,
digital security has now caught the attention of the general
public. The weaknesses of cur- rent encryption protocols, such as
the existence of back-doors or the predicted breakdown of popular
algorithms such as RSA, reveal the need for alternative encryption
schemes ensuring unconditional security on all types of devices.
Quantum Key Distribution (QKD) has emerged as a powerful option to
ensure a private communication between two users. Based on the laws
of quantum mechanics, this class of protocols offers the
possibility to detect the presence of a third party trying to
intercept the key during its distribution, and even to quantify the
amount of leaked information. While most research projects focus on
long distance applications, little attention has been devoted to
short distance schemes such as wireless payment, network access and
authentication, which could highly benefit from QKD-enhanced
security. This thesis focuses on the development of a miniature QKD
sender add-on that could be embedded either in mobile devices or in
existing optical communication platforms, thus allowing for a
secure key exchange with a shared dedicated receiver over a free-
space link. The proposed optics architecture (35 × 20 × 8 mm 3 ) is
optimised for BB84-like protocols and uses an array of four
Vertical-Cavity Surface-Emitting Lasers with highly similar
properties to generate 40 ps long near-infrared faint coherent
pulses at 100 MHz repetition rate. Under strong modulation, the
polarisation of the pulses is not well defined and enables an
external control of each diode’s emission by a wire-grid polariser.
The four beams are spatially overlapped in a
polarisation-insensitive femtosecond laser written waveguide array,
and combined with a red beacon laser using an external beamsplitter
to ensure a stable, synchronised optical link with the receiver.
The complete module is compatible with current smartphone
technology, allowing to run the classical post-processing over WLAN
in the future. First tests with a free-space receiver indicate an
average error ratio of 3.3 % and an asymptotic secure key rate of
54 kHz under static alignment. For the first time, a secure key
exchange between a mobile platform held by a user and a receiver
equipped with a dynamic alignment system could be demonstrated with
an error ratio of 4.1 % and a secure key rate of 31 Hz. The further
optimisation of the experimental parameters and the implementation
of a decoy protocol will enhance the key generation rate as well as
the general security of the system. The results of this thesis pave
the way towards unprecedented security in wireless optical
networks, as examplified for the communication between a mobile
device and a dedicated receiver.