The Bridge - February 2018 - 22

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Upon receiving the encrypted message from Alice,
Bob can perform another bitwise modulo-2 sum
using the shared secret key to decrypt the message:
There are additional error correction and privacy
amplification protocols that Alice and Bob can
perform to increase the secrecy and accuracy of their
secret key, at the expense of reducing the number
of bits in the key. The majority of those protocols
are classical, and take place after the transmission of
information through the quantum channel [1].
Key distribution via QKD has been proved
theoretically secure against all attacks due to the
probabilistic nature of quantum measurement. Any
attempts by an eavesdropper to secretly measure
the quantum bits mid-transmission will irrevocably
alter those quantum states such that Alice and Bob
will be aware that their quantum bits have been
tampered with by measuring the QBER. Even with a
quantum computer an eavesdropper cannot nondestructively measure or clone quantum states due
to the quantum no-cloning theorem [17].

4 QKD IMPLEMENTATION
In addition to the well-known BB84 protocol,
researchers have developed a number of alternative
protocols. Some of them are variations of BB84,
with differences in procedures and states used [1].
For instance, the E91 protocol, introduced by Arthur
Ekert in 1991, involves creating entangled pairs of
photons (Einstein-Podolsky-Rosen states) at a central
source and sending each photon to Alice and Bob
for measurement [19].

THE BRIDGE

Due to the variety of available QKD protocols and
two-state quantum systems, there are countless
possible approaches to implementing QKD. Each
has its own unique design corresponding to the
requirements of the protocol and the use case, and
comes with its own strengths and weaknesses in
terms of QBER tolerance thresholds, key generation
rate, feasibility, and other factors. For instance, while
in the previous chapter we considered a general
polarization-based implementation of BB84 using
optical fibers, it is also possible to implement BB84
using photon phase and frequency encoding [1].
Additionally, one is not restricted to the use of optical
fibers for qubit transmission; free-space QKD where
photons are transmitted kilometers through the
atmosphere along a shared line of sight have been
demonstrated [20]. Commericial QKD systems are
currently available for purchase, with many field tests
conducted in recent years proving their feasibility
[22, 23].
4.1 Exploiting Implementation Flaws
In the ideal QKD implementation, guaranteed
security against eavesdroppers relies on the
assumption that only idealized equipment is used
where every component functions precisely as
expected. However, real-world implementations tend
to be far from ideal due to a variety of technical and
engineering challenges. Such implementation flaws
could significantly threaten the security of QKD as
they offer side channel exploitation opportunities for
Eve to increase her eavesdropping abilities without
detection [1].
An example of a side channel attack is the
backflash leakage attack, which exploits the physical
implementation of a typical BB84 fiber-based system
with polarization encoding . In such a system, the
quantum state of photons transmitted by Alice is
typically measured by Bob by routing the photons
through a series of beamsplitters (non-polarizing
and polarizing) and additional optics such that a
photon in one of the four polarization states used



Table of Contents for the Digital Edition of The Bridge - February 2018

Contents
The Bridge - February 2018 - Cover1
The Bridge - February 2018 - Cover2
The Bridge - February 2018 - Contents
The Bridge - February 2018 - 4
The Bridge - February 2018 - 5
The Bridge - February 2018 - 6
The Bridge - February 2018 - 7
The Bridge - February 2018 - 8
The Bridge - February 2018 - 9
The Bridge - February 2018 - 10
The Bridge - February 2018 - 11
The Bridge - February 2018 - 12
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The Bridge - February 2018 - 17
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The Bridge - February 2018 - 20
The Bridge - February 2018 - 21
The Bridge - February 2018 - 22
The Bridge - February 2018 - 23
The Bridge - February 2018 - 24
The Bridge - February 2018 - 25
The Bridge - February 2018 - 26
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The Bridge - February 2018 - 28
The Bridge - February 2018 - 29
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The Bridge - February 2018 - 31
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