IEEE - Aerospace and Electronic Systems - March 2021 - 31

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of the adversary. QKD attains this goal by the careful
utilization of a quantum communication channel, in
addition to several classical postprocessing methods.
Indeed, QKD protocols operate, necessarily, through
careful mixture of new quantum communication methods, and already established classical cryptographic
primitives.
Originally developed in 1984 by Bennett and Brassard [3] (though quantum cryptography was actually first
proposed in the 1970s by Wiesner [4]), and also independently in 1991 by Ekert [5], QKD protocol design and
analyses has flourished as a field yielding numerous protocols, security analyses, and practical implementation
methodologies. However, despite this, all QKD protocols
follow the same general methodology involving a quantum communication stage (QCS) followed by a classical
post processing stage to yield the secret key.
At its core, a QKD system, which allows a communication network to establish a secure secret key between
two parties A and B, consists of a quantum communication channel along with a classical communication channel. One of the fascinating, and from a cryptographic
perspective, useful, properties of quantum communication
is that any attempt by a party (say an adversary) to gain
information on the data sent through a quantum channel,
necessarily causes a disturbance which may be detected
by honest parties. The more information an adversary
attempts to gain, the more noise in the channel she will
create. Unlike in classical communication, with quantum
communication, there is a direct correlation between
observed quantum channel noise, and adversarial information gain. Furthermore, any attack on a quantum channel
cannot be performed passively; instead it must be active
as one cannot copy quantum bits with certainty due to the
" no-cloning theorem " [6].
Finally, unlike quantum computers which are difficult
to implement in practice due, in large part, to the requirement that many hundreds or thousands of quantum bits
must interact with one another, QKD is much easier to
implement as it requires only single qubits sent. Due to
this, there are many experimental and commercial applications of QKD. This also includes satellite communication [7] and mobile freespace QKD, even between a
MARCH 2021

ground station and an airplane in flight [8]. These are just
a few of the numerous instances of QKD put into practice
recently which we will review in this work.
This survey will analyze QKD technology, both in theory and in practice. Unlike other surveys of QKD, we
focus primarily on giving a high-level overview of quantum communication for readers not familiar with quantum
physics or information theory. We also focus on practical
applications of QKD technology. We will begin with an
introduction to the basic resources necessary for QKD to
operate. We will investigate, in detail, source and receiver
methods along with various methods of encoding data in
practice. We will also discuss free space and fiber communication. A discussion on classical-quantum network
architectures and protocols is also covered. Finally, we
cover current experimental and commercial efforts in
QKD technology along with current telecom company
investments. This article aims to be a high-level overview
of QKD technology; for technical details, we provide
references throughout which provide more in-depth overview of individual aspects of the topic covered. For more
detailed, technical, surveys of QKD, the reader is referred
to [9]-[12] while for a general introduction to quantum
communication, the reader is referred to [13].

COMMUNICATION RESOURCES
At a high-level, QKD protocols require two communication channels: a quantum communication channel and an
authenticated classical channel.
Quantum communication channel: allows quantum
information to be sent from Alice (A) to Bob (B). Practical quantum channels are often photon channels, however,
even here there are different encoding methods (and may
also lead to higher dimensional quantum systems depending on the encoding used). A qubit is simply a mathematical abstraction for a two-level quantum system; physically
a qubit may be implemented using photons in a variety of
encoding manners. Common methods including a photon's polarization, phase encoding, or time-bin encoding.
We will discuss these methods in detail in the " Quantum
Key Infrastructure " section.

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IEEE - Aerospace and Electronic Systems - March 2021

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