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information theory. The basic elements of a QKD are a 3. STANDARDS FOR QKDN IN ITU-T SG13
transmitter (QKD-Tx) and a receiver (QKD-Rx), each of
which is referred to as a QKD module. A QKD link connects ITU-T SG13 started the first initiative on QKDN as a
the QKD modules, potentially with the help of a quantum framework document in July 2018 and Recommendation
relay point (e.g. optical switch) for relaying quantum signals. ITU-T Y.3800 was approved in 2019 as the first
The keys are shared via the QKD link. The QKD link usually Recommendation. So far, ITU-T SG13 has completed the
consists of a quantum channel and a classical channel. The development of six Recommendations and one Supplement
quantum channel for transmitting a quantum signal is as shown in Figure 2. There are some ongoing work items
reserved for quantum signals, such as a single-photon-level (see Table 1) including recently created new work items on
coherent state of light, to transmit random bit strings. The interworking, ML, and resilience, etc. in QKDNs.
classical channel for exchanging data is reserved for
synchronization and data exchange between the QKD
modules.

As shown in Figure 1, the QKD systems are expanded to a
QKDN comprised of two or more QKD nodes connected
through QKD links. A QKDN allows sharing of keys
between the QKD nodes by the key relay function providing
keys between QKD nodes via intermediate QKD node(s)
when they are not directly connected by a QKD link. The
user network is a network in which cryptographic
applications consume keys supplied by a QKDN.

Figure 2 – Core Recommendations on QKDN
developed in ITU-T SG13

3.1 Core Recommendations on QKDN in ITU-T SG13

3.1.1 ITU-T Y.3800 – Overview on networks supporting
quantum key distribution

ITU-T Y.3800 [10] gives an overview of the networks
supporting QKD. It aims to provide support for the design,
deployment, operation, and maintenance for the
implementation of QKDNs, in terms of standardized
technologies, along with the conceptual structures of a
Figure 1 – QKDN concepts and their relation to QKDN and a user network. As shown in Figure 3, the layered
a user network
structure of QKDN consists of a quantum layer, a key
Considering both the communication network and security management layer, a QKDN control layer and a QKDN
service requirements, a QKDN as both a new kind of security management layer. The user network is described by a
solution and a new form of network infrastructure requires a service layer and a user network management layer.
systematic set of standardization work.

Taking the organization of the ITU-T as an example, the
work on QKDN is already ongoing in SG13 on networking
aspects and in SG17 on security aspects. Future work may
involve protocols and signalling for networks, users and
device interconnection (related to SG11), network operation
related specifications for QKDNs (related to SG2),
integration of QKD with classical optical communication
networks (related to SG15) and on QKD applications in data
centre interconnection and computing, Internet of Things,
mobile networks, etc. (related to SG16 and SG20).
Figure 3 – The conceptual structures of a QKDN and a
user network

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