# Quantum Key Distribution in Kubernetes Clusters
## 2 Background
### Quantum Key Distribution  
QKD is a well-known technique of Quantum Cryptography and allows exchanging keys among parties by leveraging principles of quantum mechanics, such as entanglement and the no-cloning theorem [5]. Broadly speaking, QKD counts two main classes: Discrete Variable QKD (DV-QKD) and Continuous Variable QKD (CV-QKD). Differences depend on the physical aspects of the key exchange process, i.e., the method to encode quantum information.  
Regardless of the specific class and protocol, QKD systems leverage physical devices such as photon sources and detectors and may use specific waveguides (e.g., optical fibre) to transmit quantum information. All those elements have intrinsic vulnerabilities and imperfections that may lead to attacks if not conveniently addressed by QKD protocols. From a high-level perspective, it is also worth noting that QKD schemes require two different channels for communication: a quantum channel and a classical authenticated channel. The first serves as a carrier for quantum information; the second bears additional classical information, which is generally required to carry out QKD protocols. Several of these protocols are available in the literature [5; 6], e.g., BB84, B92, SARG04, E91, BBM92, and some of them are implemented in commercial devices, e.g., BB84, COW, T12 [7]. As a tangible example of a QKD system, ID Quantique [8], a renowned QKD devices manufacturer, recently presented the _Cerberis XG QKD System_[9] with a secret key rate of approximately 2 kbit/s and a maximum quantum channel length of 50 km. Moreover, Toshiba commercialises QKD devices [10] based on a modified version of BB84 (with decoy states and phase encoding) with a range of up to 120 km and an exchange rate of 300 kbit/s (at 10 dB loss). Those products provide a good outlook on commercial devices' state-of-the-art.  
Despite the availability of these commercial devices, QKD represents a vibrant research field, where great effort is dedicated to enhancing current QKD systems in terms of security, distance range, and key exchange rate. Some recent proposals and experimental developments related to the so-called twin-field QKD (TF-QKD) protocol [11] are also worth mentioning. This protocol is based on a measurement-device-independent QKD (MDI-QKD) scheme [12]. The TF-QKD protocol is remarkable because, as the basic MDI-QKD, it has the advantage of removing all detector side-channels but additionally it proposes a novel approach to overcome the rate-distance limit of QKD. This method is relevant for developing future long-distance QKD exchange systems. This paper does not go into the details of QKD devices since we treat QKD as a \"black box\". We assume that Point-to-Point (PTP) key exchange between QKD devices is feasible leveraging commercial devices, and it comes with some limitations regarding endpoints distance and key rate. For the rest of the paper, we rely on the concept of the _QKD network_, a network where nodes leverage QKD devices capable of exchanging keys with their direct neighbours. In such a network, long-distance exchanges are possible by means of trusted repeaters (Section 3.2).