1. Introduction
For secure communications, information is encrypted by a sender and decrypted by a recipient using a cryptographic key that is shared only between them [1].
The purpose of quantum key distribution (QKD) is to transmit this key in a way that it cannot be intercepted without the recipient being alerted [1]. This is achieved by encoding the key in a series of quantum bits, called qubits. Due to the no cloning theorem [1], any attempt to read a qubit causes it to be destroyed. This means that the interceptor can no longer recreate that qubit leading to the information in the key being lost and the receiver being alerted. This removes the possibility of eavesdropping in the transmission of the cryptographic key. Then, once this key is shared, it can be used only once, using the one-time pad algorithm, to encrypt information transmitted through a classical channel in a way that is totally secure [1].
The quantum key is typically transmitted via single photons in an optical fiber [1] [2]. The key is created by an optical circuit called the transmitter that encodes it in a series of pulses [2]. Then, the pulses are detected by a second optical circuit, called a receiver, that reads the secret key from the series of pulses.
The use of integrated photonics for QKD enables higher miniaturization and lower fabrication costs compared to conventional bulk optical systems. It has been argued that this would enable widespread adoption of QKD systems [2]. For this reason, a number of systems have been developed that implement the transmitter and receiver in integrated photonic circuits [2] [3].
In this tutorial, we will show how to use Luceda IPKISS to design a receiver for QKD and simulate pulse propagation through it.