Quantum Key Distribution, or QKD, is one kind of quantum cryptography protocol. It harnesses the fundamental uncertainty of quantum particles such as photons to encode information into them to produce a secure key. This can be a tricky subject to explain, so let’s introduce Alice and Bob to help. Let us say that Alice and Bob want to share a secret key between themselves without an eavesdropper, Eve, knowing. Using QKD, Alice prepares her photons in certain quantum states and then sends them to Bob. Diagrammatic representation of QKD taken from here. A fundamental tenet of quantum mechanics is that any kind of measurement alters the quantum state of a particle. And thus, if Eve tries to measure the quantum state of the photons Alice has sent, she would inevitably alter them, thus making her presence known. If she tries to make a copy of them without altering them, she would find that she is blocked by something known as a quantum “No cloning theorem” if Alice and Bob have used the right protocol. And thus, there you have it, a secure means of setting a secret key between two parties! Once this secret key or an OTP (one time pad) is set, this can be used by Alice and Bob to securely and secretly communicate without the fear of any third party overhearing them or stealing their information. There are various kinds of QKD protocols. Some protocols require single photons to be transmitted between Alice and Bob (Single Photon QKD). But producing single-photon states is hard, and thus, many times, weak-coherent laser pulses are used in their stead (Weak-Coherent Laser Pulse QKD). A second kind of QKD requires the use of an entangled photon source which sends pairs of entangled photons to both Alice and Bob (Entangled Photon QKD). There are various missions planned around the world currently in order to demonstrate QKD in space. In the UK, a consortium comprised of Craft Prospect, the University of Strathclyde and the University of Bristol are working on the ROKS mission which will demonstrate satellite to earth Weak Coherent Laser Pulse QKD.