(Medium) This explanation of Space Based QKD is from Crafts Prospect, a NewSpace R&D company concentrating on  neural networks, quantum encryption.
QKD is a cryptography method which uses the fundamental quantum states of particles such as photons to encrypt information securely. At the core of QKD is the fundamental principle that, generally, quantum states cannot be copied exactly and become altered in the process of attempting to copy them.
This makes QKD more lucrative and advantageous than current classical key services. Unlike the latter, which can be compromised by quantum computers, QKD is provably secure forever from any attacks in the future, thus making it highly desirable.
In theory, this is all well and good, but in practice, the transmission of photons large distances across the Earth faces numerous challenges. Optical fibres, as well as terrestrial atmosphere, lead to loss of photons during transmission. This reduces the long-distance feasibility of QKD and limits QKD to a few hundred kilometres only (max: 400–500 km). This is too low a limit to be useful if we want to establish a worldwide QKD network.
But what if we take the quantum source into space? Say onboard a satellite which is on a low Earth orbit (LEO), typically around 500 km above the Earth.
Photons mostly travel through empty space before they enter the last few hundred kilometres of atmosphere. The nature of space allows negligible photon loss and decoherence which prevents loss of transmission. And voilà! Quantum mechanics and space exploration have combined together to take us forward into a new era of space-enabled quantum technology to improve our lives.

**Crafts Prospects is participating in ROKS — Responsive Operations and Key Services: ROKS will be a proof of concept mission, targeted to be launched by 2021 by Craft Prospect. It will demonstrate satellite-to-Earth Quantum Key Distribution (QKD) for augmentation of future encryption services. ROKS will also carry a responsive operations payload which will demonstrate neural networks working in-orbit for cloud detection and decision making without always relying on ground station to send commands to the satellite. ROKS will pave the way for groundbreaking changes in data protection and encryption and onboard automation.