Quantum key distribution is costly and, at the moment, offers low performance
in space applications. Other more recent protocols could offer a potential
practical solution to this problem. In this work, a preliminary optical payload
design using commercial off-the-shelf elements for a quantum communication
downlink in a 3U CubeSat is proposed. It is shown that this quantum state
emitter allows the establishment of two types of quantum communication between
the satellite and the ground station: quantum key distribution and quantum
keyless private communication. Numerical simulations are provided that show the
feasibility of the scheme for both protocols as well as their performance. For
the simplified BB84, a maximum secret key rate of about 80 kHz and minimum QBER
of slightly more than 0.07% is found, at the zenith, while for quantum
private keyless communication, a 700 MHz private rate is achieved. This design
serves as a platform for the implementation of novel quantum communication
protocols that can improve the performance of quantum communications in space.Comment: 24 pages, 9 figure