Compact Transmitters for Free Space Quantum Communication

Abstract

To extend the information-theoretically perfect security benefits of quantum key distribution (QKD) to global distances, free-space QKD links must be used due to their lower loss and greater flexibility than fibre-based networks. In this thesis, I develop three polarisation-encoded QKD transmitters optimised for different types of free-space QKD links within a quantum network. For satellite-to-ground QKD, I built a compact Discrete-Variable QKD (DVQKD) transmitter for the Satellite Platform for Optical Quantum Communication (SPOQC) CubeSat mission of the UK’s Quantum Communications Hub. The transmitter fits within a 0.5 U CubeSat (100 mm×120 mm×42 mm), requiring ≤ 8 W of power. The design is miniaturised by reducing the number of optical components needed to generate polarised photons for the decoy-state BB84 protocol, and utilises wavelength multiplexing (785 nm and 808 nm) to overcome modulation rate limitations of the laser diodes, generating photon pulses at 200 MHz. Extensive environmental testing — vibration, shock, and thermal vacuum — confirmed survivability and performance. Security analysis showed that small spectral shifts at extreme temperatures could introduce side-channel information leakage. This leakage is limited to the order of 10−4 bits per transmitted pulse through thermal stabilisation of the laser diodes. Narrowband spectral filtering, careful laser diode spectral characterisation and conservative privacy amplification can bound Eve’s information to produce secure key. For user-level integration into QKD networks, a handheld QKD device is developed. This low-cost, portable transmitter employs LEDs, micropolarisers, and spectral filters for a compact design that operates at 80 MHz with a secure mean photon number of 0.025 photons/pulse for BB84 protocol to mitigate multi-photon emission probability. A single-mode fibre co-aligns the 650 nm quantum signal with a 517 nm beacon for alignment and timing synchronisation. Real time beam steering and polarisation compensation setups are embedded with the receiver (quantum key point), where the user can connect to the quantum network with an estimated key rate of 103 kbps. Lastly, novel UV-C-based DVQKD transmitter is proposed that leverages low divergence loss for inter-satellite QKD links. UV-C potentially offers a 20 dB advantage over distances ≥ 1000 km in comparison to near-infrared wavelengths with congruent satellite parameters to SPOQC. Though limited by availability of current UV-C components, UV microLEDs at 280 nm produces photon pulses at 1 GHz modulation rate, yielding an estimated key rate of 73 kbps. These optimised transmitters demonstrate secure, scalable quantum communications adaptable to specific QKD scenarios across satellite-ground, user-network, and intersatellite links.

Date of Award	13 May 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Siddarth K Joshi (Supervisor), John G Rarity (Supervisor) & Daniel Oi (Supervisor)