Secure Real-Time Transmitter for Continuous Variable Quantum Key Distribution

Quantum key distribution (QKD) is a cryptographic technique which utilizes quantum phenomena, such as superposition and entanglement, to enable the sharing of a secret random key for encrypted communication. Continuous variable QKD (CV-QKD) offers a practical way for performing a secure key exchange by means of broadband modulation of laser light, where the information is encoded in the light field quadratures.
We focus on building a real-time high-rate CV-QKD transmitter using flexible field programmable gate array (FPGA) chip technology. Composable security for CV-QKD has been proven only for Gaussian modulated quantum states. We implement a high-rate Gaussian sampler using cumulative distribution table (CDT) method. Important security parameters are calculated and the algorithms are realized with respect to maintaining the overall security of the CV-QKD system. Furthermore, digital signal processing (DSP) blocks are implemented in order to support fast (50 MBd/s) exchange of quantum states.
A necessary resource in any QKD protocol is a supply of uniformly distributed random numbers. Such a resource is particularly facilitated by quantum processes, which can be used to generate provably secure random numbers. We employ a high-rate vacuum fluctuation-based quantum random number generator (QRNG) with 8 Gb/s random number output. Our standalone QRNG system includes a real-time entropy testing based on monitoring the power spectral density according to a rigorous security model, instead of using common statistical tests that do not prove security. We address the issue of traditionally slow post-processing with a fast randomness extraction method based on Toeplitz hashing. To our knowledge, this is currently the fastest real-time QRNG implementation. A consequence of this is the fastest implementation of the Gaussian sampler used in CV-QKD. The thesis concludes with the evaluation of the transmitter as a part of our CVQKD setup. It demonstrates for the first time an all-in-one implementation of transmitter functions for high-rate CV-QKD links. We recognize the potential for wide adoption of CV-QKD in near-future secure communication networks.