Speaker
Description
We present a System-on-a-Chip (SoC) architecture, based on Field Programmable Gate Array (FPGA), suitable for satellite quantum communication which exploits a COTS board, the ZedBoard by AvNet. Beside granting a very high flexibility thanks to its Zynq-7000 SoC, including both an FPGA and a CPU, this architecture allows to implement a “1-random-1-qubit” encoding where a unique random number, generated with no expansion from a Quantum Random Number Generator (QRNG), is used to encode a single qubit in a discrete variable Quantum Key Distribution (QKD) scheme exploiting the polarization degree-of-freedom of single photon.
The architecture has two main layers: one is implemented at CPU level and is responsible for the high-level functions (data transfer, parameters setting); the other is implemented on the FPGA to handle the high speed and deterministic functions (generation of the output signals driving the electro-optical setup). This offers a very high flexibility as the FPGA design is strictly reserved only to specific functions that require precise timing and high speed.
On the FPGA, 3 signals (5ns width) are generated with a repetition rate of 50 MHz (due to analog bandwidth limits). We also exploit both the CPUs to realize a continuous stream from an external source (QRNG/PC) through a 1-Gbps TCP connection which allows to have a continuous qubit transmission with no interruptions. Given a 4-bit encoding for each qubit (2 bits for polarization, 2 for intensity), we implement the architecture to sustain more than 200 Mbit/s. The data transfer is organized in two processes: from QRNG/PC to CPU0 where the data is stored into the on-board RAM memory; from CPU1 to FPGA where the data is moved from RAM to Block-RAM. The whole procedure is synchronized through interrupts (FPGA<->CPU1; CPU1<->CPU0; CPU0<->QRNG/PC). Over recent years, this system (or variation of it) was successfully used in several (satellite) QKD/QRNG experiments realized by the QuantumFuture research group. It was also implemented in the commercial QKD systems provided by ThinkQuantum, a spin-off company from University of Padova. Results were recently published in a peer-reviewed article (A. Stanco et al., DOI: 10.1109/TQE.2022.3143997).
The system was tested with a QRNG device, able to provide >200 Mbit/s, for 55 hours showing no interruptions and correctly delivering the data for the qubit transmission. Most of nowadays systems exploits a low-rate QRNG (~Mbit/s) and algorithm expansions to reach the required bitrate but with a major drawback in security as the transmitted qubit sequence is not fully random due to the expansion algorithms. Thus, our system offers a higher level of security for QKD thanks to the true randomness of the qubit sequence. According to the current state of the European and Italian satellite QKD missions, this represents a relevant result since such missions are considering payloads with both a QKD transmitter and a QRNG. Furthermore, as Cubesat technology is becoming more prominent and COTS components have started to find their place in space mission, a COTS-based system for a QKD-QRNG apparatus can be considered a valid baseline for satellite quantum communication.
