# Secure Quantum Communication Technologies and Systems: From Labs to Markets
## 3 Devices for QKD Systems
### Quantum Random Number Generators
Compared to other QRNG approaches, generators based on phase diffusion have been shown to provide higher bit rates owing to the use of conventional photodetectors instead of SPADs [103]. In addition, PN-QRNGs are also more robust against detector noise and have been successfully evaluated versus extensive randomness tests, thus ensuring both high data rate and reliability [104]. However, most of these demonstrations use bulk fiber or free space components, which are large, costly, and exhibit long-term instability. Significant reduction in size is essential for the integration into complex systems such as QKD receivers. Similarly, larger size and higher cost reduce scalability and limit widespread commercial use. Instability issues strongly affect the reliability of these devices. To achieve low-cost, scalable solutions, there has been a huge interest in photonic integrated circuit (PIC) technologies for quantum optics recently. A number of on-chip QRNG solutions for well-investigated schemes (referenced above) have been reported in the literature, demonstrating different levels of complexity and using different integration technologies, e.g., Lithium Niobate (LiNbO3) or Indium Phosphide on Silicon (InP/Si) [106, 107, 108, 109, 110, 111, 112, 113]. Authors in [106] demonstrated a monolithtically integrated QRNG on a Si platform using a light emitting diode (LED) as a quantum entropy source and a SPAD, achieving a 1 Mbit/s rate. [PERSON] et al. demonstrated a photonic integrated scheme for multiplexed QRNG on a lithium niobate platform to achieve higher generation rates [107]. However, the relatively large footprint of lithium niobate devices restricts widespread use in complex applications. Two fully integrated PN-QRNGs, including entirely passive and active elements and providing Gbit/s rates using InP integration technology, have been demonstrated recently [108, 109]. Both these implementations employed accelerated phase diffusion exploiting gain switching in lasers. These schemes use two lasers to reduce correlations between successive pulses. Indium phosphide technology has the advantage of monolithic on-chip integration of active (lasers, photo-detectors) and passive (couplers, multiplexers, etc.) components on a single platform. On the other hand, silicon photonic systems have the advantage of compatibility with cost-effective complementary metal oxide semiconductor (CMOS) processes, hence increasing their deployment prospects in advanced semiconductor applications. [PERSON] et al. [110] demonstrated optical integration of a homodyne detector on a silicon-on-insulator (SOI) chip for QRNG based on vacuum fluctuations and achieved a 1.2 Gbit/s rate. An integrated Gbit/s QRNG based on improved homodyne detection of vacuum fluctuation using a silica planar light circuit was reported in [111]. [PERSON] et al. recently demonstrated another SOI-integrated QRNG based on phase fluctuations from an off-chip laser diode, with all other components integrated on a SOI chip [112]. [PERSON] et al. demonstrated interferometric photo detection on a Si chip using an integrated uMZI interferometer for a quantum entropy source based on accelerated phase diffusion. The input multi-mode interference (MMI) coupler of the uMZI was designed to provide an unbalanced splitting ratio (2% and 98%) and 1 ns delay [113]. Photonic integration certainly helps to provide a more compact uMZI compared to bulky discrete devices. However, the performance is strongly impacted by stability issues associated with long delays in uMZI caused by intrinsic phase noise and temperature drifts, and therefore requires strong stability control [111]. Improved visibility and signal-to-noise ratio (SNR) and higher entropy generation rates (short delays require faster modulation) can be achieved by reducing the waveguide losses and difference in length of the two arms of the uMZI [113]. In conclusion, QRNGs employing different quantum phenomena have been extensively investigated and they can be considered as relatively mature quantum technologies.