# Secure Quantum Communication Technologies and Systems: From Labs to Markets
## 3 Devices for QKD Systems
### Quantum Random Number Generators
The time interval between two photodetection events is the difference of two exponential random variables, which is also exponential [105]. Therefore, the time difference between the arrival times of consecutive pulses can be compared to generate random bits. This type of QRNG can generate random bits at Mbit/s rates. Major design challenges are the time precision and the detector dead time. In optical communications, strong amplification spontaneous emission (ASE) noise is a limiting factor, but it is a good source of entropy for QRNGs. The main advantage is that ASE provides a readily available strong signal of quantum origin that can be measured with conventional optical devicesat fast rates [105]. QRNGs based on amplified spontaneous emission use amplitude fluctuations as entropy sources. [PERSON] et al. [106] demonstrated an ASE-based QRNG that uses a 915 nm pumped erbium/ytterbium co-doped fiber as an optical randomness source. The output ASE spectrum is band-pass-filtered and amplified again by a conventional erbium-doped fiber amplifier (EDFA). A fiber polarization splitter is used to separate the two independent orthogonal polarization components prior to photo-detection. The detected signals in each arm, v1(t) and v2(t), have random amplitude whose distribution depends on the shape of the filter. To generate random bits, the two detected signals are compared at a sampling instant, generating a \"1\" when v1(t) > v2(t) and a \"0\" otherwise. Alternative schemes have also been proposed [8]. These devices can achieve generation rates of Gbit/s. The rate of change of amplitude of the ASE field is usually much faster than the speed of the detection mechanism, thus making the speed of the detector the limiting factor for higher generation rates. QRNGs have also been demonstrated to exploit the fluctuations in the quantum vacuum state [107]. The vacuum state can be considered as a superposition of amplitude quadrature states. These generators use homodyne detection to measure one of the quadrature fluctuations. In homodyne detection, the vacuum state is mixed with a reference field of a local oscillator (LO) laser in a balanced optical beam splitter and photo-detected with balanced detectors at the splitter outputs. The detector outputs are subtracted and processed, generating a current output proportional to the quadrature amplitude of the vacuum field. Its values are a measure of the fundamental uncertainty in the vacuum state. This signal is then digitized and processed to produce random number sequences [105]. The achievable output generation rate is dependent on the characteristics of the local oscillator, the detector noise and bandwidth, and the characteristics of the digitizer. Other challenges are the removal of classical noise and the post-processing complexity. These QRNGs can achieve rates in the Gbit/s range. Semiconductor laser output field fluctuations are caused by spontaneous emission in the laser cavity. The resultant random phase noise, also referred to as phase diffusion (PD), has a quantum mechanical nature [93] and is exploited to generate random numbers in phase noise-based QRNGs (PN-QRNG). PN-QRNGs basically measure field quadrature fluctuations of phase-randomized weak signal states [23]. This is achieved by translating the random phase fluctuations of the laser into amplitude fluctuations, using an unbalanced Mach Zehnder interferometer (uMZI). Two different schemes have been reported for PN-QRNGs. The first approach directly detects phase fluctuations from a continuous wave laser using a delayed self-heterodyne detection system [98, 104], whereas the second scheme strongly modulates the laser in the gain switching regime to produce a pulsed light output. The uMZI delay is matched to the pulse repetition rate to translate the phase difference in successive pulses into amplitude variations [99, 100, 101]. Pulsing helps to increase the phase diffusion rate for the same mean power, which is proportional to the spontaneous emission rate over the intra-cavity photon number [101].